Vibrational Spectra of Organometallics: Theoretical and Experimental Data

By Edward Maslowsky, Jr.

A comprehensive compilation of the available experimental and theoretical vibrational data for organometallic compounds and its role in evaluating the structures, bonding, and properties of these key compounds

This unique book offers a thorough review of the literature dealing with vibrational data obtained using various phases, including matrices, reported for organometallic compounds from infrared spectra, Raman spectra, and several other techniques. It is the only one that compiles the available experimental and theoretical vibrational data on these compounds, and which discusses the importance of this information and its role in evaluating structures, bonding, and other important properties. It also treats the use of DFT and other theoretical calculations to analyze the vibrational data and to predict properties associated with these compounds. The book also includes vibrational data for organic species that form on metal and other surfaces.

Vibrational Spectra of Organometallics: Theoretical and Experimental Data offers complete coverage of: Carbide, Alkylidyne, Alkylidene, Alkyl, and Alkane Derivatives; Noncyclic Carbon Clusters and Unsaturated Hydrocarbon Derivatives; and Cyclic, Unsaturated Organometallic Derivatives. By summarizing work that has already been done on organometallic compounds, it serves as an important reference for those studying their vibrational spectra and will, in the end, lead to a clearer understanding of other research that needs to be done in order to help researchers determine new research directions.

• An important reference for those studying the vibrational spectra of organometallic compounds
• Gathers the existing experimental and theoretical vibrational data and discusses its significance in assessing structures, bonding, and other principle properties
• Includes DFT methods for the interpretation of spectra, which has been one of the major developments of the last two decades 

Vibrational Spectra of Organometallics: Theoretical and Experimental Data is an important reference for researchers and practitioners in the areas of inorganic, organometallic, organic, and surface chemistry who have an interest in using vibrational data to characterize the bonding, composition, reactions, and structures of organometallic compounds, and organic species that are formed on various surfaces.

• Language: English
• Publisher: Wiley
• Release date: Dec 13, 2018
• ISBN: 9781119404682

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Part I

Carbide, Alkylidyne, Alkylidene, Alkyl, and Alkane Compounds

1

Monocarbide Complexes

The ν(AlC) mode of AlC has been assigned to infrared bands at 629.8 and 640.1 cm−1 in an Ar matrix study [1], and a formal bond order of 1.5 has been determined for the AlC species [2]. It was proposed that the 629.8 cm−1 band might be from AlC molecules that are surrounded by Ar and the 640.1 cm−1 band, which is close to the value of 654.8 cm−1 in the emission spectrum of AlC vapor [3], from molecules on the grain surface. Both assignments are close to a calculated ν(AlC) value of 629 cm−1 [4].

The ν(WC) features at 636, 582, and 392 cm−1 in a high resolution electron energy loss spectroscopy (HREELS) study of a clean carbide‐modified W(211) surface have been attributed to several different C atom sites on that surface [5] and an HREELS band at 581 cm−1 to a WC species on a W(110) surface [6]. An HREELS loss at c. 600 cm−1 has been assigned to the ν(RuC) mode of adsorbed C atoms on a Ru(001) surface [7]. A Raman band at 552 cm−1 and an HREELS loss at 520 cm−1 have been attributed to adsorbed atomic C on stepped Ni(111) [8] and silica‐supported Ni catalyst surfaces [9], respectively, and an HREELS band at 520 cm−1 to atomic C adsorbed on a Pt(210) surface [10]. The slightly lower ν(RhC) frequency of Rh≡C in Ne and Ar matrices (Table 1.1) compared with that of 1040.0 cm−1 in the vapor phase [11] shows a slight influence of the matrix atoms on the frequency [12]. The larger frequency shift in an Ar than a Ne matrix was attributed to weak van der Waals interactions between Rh≡C and the more easily polarized matrix Ar atoms [12]. Although infrared bands were not detected in Ar matrix isolation studies due to expected low species concentrations, band intensities and ν(MC) (M = Cu, Ag) frequencies were calculated for CuC (585.6 cm−1) [13] and AgC (477.2 cm−1) [14]. The lower ν(UC) frequency of U≡C in an Ar than a Ne matrix (Table 1.1) was attributed to a stronger interaction of U≡C molecules with the Ar than the Ne matrix, and the frequency in the Ne matrix is closer to the calculated value of 908 cm−1. It was also suggested that the higher than expected ν(UC) frequency difference of 44 cm−1 between the Ar and Ne matrices might be from different electronic ground states of U≡C molecules in these matrices [15, 16]. Table 1.1 summarizes infrared assignments for diatomic metal carbides.

Table 1.1 Infrared diatomic metal carbide stretching mode assignments (cm−1).

a) It was suggested in Ref. [1] that the two bands might be from two different environments for the AlC molecules, with the lower‐frequency band from AlC molecules that are surrounded by Ar and the higher frequency one from those on the grain surface.

The ν(MO) mode in the Ne matrix infrared spectrum was assigned for CMO− (M = Nb, 877.8 cm−1; Tb, 902.0 cm−1), and the ν(NbC) and ν(NbO) modes of CNbO have been assigned at 783.7 and 919.8 cm−1, respectively, but CTbO was not detected [19].

Initial infrared data and discrete Fourier transform (DFT) calculations indicated that isolated CUO molecules (ν(UC) = 1047.3 cm−1, ν(UO) = 872.2 cm−1) formed in a Ne matrix but that the U atoms of CUO molecules formed weak chemical bonds with the atoms of Ar, Kr, and Xe matrices [20]. Indeed, in a 1% Ar in Ne matrix, the lower ν(UC) (854.0 cm−1) and ν(UO) (806.5 cm−1) values were given as evidence for CUO·Ar. Later matrix isolation infrared studies and DFT calculations concluded that triplet state CUO was present in pure Ar (ν(UC) = 887 cm−1, ν(UO) = 834 cm−1), Kr, and Xe matrices [21]; the CUO molecule is linear and the singlet state (ν(UC) = 1047.3 cm−1, ν(UO) = 872.2 cm−1) in a Ne matrix [22] and that the CThO molecule is nonlinear with a 109° CThO angle and a triplet ground state (ν(ThC) = 617.7 cm−1, ν(ThO) = 812.2 cm−1) [23]. There was also infrared evidence for the complexes CUO(Ar)4−n(Ng)n (Ng = Kr, Xe; n = 1–4) [21]. In addition, the ν(ThO) mode of CThO− was assigned to a weak intensity infrared doublet at 761.7/759.6 cm−1 [23] and the ν(UC) and ν(UO) modes at 929.3 and 803.3 cm−1, respectively, for CUO− [22].

In an Ar matrix infrared study, it was concluded that the triplet ground state C≡U≡C molecule is linear, and the νa(UC) mode was assigned at 891.4 cm−1 [15, 16].

Ar matrix infrared data and frequency and intensity calculations of the three vibrational modes of M2C (M = B [24], Al(1), Si [25, 26]) indicated nonlinear structures of C2v symmetry. The inability to detect some of the expected bands was consistent with the low intensities predicted in theoretical calculations, and the calculated kSiC value indicated a bond order of between one and two for Si2C [25]. Results from these studies are shown in Table 1.2. A linear structure has been proposed in vibrational studies of Se2C in various phases [17, 27, 28], and the νs(Se2C), δ(Se2C), and νa(Se2C) modes have been assigned at c. 364, 313, and 1303 cm−1, respectively [27]. An ab initio study of the structure and vibrational spectrum of Si3C predicted a rhomboidal C2v symmetry ground state with two equivalent Si atoms in a nonlinear SiCSi framework and a transannular Si─C bond formed by the third Si atom [29]. These conclusions were supported by the assignment of five of the six fundamental vibrational modes of Si3C in an Ar matrix infrared study [30].

Table 1.2 Theoretical and experimental vibrational assignments (cm−1) for nonlinear CM2 compounds.

a) Band not observed.

Ar matrix infrared data and calculations indicate a linear structure for HB═C═BH (νa(¹⁰BC¹⁰B) = 1895.2 cm−1) and its isotopomers [31]. The compound Al3BC3 has Al5C trigonal bipyramids that are linked by common corners of the basal plane to produce linear C═B═C⁵− groups located between Al3C layers [32]. In addition to vibrational assignments for the C═B═C⁵− unit (νa(CBC) = 1580 cm−1, νs(CBC) = 1041 cm−1, δ(CBC) = 735 cm−1), Raman bands at 421 and 560 cm−1 were assigned to ν(AlC) modes of Al5C [32]. Some infrared assignments were given for Ga2I2{C[Si(CH3)3]3}2 (νa(SiC) = 658 cm−1, νs(SiC) = 620 cm−1) and Ga2I4{C[Si(CH3)3]3}2 (νa(SiC) = 660 cm−1) [33]. Infrared and Raman data have been assigned for (CH3Hg)4C (νa(Hg4C) = 620 cm−1; νs(Hg4C) = 137 cm−1) [34], (RHg)4C (R = CN−, HCOO−, CH3COO−, CF3COO−; νa(Hg4C) = 720–632 cm−1; νs(Hg4C) = 147–129 cm−1) [35], and (XHg)4C (X= F, Cl, Br, I; νa(Hg4C) = 690–623 cm−1; νs(Hg4C) = 165–98 cm−1) [36]. Infrared assignments, including the ν(GeC) mode, have been made for (X3Ge)4C (X3 = H3, 745 cm−1; Cl2Br, 701 cm−1; Br3, 672 cm−1) [37].

The νs(FeC) mode was assigned at 443 cm−1 from resonance Raman data for a linear FeIV═C═FeIV unit in a μ‐carbido Fe tetraphenylporphyrin dimer and the νa(FeC) mode at 939/885 (sh) cm−1 from infrared data [38, 39]. It is the first reported complex with a formally dicarbenic C atom bridging a pair of transition metal atoms [39].

Vibrational data for complexes of interstitial carbide atoms in transition metal clusters with CO, or CO and other ligands have mainly included the assignment of ν(CO) and ν(metal–C) modes. A 1980 review compares ν(CO) assignments as a function of cluster size and charge for clusters with five, six, and eight Fe, Ru, Os, Co, Rh, Fe/Rh, and Fe/Mo atoms [40]. Later studies included ν(CO) assignments for carbide CO complexes with Re6 [41], Re7 [42], Re8 [43], Re7M (M = Rh, Ir, Pd, Pt) [44], Fe5 [45], Ru5 [46–49], Os5 [49], Ru6 [50], and Nin (n = 8, 9) [51] clusters. Changes in the number and frequency of the infrared ν(CO) bands in CH2Cl2 solutions have been followed as {[Co5C(CO)12]Au[Co(CO)4)]}− was reduced to 2‐ and 3‐oxidation states and {[Co5C(CO)12]2Au}− was oxidized to the 0 and reduced to the 2‐ and 3‐oxidation states [52].

Infrared and Raman assignments at both 100 and 300 °C have been reported for the four vibrational modes (illustrated in Table 1.3) associated with the Fe─C─Fe fragment of three tetrairon carbide CO clusters with butterfly Fe4 cluster arrangements of C2v symmetry [53]. Assignments for the ¹²C/¹³C isotopomers of these modes recorded at 100 °C are also given in Table 1.3. This study included carbide mode assignments for the [(C6H5)3P]2N+ salts of the related Cs symmetry mixed metal carbide CO complexes [Fe3WC(CO)13]²− and [Fe3RhC(CO)12]−, with the highest carbide mode frequency at 899 and 953 cm−1, respectively, a second (equivalent to the A1(2) mode of the C2v complexes) at 651 and 668 cm−1, respectively, and a third at 263 and 274 cm−1, respectively [53].

Table 1.3 Vibrational assignments (cm−1) associated with carbide motion in butterfly tetrairon carbide CO clusters of C2v symmetry at 100 °C.a)

a) Reference [53].

b) Data are for the anions of the [(C6H5)3P]2 N+ salt.

An X‐ray diffraction study of Fe5C(CO)15 showed a C atom at the center of an approximate equilateral tetragonal pyramid formed by the Fe atoms of five Fe(CO)3 fragments [54]. It has also been concluded [55] that previously suggested assignments [54] of infrared bands at 790 and 770 cm−1 to semiinterstitial C atom vibrations in Fe5C(CO)15 are likely correct. It was later noted that solid Fe5C(CO)15 exists as needle and plate polymorphs that had slightly different carbide ν(FeC) frequencies at 805 and 765 cm−1 and 805 and 780 cm−1, respectively [45]. At 298 K, isostructural Ru5C(CO)15 exhibited three semiinterstitial C atom bands (730, 738, and 757 cm−1) that became five bands (735, 745, 752, 769, and 772 (sh) cm−1) on cooling to 90 K [56]. Interstitial C atom vibrations have been assigned for Os5C(CO)15 (795, 777, 770, and 755 cm−1) [57]. It has been reported that the complexity of the infrared spectra of M5C(CO)15 (M = Ru, Os), as was also seen for Fe5C(CO)15, is from two nonequivalent sets of molecules in the unit cells [45]. An infrared study has concluded that the dependence of the carbide ν(MC) frequencies on the cluster geometry can be used as a structural indicator for M5C(CO)15 (M = Ru, Os) and several of their derivatives [58]. The ν(CO) frequency for [Fe5C(CO)15]²− (1735 cm−1) has been noted [45, 59] to be consistent with a structure with two semi‐edge‐bridging CO ligands over two adjacent basal Fe─Fe bonds. This complex also has an infrared carbide ν(FeC) band at 815 cm−1 [59].

In contrast to Ru5C(CO)15, the complexes HRu5C(CO)15X (X = Cl, Br) have a μ‐hydrido‐bridged butterfly Ru5 geometry with carbide ν(RuC) bands at c. 690 and 825 cm−1 at 298 K and a c. 5 cm−1 splitting of the 825 cm−1 band on cooling to 90 K [56]. This study included ν(RuRu) and ν(RuX) assignments for HRu5C(CO)15X (X = Cl, Br). A similar bridged butterfly geometry has been found for the Ru atoms in Ru5C(CO)15 · CH3CN, where the interstitial C stretching modes were assigned at 674 and 817 cm−1 [46]. Interstitial carbide ν(MC) bands have been assigned for the ¹³C‐enriched carbide CO complexes [M6C(CO)n]m− (M = Fe, n = 16, m = 2; Ru, n = 16, m = 2, and n = 17, m = 0) (900–600 cm−1) [60], H2Os10C(CO)24 (735.4, 760.3, and 772.8 cm−1), and [(CH3)4N]2[Os10C(CO)24] (753 cm−1) [61].

Comparison of the infrared spectrum of Co6C(CO)12S2 to that of the same complex containing 90% ¹³C led to the assignment of bands at 819 and 548 cm−1 in the unlabeled compound and 790 and 535.5 cm−1 in the ¹³C isotopomer to the A2″ and E′ vibrations, respectively, for a C atom encapsulated at the center of a trigonal prism formed by the six Co atoms [55]. Enrichment with ¹³C also allowed for the assignment of the interstitial ν(MC) modes for [M6C(CO)15]²− (M = Co, Rh) [62], and the calculation of the axial and equatorial metal carbide force constants [62]. An infrared study that used ¹³C labeling to assign the carbide ν(MC) frequencies of both the metal‐capped and uncapped Re6(μ‐C) and Ni6(μ‐C) cores of CO clusters concluded that while a single force constant could account for the frequencies for the capped and uncapped Ni carbide clusters and the uncapped Re carbide clusters, vibrational assignments for the capped Re clusters required slightly different equatorial and axial ReC force constants [63].

2

Terminal and Bridging Methylidyne (CH) and Other Alkylidyne Complexes

A theoretical study of the equilibrium geometry and vibrational frequencies has concluded that the structure Al≡CH with a triple bond between the Al and C atoms is not the most stable ground‐state structure [64]. The ν(Si≡C) mode of Si≡CH has been assigned from FTIR (1010.4 cm−1) [65] and electronic (1013 cm−1) [66] spectral data. The ν(P≡C) mode has been assigned at 1276 and 1110 cm−1 for P≡CH and P≡CH+, respectively [67], and 1104 cm−1 has been calculated for the ν(As≡C) mode in a theoretical study of As≡CH [68]. The ν(P≡C) mode for P≡CC6H5 has been assigned at 1565 cm−1 in an infrared study that included normal coordinate calculations [69].

Infrared and Raman assignments have been made for (CH3Hg)3CH (νa(Hg3C) = 615 cm−1, νs(Hg3C) = 450 cm−1, ν(CH) = 2900 cm−1) [34].

The infrared frequencies from the bis(trimethylsilyl)methyl group, {[(CH3)3Si]2CH}−, in organometallic complexes of this group, including those from 2960 to 2890 cm−1 (ν(CH)), 1295 to 1250 cm−1 (δ(CH3)), 1070 to 1005 cm−1 (ρ(CH3)), 2960 to 2890 cm−1 (ν(CH3)), and 340 to 280 cm−1 (δ(SiC3)), have been noted to be relatively constant [70]. Vibrational data and some assignments have been reported for B and Al [70], Cd and Hg [70], and Sb and Bi complexes [71] of this group.

All but the ν(CH)/ν(CD) modes have been assigned from the laser‐induced fluorescence spectra of gaseous M≡CH/M≡CD (M = Ti [72], V [73]). Assignments have been made from the vapor‐phase fluorescence spectrum for W≡CH [74] and the Ar matrix infrared spectrum of U≡CH [16]. These assignments are summarized in Table 2.1.

Table 2.1 Vibrational assignments (cm−1) for transition and actinide metal methylidyne complexes.

a) No evidence was found for the ν(CH) mode.

A complete Raman (polycrystalline) and infrared (benzene‐d6 solution) study and normal coordinate analyses have been reported for trans‐W(≡CH)[P(CH3)3]4Cl (ν(W≡C) = 909 cm−1, ν(CH) = 2994/2978 cm−1, δ(W≡CH) = 788/755 cm−1) and its ≡CD and d36 P(CD3)3 isotopomers [75]. Normal coordinate calculations show that the low ν(W≡C) frequency in this complex relative to that for other metal–alkylidyne M≡CR complexes (R = CH3, C6H5; ν(M≡C) = c. 1270–1600 cm−1) [75, 76] is from the lack of mixing of this mode with the ν(CR) (when R is heavier than H) and internal vibrational modes of the R group. Adsorbed CH (νs(W≡C) = 581 cm−1, ν(CH) = 2930 cm−1, δ(CH) = 925 cm−1) has also been reported to form from ethyne on a W(110) surface in a high resolution electron energy loss spectroscopy (HREELS) study [6]. Although an infrared and Raman study of trans‐X(CO)4W≡CCH (X = Cl, Br, I) and trans‐Br(CO)4W≡CCD3 assigned the ν(W≡C) mode at 1315 cm−1 for the d3 complex, strong vibrational coupling of the ν(W≡C) and δ(CH3) modes that are expected in the same region for the nondeuterated complexes led to Fermi resonance and gave two bands at c. 1355 and 1275 cm−1 [77]. Limited infrared assignments have also been given for trans‐X(CO)4M≡CC6H5 (M = Cr, X = Br, I; W, I) [78].

A unique η²‐CH ligand has been found in an X‐ray diffraction study of the Fe4 butterfly cluster compound HFe4(η²‐CH)(CO)12 [79]. Vibrational modes associated with the carbide fragment of the methylidyne ligand in HFe4(η²‐CH)(CO)12 have been assigned at 824/800, 657/645, and 248 cm−1 [53]. Infrared and Raman assignments and approximate normal coordinate calculations have been given for the HC≡Ru3 fragment of H3Ru3(μ3‐CH)(CO)9 (ν(CH) = 2988 cm−1, δ(Ru≡CH) = 894 cm−1, νa(RuC) = 427 cm−1, νs(RuC) = 670 cm−1) [80]. The ethylidyne modes were assigned [81] from infrared data for (μ‐H)3M3(μ3‐CCH3)(CO)9 (M = Ru, Os) in a comparison with similar assignments noted below for Co3(μ3‐CCH3)(CO)9 [82]. Infrared assignments have also been made for [(η⁵‐C5H5)2Ru3(μ2‐CCH3)(CO)3]BF4 where the μ2‐CCH3 ligand is more electron deficient than the μ3‐CCH3 ligand in (μ‐H)3M3(μ3‐CCH3)(CO)9 (M = Ru, Os) [81].

Infrared and Raman assignments for the HC≡Co3 fragment (ν(CH) = 3041 cm−1, δ(Co≡CH) = 850 cm−1) of Co3(μ3‐CH)(CO)9 and its d1 isotopomer, where the C atom is bonded to the equilateral triangle formed by the Co3 cluster, were supported by HREELS assignments noted below for C≡H groups adsorbed on triangular sites of a Ni(111) surface [83]. Likewise, vibrational assignments noted below and a normal coordinate analysis for Co3(μ3‐CCH3)(CO)9 and its d3 isotopomer supported assignments made in HREELS studies of C≡CH3 on a Pt(111) surface [82]. Adsorbed CH was also found (ν(CH) = 2928 cm−1, δ(Rh≡CH) = 930 cm−1, νs(RhC) = 750 cm−1) in an HREELS study of the decomposition of benzene on a Rh(111) surface [84]. Both IRMPD spectroscopy and DFT calculations have been used to identify the methylidyne complex HIrCH+ in a reaction of methane and gas‐phase Ir+ cations [85].

HREELS has proven useful in studying adsorbed methylidyne and ethylidyne on transition metal surfaces. Although it was originally suggested that HREELS data showed CH on a Pd(111) surface [86, 87], the same authors later concluded [88] that the data favored the formation of a CCH species and that assignments made for CH on Ni(111) [89, 90], Pt(111) [91], and Rh(111) [92] surfaces might be better reinterpreted as consistent with the CCH species. HREELS vibrational assignments for methylidyne and ethylidyne species on metal surfaces are summarized in Tables 2.2 and 2.3, respectively.

Table 2.2 HREELS vibrational assignments (cm−1) for methylidyne adsorbed on transition metal surfaces.

a) Reference [88] has concluded that the HREELS data on this surface might be more consistent with the CCH and not the CH species.

Table 2.3 HREELS vibrational assignments (cm−1) for ethylidyne adsorbed on transition metal surfaces.

a) Assignments of data in Ref. [103] were made in Ref. [96].

An HREELS study that included normal coordinate calculations and ¹³C and ²H substitution identified the formation of ethylidyne on a threefold hollow adsorption site of Ni(111) [106]. The HREELS spectrum has been illustrated for ethylidyne and its d3 isotopomer that formed on a Pt(111) surface [116]. DFT [111], and infrared frequency and intensity calculations [117] were used to assign the vibrational modes for ethylidene on this surface and to differentiate it from other possible vinylidene reaction intermediates.

The kinetics of ethylidyne buildup on Pd/Al2O3 surfaces from the reaction of ethene and H2 gas mixtures has been followed using the δs(CH3) mode (1333 cm−1) [118]. Vibrational coupling of neighboring adsorbed ethylidyne species in an equimolar mixture of adsorbed ¹²CH3C and ¹³CH3C on a catalytic Pt/Al2O3 was found by the increased intensity sharing of the δs(CH3) mode of these species with increased ethylidyne coverage [119]. The infrared spectra have been assigned for ethylidyne formed in the reaction of ethene on M/Al2O (M = Ru, Rh, Pd, Pt) catalytic surfaces [120].

The propylidyne ligand infrared spectrum has been assigned for C3H5Co3(CO)9 [121]. The infrared spectrum of propane adsorbed on a Pt/SiO2 catalyst at room temperature indicated mainly surface propylidyne [122]. HREELS assignments have also been made for propylidyne on Rh(111) [123], Ru(0001) [124], and Pt(111) [123] surfaces, and RAIRS has been used to identify propylidyne [125] and cis‐ and gauche‐butylidyne on Pt(111) surfaces [126], propylidyne [127] and cis‐ and gauche‐butylidyne on Ru(001) [128] surfaces, and 1‐butylidyne and i‐butylidyne on single‐crystal Ru(0001) surfaces [129]. RAIRS has also identified different rotational conformers of hexylidyne produced from 1‐hexyne on a Ru(0001) surface [130] and from hexene on Ru(0001) and Pt(111) surfaces [131].

3

Terminal and Bridging Methylidene (CH2) and Other Alkylidene Complexes

The normal mode frequencies and intensities of CH2M (M = Be, Mg) have been calculated and the Ar matrix infrared spectra assigned for CH2Mg (ν(CH2) = 2787 cm−1, ν(MgC) = 472 cm−1) and its d2 isotopomer [132]. This study concluded that it is inaccurate to represent the structures as CH2═M (M = Be, Mg); rather, the metal–carbon bond was descried as a polar one‐electron σ‐bond from metal to carbon atom donation with no significant π‐bonding [132]. A theoretical study of the equilibrium geometry and vibrational frequencies has similarly concluded that the structure CH2═Al with a double bond linking the Al and C atoms is not the most stable ground state [64].

Vibrational assignments have been made from the laser‐induced fluorescence spectra of CH2═M (M = Si, ν(Si═C) = 930 cm−1 [133, 134]; Ge, ν(Ge═C) = 782 cm−1 [135]). Infrared assignments have been given for H2Si═CH2 (ν(Si═C) = 985 cm−1) [136] and CH3(H)Si═CH2 [137], and Ar matrix infrared data have been reported for (CH3)2Si═C(H)(CH3) [138] and (CH3)(H)Si═CH2 [139]. The original ν(Si═C) assignment to an infrared band at 1407 cm−1 for a compound formulated as (CH3)2Si═CH2 [140] was withdrawn [138]. Later Ar matrix infrared studies have been reported for (CH3)2Si═CH2 and various deuterated isotopomers [141–144]. Comparison of the observed and computed frequencies led to the conclusion that the ν(Si═C) mode contributes to two bands at 1117.5/1112.5 and 895.1/866.5 cm−1 for (CH3)2Si═CH2/(CD3)2Si═CD2, with the higher‐frequency band in each pair having a higher ν(Si═C) contribution [141]. The complex (CH3)2Ge═CH2 was found in an Ar matrix infrared study, and the ν(Ge═C) mode, which was coupled with the CH3 rocking mode, was tentatively assigned at 847.3 cm−1 [145].

An infrared and Raman study of (CH3)3P═CH2 assigned the ν(P═C) mode to a Raman band at 998 cm−1 [146]. The ν(P═C) mode has been assigned at c. 1190 cm−1 in solvent studies of model phosphorus ylides [147].

Vibrational assignments have been made from the laser‐induced fluorescence spectra of CH2═Se (ν(Se═C) = 662 cm−1 to a singlet electronic state transition; 704 cm−1 to a triplet electronic state transition [148]). The six fundamental modes and two combination bands have been assigned for CH2═Se that was isolated in an Ar matrix, and the isotopic pattern was resolved for the ⁷⁷Se, ⁷⁸Se, ⁸⁰Se, and ⁸²Se isotopomers of the ν(Se═C) mode at c. 854 cm−1 [149]. This study included assignments of Ar matrix infrared bands for CH2Se2 and a listing of Raman and infrared bands for (CH2Se)3.

HREELS assignments have been made for adsorbed CH2 on GaAs(100) (ν(CH2) = 2900, ω(CH2) = 1340 cm−1, ρs(CH2) = 860 cm−1) [150], Mo(110) (ν(CH2) = 2920, 2780 cm−1, ω(CH2) = 1320 cm−1) [151], W(110) (νs(CH2) = 2930, ω(CH2) = 1436, ν(WC) = 581 cm−1) [6], Fe(110) (νs(CH2) = 2970, δ(CH2) = 1430 cm−1, ω(CH2) = 1020 cm−1, νs(FeC) = 650 cm−1) [94, 152], Ru(001) (νa(CH2) = 2945 cm−1, νs(CH2) = 2870 cm−1, δ(CH2) = 1295 cm−1, ω(CH2) = 1065 cm−1, ρs(CH2) = 890 cm−1, νs(RuC) = possibly 590 cm−1) [96, 98], Pt(111) (ν(CH2) = 3030, 2906 cm−1, ω(CH2) = 1380 cm−1) [108], and Pd(111) (ν(CH2) = 2906 cm−1) [108] surfaces.

Selected Ar matrix isolation infrared data for neutral organometallic methylidene complexes of the d‐block elements are given in Table 3.1. Assignment of the ν(Ni═C) mode in an Ar matrix FTIR study of Ni═CH2 was aided by the doublet splitting of this band in a 3 : 1 ratio that is consistent with the natural abundances of the ⁵⁸Ni (67.8%) and ⁶⁰Ni (26.2%) isotopes [153]. Doublet splitting or shouldering was also found in an Ar matrix for the ν(Cu═C) mode of Cu═CH2 due to different Cu isotopes [154], and a bond order less than two has been reported for CH2─Au using Ar matrix infrared data [155].

Table 3.1 Ar matrix isolation infrared assignments (cm−1) for neutral organometallic d‐block element complexes of methylidene.

a) It was not specified if it was the νs(CH2) or νa(CH2) mode.

Using mass‐analyzed threshold ionization (MATI) spectroscopy data, the ν(La═C) (670 cm−1), CH2 scissor (1258 cm−1), and ρr(CH2) (446 cm−1) modes have been assigned for LaCH2+, and DFT structural calculations for this species showed unequal C─H bond lengths and La─C─H bond angles from an agostic La⋯H─C group interaction [160]. IRMPD spectroscopy and DFT calculations have been used to study the dehydrogenation of methane on reaction with gas‐phase metal cations and to identify the methylidene complexes MCH2+ (M = Ta, W) that had structures that were distorted from C2v symmetry by agostic interactions involving a C─H bond [161]. This reaction produced a more complex spectrum with Ir that indicated mainly HIrCH+ and other minor features that were attributed to the possible presence of IrCH2+. The same study also identified the methylidene complex PtCH2+ that formed in a reaction of CH4 and gas‐phase Pt+ cations and had an undistorted C2v symmetry and no agostic interactions [85]. An IRMPD spectroscopy study of the activation of CH/CD bonds in Pt+ + nCH4/CD4 (n = 1–4) identified the initial formation of PtCH2+/PtCD2+ [162].

Bands have been assigned in HREELS studies of CH2 adsorbed on W(110) [6] and Ni(111) [89] surfaces, and an infrared band at 2914 cm−1 has been assigned to a ν(CH) mode of CH2 adsorbed on a Ag(111) surface [163].

Infrared data have been listed for (η⁵‐C5H5)2(R3P)Ti═CH2 (R3 = (CH3)3, (C2H5)3, (CH3)2C6H5), and the ν(CH)/ν(CD) bands for the CH2/CD2 group of the (CH3)3P complex were assigned at 2940/2165 and 2885/2095 cm−1) [164]. Thermal decomposition of (CH3)2Cl2Ta(OSiO≡) gave (CH2═)Cl2Ta(OSiO≡) (νs(CH2) = 2961 and 2858 cm−1) [165].

The normal modes for compounds with a bridging methylene group (─CH2─) are illustrated in Figure 3.1, and frequency ranges for these modes are summarized in Table 3.2. The CH2 frequencies are generally lower than those of related CH3 modes. The ν(Si2X) frequencies of [(CH3)3Si]2X (X = O, NH, CH2) have been interpreted as implying that the Si─X─Si angle is relatively large [166]. Using similar arguments based on the ν(Si2C) and ν(CH2) frequencies, a minimum Si─C─Si angle of 116° was calculated for (H3Si)2CH2, and the H─C─H angle was concluded to be less than the tetrahedral angle [167]. Data have been reported for several compounds with the SiCH2Ge unit [168].

9 Normal modes of vibration for the bridging methylene group, M─CH2─M: νs(CH2), δ(CH2), νs(M2C), δ(M2C), ρt(CH2), νa(CH2), ρr(CH2), ρw(CH2), and νa(M2C).

Figure 3.1 Normal modes of vibration for the bridging methylene group, M─CH2─M.

Table 3.2 Frequency ranges (cm−1) for the CH2 modes of bridging methylidene groups.

The infrared and Raman band frequencies from overlapping ν(CH3) and ν(CH2) (3000–2900 cm−1), in‐plane CH2 deformation, and, to some extent, CH3 deformation modes are fairly constant for organometallic complexes of the trimethylsilylmethyl group ((CH3)3SiCH2) [169]. Bands from 1200 to 950 cm−1 are more sensitive to the nature of the metal bonded to this group [169]. Relatively complete infrared and Raman data and approximate assignments have been given for [(CH3)3SiCH2]2M (M = Cd [170], Hg [171]), [(CH3)3SiCH2]HgX (X = Cl, Br, I) [171], [(CH3)3SiCH2]4M (M = Sn, Pb, V, Cr) [169], [(CH3)3SiCH2]3VO [169], and [(CH3)3SiCH2]6M2 (M = Mo, W) [169]. Less complete vibrational data are found for complexes of Zn [172], Al [70, 173, 174], Ga [175–176], In [176, 178, 179], Tl [180, 181], Sb [182], Ti [183, 184], Zr [183], Hf [183], Cr [185], and Re [186].

Replacement of alkyl with trimethylsilylmethyl groups can sometimes give organometallic compounds considerable stability. It has been reported that the (CH3)3P═CH2 group has a greater stabilizing effect and the metalated ylides [(CH3)2P(CH2)2]2M2 (M = Cu [187], Ag [187], Au [188]) (1) have been isolated and infrared bands at 557 and 509 cm−1, 518 and 470 cm−1, and 551 cm−1 assigned to ν(MC2) modes for M = Cu, Ag, and Au, respectively. The transformation of square planar {[(CH3)3PCH2]2AuX2}Br (X = Br, I) from the trans to the cis isomer increased the infrared ν(AuC) frequency from 549 to 560 cm−1 for X = Br and from 543 to 556 cm−1 for X = I and shifted the ν(AuBr) frequency for the X = Br complex (256 cm−1) below the experimental limit of 250 cm−1 [189]. An infrared band at 360 cm−1 is characteristic of the AuCH2Au bridge in three homologous cyclic ylide complexes [190]. Infrared data have been listed for [(CH3)2P(CH2)2]2Rh(cyclooctadiene) and a mixture of [(CH3)2P(CH2)2]2Rh(CO)2 and (CO)2Rh[μ‐(CH3)2P(CH2)2]2 Rh(CO)2 [191]. Assignments have been made of infrared data for (I2As)2CH2 and (X2AsCH2)4C (X = CH3, I) [192] and infrared and Raman data for [(CH3)2Sb]2CH2 and [(CH3)2X2Sb]2CH2 (X = Cl, Br) [193]. Vibrational assignments have been made for several other complexes with the SbCH2Sb unit [164, 194, 197], and a linear relationship found between the ρr(CH2) and the average Sb2C unit frequencies, νav (νav = 0.5[νa(Sb2C) + νs(Sb2C)]), for these complexes has been attributed to the amount of Sb s‐orbital character in the Sb─C bonds [197]:

Structural formula of metalated ylides [(CH3)2P(CH2)2]2M2 (M = Cu [186], Ag [186], Au [187]) (1).

The vibrational spectra have been assigned for (CH3Hg)2CH2 (νa(Hg2C) = 577 cm−1, νs(Hg2C) = 465 cm−1) [34]. Polycrystalline infrared and Raman and single‐crystal Raman data for (ClHg)2CH2 (νa(Hg2C) = 643 cm−1, νs(Hg2C) = 512 cm−1) and its d2 isotopomer have been assigned [198].

RAIRS assignments have been made for ethylidene that formed through the carbonyl‐specific dissociation of acetaldehyde on β‐Mo2C, and DFT calculations showed coupling of the CH deformation and ν(C═C) modes in this complex with the νa(Mo═C) mode to give a doublet at 1132/1120 cm−1 [199].

Infrared ν(CO) frequencies have been listed for hexane solutions of Fe2(CO)8(μ‐CH2) and several analogous μ‐alkylidene complexes [200]. The complete infrared spectra of Fe2(CO)8(μ‐CH2) in Ar (ν(FeC) = 446.5 cm−1) and N2 (ν(FeC) = 646.3 cm−1) matrices and as a KBr pellet has also been assigned [201], and infrared assignments have been made for Ru3(CO)10(μ‐CH2)(μ‐CO) [201, 202], Os3(CO)10(μ‐CH2)(μ‐H)2 (ν(OsC) = 660 cm−1) [203], Os3(CO)10(μ‐CH2)(μ‐CO) [203], and Os2(CO)8(μ‐CH2) [204]. Three carbonyl ν(CO) bands in the THF solution infrared spectra were given as evidence that the [M(CO)4(CO2CH3)]− (M = Fe, Ru, Os) complexes are axially substituted [205]. The two resonance structure extremes of the methoxycarbonyl ligand are 2 and 3. The lower methoxycarbonyl group ν(CO) frequency for the Na+ (1594–1585 cm−1) than the bis(triphenylphosphine)nitrogen (PPN+) ion salts (1630–1621 cm−1) implies that resonance form 3 is stabilized more by Na+ than by PPN+ ions through an interaction with the methoxycarbonyl oxygen atom [205]. The ratio of the areas of the overlapping infrared ν(CO) bands of (CO)4Fe(CO2X)− (X = H, acid ν(CO) = 1604 cm−1; X = CH3, ester ν(CO) = 1621 cm−1) led to a calculated equilibrium ratio of 95 : 5 for a mixture of the acid–ester species in a THF solution at −78 °C [206]:

Reaction schematic with double-headed arrow between compounds 2 and 3 as the resonance structure extremes of the methoxycarbonyl ligand.; Structural formula of anti conformer (4).

Infrared assignments have been made for the ethylidene complexes Fe2(μ‐CHCH3)(CO)6(μ‐dppm) [81, 207], cis‐[(η⁵‐C5H5)2Ru2(μ‐CHCH3)(CO)3 [207], and infrared and Raman data for Os2(μ‐CHCH3)(CO)8 and its d1 and d4 isotopomers of [207]. Vibrational assignments have also been given for ethylidene species that likely formed from H2 reduction of CO adsorbed on a Rh/Al2O3 surface using inelastic tunneling spectroscopy [208] and from adsorbed ethene on a Pt(111) surface with 0.12 monolayers of K at 300 K [113] and 0.23 monolayers of preadsorbed oxygen that was annealed at 325 K [112] using HREELS. Table 3.3 gives selected assignments from these studies.

Table 3.3 Vibrational assignments (cm−1) for the ethylidene ligand in coordination complexes and on transition metal surfaces.

a) Some assignments for this complex in Ref. [81] were revised in Ref. [207].

Although the conversion of ethene to ethylidyne on a Pt(111) surface has been interpreted using a mechanism with an ethylidene intermediate, theoretical investigations [209, 210] suggested that another possible mechanism might involve adsorbed vinylidene. And a calculation of the infrared band frequencies and intensities of possible intermediates and their ²H isotopomers in the conversion of ethene to ethylidyne on a Pt(111) surface [117] concluded that vinylidene rather than ethylidene modes were a better match in both the frequency and intensity for bands at c. 2960 and 1387 cm−1 that had previously been attributed to [211, 212] ethylidene. The possibility that some bands initially assigned to ethylidene might be from a vinylidene intermediate was also proposed in a RAIRS study of vinyl iodide decomposition on a Pt(111) surface [213].

Ten Ta and two Nb neopentylidene (CHC(CH3)3) complexes showed a single low‐frequency infrared ν(CH) band (2535–2420 cm−1), or a doublet for some of the Ta complexes, that has been attributed to an agostic interaction of the neopentylidene ligand α C─H bond and the Tb and Nb atoms [214]. Formation of a mixture of (≡SiO)Ta[═CHC(CH3)3][CH2C(CH3)3]2 and (≡SiO)2Ta[═CHC(CH3)3][CH2C(CH3)3] through the reaction of Ta[═CHC(CH3)3][CH2C(CH3)3]2 with silica dehydroxylated at 500 °C has been followed using infrared spectroscopy [215]. Similar to the doublet at 1132/1120 cm−1 noted above for ethylidene that formed through the carbonyl‐specific dissociation of acetaldehyde on β‐Mo2C [197], a band at 1121 cm−1 in HREELS and RAIRS studies has been tentatively attributed to neopentylidene species formed through the thermal decomposition of [(CH3)3CCD2]4Ti adsorbed on a Cu(111) surface [216].

Photochemical reactions of (CO)5Cr═C(OCH3)C6H5 using different solvents and alkynes have been studied using infrared spectroscopy (2100–1800 cm−1) [217]. Matrix isolation infrared spectra of the photochemical reaction products of (CO)5M═C(OCH3)C6H5 (M = Cr, W) identified the isomerization of the anti (4) to the syn (5) conformer as the primary process and had a ν(Ccarbene─OR) band at 1235 cm−1 for the anti and 1271 cm−1 for the syn conformers of both complexes [218]. The loss of a CO ligand to give (CO)4M═C(OCH3)C6H5 (M = Cr, W) was identified as a secondary process, and the data also indicated that (CO)4W═C(OCH3)C6H5 reacted further to give a species where the C─H bond of the methoxy CH3 group formed a two‐electron three‐center bond with the W atom [218]. Solution time‐resolved resonance Raman spectroscopy and Ar matrix isolation infrared spectroscopy have been used to study photoinduced anti–syn isomerization of the three alkoxy carbenes (CO)5W═C(OR)R′ (R = R′ = CH3; R = CH3, R′ = p‐tolyl; R = C2H5, R′ = C6H5) [219]. Although the Raman ν(Ccarbene─OCH3) frequencies for the anti and syn isomers were almost identical for the first complex, complexes two and three both had a ν(Ccarbene─OR) band at 1235 cm−1 for the anti isomer and 1270 cm−1 for the syn isomer [219]. Ar matrix infrared data indicate a similar equilibrium for (CO)5W═C(OCH3)R (R = CH3, C6H5), and solution Raman bands at 1270 and 1235 cm−1 for (CO)5Cr═C(OCH3)C6H5 also suggested an anti–syn equilibrium [219]. UV–vis and infrared spectra of di‐n‐butyl ether and n‐hexane solutions have been used to study photochemical reactions of the anti isomer of (CO)5W═C(OCH3)C6H5 that produced the syn isomer in the primary process and products that have lost a CO ligand in the secondary process [161]. Raman spectroscopy has been used to study the photochemical conversion of the anti isomer of (CO)5W═C(OC2H5)Si(C6H5)3 to the syn isomer [220]. Infrared and Raman data indicate that photochemical irradiation of (CO)5W═C(NC4H8)Si(C6H5)3 in various solvents causes the loss of a CO ligand to give (CO)4W═C(NC4H8)Si(C6H5)3 [220]. Infrared data have been listed for the methoxycarbenes η⁵‐C5H5Re(NO)[P(C6H5)3][═C(OCH3)R]+ (R = CH3, C6H5) and hydroxycarbenes η⁵‐C5H5Re(NO)[P(C6H5)3][═C(OH)R]+ (R = CH3, C2H5) [221]:

Structural formula of syn conformer (5).

Infrared assignments of primarily the ν(CO) and, when appropriate, ν(CN) modes, have been used to determine if the R groups (R = cyanoaminoalkylidene, aminoalkylidene, diaminoalkylidene) are terminal or bridge the Fe atoms in (η⁵‐C5H5)2Fe2(CO)3R complexes [222].

4

Methyl Complexes

4.1 Neutral Monomeric Complexes

Among the earliest vibrational studies of an organometallic compound was that of the Raman spectrum of (CH3)2Zn by Venkateswaren in 1930 [223]. Since this study, vibrational data have been reported for neutral covalent methyl (CH3) compounds of nearly every main group metal and metalloid and some transition metals.

Figure 4.1 illustrates the vibrational modes for covalent CH3M groups, and Table 4.1 summarizes the frequency ranges for the CH3/CD3 modes for these compounds. Vibrational studies of CD2H groups in organometallic complexes have allowed the isolated C─H stretching mode, νis(CH), to be assigned and used in structural analyses [224–226]. The metal–carbon stretching mode frequency ranges are not included in Table 4.1 and are discussed in conjunction with the other metal–carbon skeletal modes.

6 Normal modes of vibration for covalent CH3M compounds: νs(CH3), δs(CH3), νs(MC), νa(CH3), δd(CH3), and ρr(CH3).

Figure 4.1 Normal modes of vibration for covalent CH3M compounds.

Table 4.1 Frequency ranges (cm−1) for terminal CH3 and CD3 modes.

Organometallic complexes with more than one CH3 group bonded to a metal atom often show more than one band in the regions for the illustrated CH3M modes. These may be from in‐phase and out‐of‐phase combinations of the modes for each CH3 group or distortions of the molecules from the isolated symmetry in the solid state. Also, intermolecular interactions might make it necessary to consider the symmetry of the entire unit cell in any accurate vibrational analysis. Vapor‐phase assignments can be aided by P, Q and R, or P and R branches for some bands; different profiles are expected for modes of different symmetry [227].

No absolute rule can be used to predict the band intensity of a mode that varies with the number of CH3 groups bonded to a metal atom, the phase in which the sample is being studied, whether the infrared or Raman spectrum is being recorded, and the nature of the metal. In general, however, the ρr(CH3) modes have relatively strong infrared intensity, weak Raman intensity, and the largest percentage of frequency variation. Although the degenerate bending mode has very strong Raman intensity for (CH3)2Se and (CH3)2Te [228], for several other compounds [229–234] it often has medium to weak intensity or is not observed in either the infrared or Raman spectra. The frequency variation of each mode within a given family of elements is shown in Figure 4.2 for Group 12 (CH3)2M (M = Zn, Cd, Hg) [234, 235], Group 13 (CH3)3M (M = B [236–238], Al [239, 240], Ga [230, 231, 240–242], In [231, 241, 243]), Group 15 (CH3)3M (M = P [244, 245], As [232, 246], Sb [241], Bi [232, 233]), and Group 14 (CH3)4M (M = Si [247, 248], Ge [249], Sn [250], Pb [249, 251]) compounds. The ν(CH3) bands are relatively stationary, with little frequency variation from one metal to another. A detailed study has been made of the ν(CH3)/ν(CD3) modes for (CH3)n(CD3)4−nM (n = 0–4; M = C, Si, Ge, Sn, Pb) [224]. The position of the δd(CH3) modes is also relatively constant. The δs(CH3) and ρr(CH3) modes, however, are at progressively lower frequencies as the metal atom mass increases in a homologous series with the exceptions of those of (CH3)2Hg.

Frequency variation of each mode within a given family of elements for Group 12 (CH3)2M (M = Zn, Cd, Hg), Group 13 (CH3)3M (M = B, Al, Ga, In), Group 15 (CH3)3M (M = P, As, Sb, Bi), and Group 14 (CH3)4M (M = Si, Ge, Sn, Pb) compounds.

Figure 4.2 Frequency variation of the CH3 modes for different groups of the periodic table.

In addition to the CH modes, skeletal ν(MC) and carbon–metal–carbon bending modes (δ(CMC)) are expected. The number and activity of the skeletal modes depend on the number of CH3 groups bonded to the metal atom. Table 4.2 summarizes the activity of the metal–carbon skeletal modes for CH3 compounds with different stoichiometries.

Table 4.2 Activity of the metal–carbon skeletal modes for (CH3)nM compounds (n = 1–6) of various symmetries.

The advantage of using both infrared and Raman data in determining the symmetry about the central metal atom is seen in Table 4.2. Therefore, the CH3 groups (assumed to be single atoms) and metal atom for (CH3)3M can be either coplanar or pyramidal. For a coplanar skeleton, the two ν(MC) modes and one δ(CMC) mode are Raman active, and one ν(MC) mode and the two δ(CMC) modes are infrared active. The two ν(MC) and two δ(CMC) modes are infrared and Raman active for a pyramidal skeleton.

It is dangerous, however, to base structural conclusions on the absence of a band for two reasons. First, the band of interest may accidentally overlap another band. Only one ν(MC) band is found in the infrared spectra of both pyramidal (CH3)3Sb and (CH3)3Bi [252]. Using only the infrared data, it is tempting to conclude that the skeletons of these compounds are planar, although they are not. The Raman spectra of each of these compounds also show one ν(MC) band. It has been concluded that both ν(MC) modes are accidentally degenerate because of the relatively heavy metal atom masses that prevent the CH3 groups from interacting with one another. As a rule, as the metal atom mass increases, the frequency separation of the expected ν(MC) and δ(CMC) modes decreases.

The other possible reason for the absence of an expected band is that the intensity can be too low to detect. The infrared νs(MC) mode of the pyramidal (CH3)3M molecule might not be observed for this reason. Compounds with a nonplanar skeleton, whose vibrational spectra appear to be consistent with planar selection rules, are called pseudoplanar. It is difficult to predict the angle required before the symmetric mode becomes observable in the infrared spectrum. This depends on the geometry of the compound and the nature of the metal atom [252]. Metal–carbon skeletal mode assignments are summarized for (CH3)nM/(CD3)nM and other isotopomers in Table 4.3.

Table 4.3 Metal–carbon skeletal mode assignments (cm−1) for (CH3)nM, (CD3)nM (n = 1–6), and other isotopomers.

a) Estimated value.

b) Calculated value.

c) δ ǁǁ was not observed.

d) The two expected δ(CMC) modes are accidentally degenerate.

e) Although monomeric and nonplanar as a vapor Ref. [286], solid (CH3)3Bi is dimeric with short intermolecular Bi⋯Bi interactions Ref. [287].

f) An axial mode.

g) An equatorial mode.

h) A trigonal bipyramidal structure with a lone‐electron pair in one of the equatorial sites.

i) Value for the ground state.

j) The value in parentheses is for the ⁵⁸Ni isotope, and that not in parentheses is for the ⁶⁰Ni isotope.

k) Value for the excited state.

l) Uncertainty was expressed in Ref. [154] over the assignment of the ν(CuC) mode in Ref. [281], and indeed DFT calculations gave values of 526 and 541 cm−1 in Ref. [282].

Normal coordinate analyses for CH3 compounds have treated either the entire molecule or used models where the CH3 group is treated as a single atom with a mass 15 or has a calculated effective mass [288]. These analyses show the extent of mixing between the modes and provide force constants for a molecule. The metal–carbon stretching force constant, kMC, is especially useful since it has been empirically related to the metal–carbon bond strength. Within a given family of main group elements, both the strength and kMC of the metal–carbon bond decrease as the metal atom mass increases. It is better to relate the bond strength to kMC rather than to the ν(MC) frequency since the latter depends not only on the strength of the metal–carbon bond but also on the metal atom mass and the extent to which this mode is mixed with other modes.

Solid methyllithium [289] and methylsodium [290] are tetramers, and the CH3 compounds of K [291], Rb [292], and Cs [292] are ionic with isolated methyl anions and metal cations. The infrared vibrational frequencies of the CH3 group and the d3 isotopomers of monomeric CH3M (M = Li [253], Na [251], K [254]) in low‐temperature matrices were among the lowest found for organometallic complexes. Only the νa(CH3) and νs(CH3) modes have been assigned for methylpotassium (2810 and 2745 cm−1, respectively) [289]. Some uncertainty has been expressed [154] over a previous assignment [281] for the ν(CuC) mode of CH3Cu. Later, the A1 symmetry δs(CH3) and E symmetry ρr(CH3) modes were assigned, and DFT calculations performed that determined all of the vibrational modes for CH3M (M = Cu, Ag, Au) and the d3 and ¹³C isotopomers that formed through CH4 activation with laser‐ablated, excited Group 11 metal atoms in an Ar matrix [282].

Although dimethylberyllium normally has an associated structure, the monomer is mainly found in the unsaturated vapor. The infrared spectra of the monomer and its ²H isotopomer have been reported [229]. A relatively high frequency that increases on deuteration is observed for the νa(BeC) mode of the monomer relative to similar frequencies for other Be alkyls. The data indicate an unusually strong Be─C bond in (CH3)2Be from hyperconjugation where the CH3 groups release electrons to vacant Be 2p orbitals (1). Hyperconjugation has also been used to explain the stability of monomeric (CH3)3B [293]. The νa(BeC) frequency is higher for (CD3)2Be than for (CH3)2Be, as was also observed for the νa(BC) mode of (CH3)3B [237]. This has been attributed to vibronic interactions and resonance effects in both compounds. The δs(CH3) (1222 cm−1) and νa(BeC) (1081 cm−1) modes of (CH3)2Be might normally be expected to shift to c. 924 and 1050 cm−1, respectively, on deuteration. Both modes, however, have A2u symmetry, and such shifts violate the noncrossing rule [294]. Therefore, since the character of the modes is switched on deuteration, it is assumed that bands at 1150 and 994 cm−1 have mainly νa(BeC) and δ(CD3) character, respectively. The shapes and intensity ratios of the relevant (CH3)2Be/(CD3)2Be bands also seem to support the above conclusions [229]. Vibrational features of the dispersed fluorescence spectrum of CH3Mg have been assigned to the A1 symmetry ν2 CH3 umbrella (1072 cm−1) and E symmetry ν6 CH3 rocking (509 cm−1) modes [295].

Reaction schematic depicting the data indicates an unusually strong Be─C bond in (CH3)2Be from hyperconjugation where the CH3 groups release electrons to vacant Be 2p orbitals (1).

Vibrational assignments have been made from laser‐induced fluorescence spectra of CH3M (M = Zn, Cd) radicals [283, 296]. The infrared and Raman spectra of the vapor [234, 235], liquid [234], and solid [234] phases and infrared spectra of the Ar matrices [297] of (CH3)2M (M = Zn, Cd, Hg) have been studied. It was noted [235] that the two ν(CH3) frequencies (ν5 and ν8) depend on the physical state of the sample. They are lower by 10–20 cm−1 in the gas than the liquid phase, but in a matrix they increase for (CH3)2M (M = Zn, Cd) and decrease for (CH3)2Hg compared with the liquid. This has been attributed to changes in the nature of the matrix cavity site that the molecules occupy [235]. A complete normal coordinate analysis has been made for (CH3)2Hg [298].

Since B has a relatively low mass, and the two natural ¹⁰B and ¹¹B isotopes have relatively high abundances of 20% and 80%, respectively, bands from both isotopes are usually observed for modes involving the motion of the B atom in its complexes. This has been observed for the νa(BC) mode of (CH3)3B [238, 257]. Only one band, however, is found for the νs(BC) mode since the B and C atoms are coplanar and the B atom does not move during this vibration. Trimethylaluminum [299], like dimethylberyllium, is found as both a monomer and a dimer as a vapor. Vapor‐phase Raman [16] and infrared [258, 283] and matrix infrared [258] data have been assigned for monomeric (CH3)3Al.

An infrared study of species produced by irradiating a matrix of SiH4 and CH4 at 10 K showed the CH3Si radical (SiC bend = 1226 cm−1, sym. CH3 def. = 1371 cm−1) [300]. The species (CH3)2Si has been isolated from photolysis reactions in both Ar matrices at 10 K and hydrocarbon matrices at 77 K and shows infrared bands at 1220 [301] and 1438 cm−1 [139]. The infrared spectrum has been tentatively identified for the CH3Ge radical (δs(CH3) = 1406 cm−1, ρa(CH3) = 536 cm−1) in a study of electron‐irradiated matrices of GeH4 and CH4 [302]. The infrared spectrum was assigned for (CH3)2Ge after isolation in an Ar matrix [263]. Isotopic structure has been observed for the νa(MC) band in the infrared spectra of (CH3)4M (M = Si, Ge, Sn) in Ar and N2 matrices [303]. Evidence has been presented for the formation of covalent bonds between a Si(111) surface and CH3 groups, with infrared bands assigned for the νa(CH3) (2989 cm−1), νs(CH3) (2916 cm−1), δa(CH) (1424 cm−1), δs(CH) (1237 cm−1), ν(SiC) (678 cm−1), and ρ(CH3) (775 cm−1) modes of the Si─CH3 adspecies [304]. A study of the vibrational force fields and amplitudes and zero‐point average structures of (CH3)3M (M = N, P, As, Sb, Bi) found a decrease of the kMC value from 5.3 mdyn Å−1 for M = N to 1.8 mdyn Å−1 for M = Bi [305].

In addition to ν(MC) and δ(CMC) skeletal modes, torsional modes are expected for CH3 compounds. Most vibrational studies have been in solution or in the liquid or vapor phases where free rotation of the CH3 groups is found. The E and A2 torsional modes have been assigned to Raman bands at 225/158 and 210/152 cm−1, respectively, for (CH3)3P/(CD3)3P [244, 245].

The A2 torsional mode was found for (CH3)2Se (175 cm−1) and the B2 torsional mode for both (CH3)2Se (207 cm−1) and (CH3)2Te (185 cm−1) in a low‐frequency infrared study of solid (CH3)2Se and (CH3)2Te at −190 °C [306]. The probable assignment of ν(TeC) modes to Raman bands of (CH3)4Te (699 and 520 cm−1) [274] was questioned in a theoretical study that calculated the vibrational spectra of (CH3)4M (M = S, Se, Te) [271]. Octahedral skeletons that are slightly distorted by steric effects have been proposed for (CH3)6M (M = Se, Te) [272]. Infrared data for (CH3)6Te were not assigned [307], and the vibrational frequencies have been calculated for (CH3)6M (M = Se, Te) [272].

The low‐temperature vibrational spectrum of (CH3)4Ti has been obtained in a diethyl ether solution [277]. The kTiC value found for (CH3)4Ti was about 20% lower than expected on the basis of a comparison with analogous force constants for the tetramethyl derivatives of the Group 14 elements.

An X‐ray diffraction study shows solid (CH3)5Mo to have a square pyramidal skeleton and also lists pentane solution Raman bands (2700–167 cm−1) [308].

Limited infrared data for (CH3)6W [279] and (CH3)6Re [280] were originally interpreted as consistent with Oh local symmetries. However, vapor‐state electron diffraction [309] and solid‐state X‐ray diffraction data [310] for (CH3)6W and X‐ray diffraction data [310] for solid (CH3)6Re show that (CH3)6W has a strongly distorted structure, and (CH3)6Re a regular trigonal prismatic structure. Unassigned Raman data have been given for solid (CH3)6Mo that also has an approximately trigonal prismatic local symmetry [311]. Figure 4.3 illustrates the infrared spectrum of tetramethylsilane [312].

Image described by caption and surrounding text.

Figure 4.3 Infrared spectrum of (CH3)4Si.

Source: Smith 1953 [312]. Reproduced with permission of AIP Publishing LLC.

4.2 Cationic and Anionic Monomeric Complexes

Figure 4.4 illustrates the infrared spectrum of solid [(CH3)4Sb]+I− [313], and Table 4.4 summarizes the skeletal mode assignments for several (CH3)mMn+ cations.

Image described by caption and surrounding text.

Figure 4.4 Infrared spectrum (5000–400 cm−1) of solid [(CH3)4Sb]+I−.

Source: Shindo and Okawara 1966 [313]. Reproduced with permission of Elsevier.

Table 4.4 Metal–carbon skeletal mode assignments (cm−1) for (CH3)mMn+ complexes.

a) Data not reported.

Water can coordinate to the metal atom of cationic organometallic (CH3)mMn+ compounds in solution. Complexes with relatively strong metal–water bonds that give observable metal–water stretching bands are discussed in Section 13.8.1.

Infrared and Raman spectra indicate that solid CH3ZnBH4 is best viewed as consisting of coupled CH3Zn+ and BH4− ions [314]. Four of the six frequencies of CH3Cd+ have been determined using ZEKE spectroscopy data [315].

The Raman spectra of water solutions of several (CH3)2Mn+ cations have been reported. Although the skeletons of (CH3)2Mn+ complexes in solution can be either linear (2) or angular (3), only linear skeletons have been found.

Structural formula of a linear (CH3)2Mn+ complex (2).; Structural formula of an angular (CH3)2Mn+ complex (3).

The first such study was of water solutions of the NO3− and ClO4− salts of (CH3)2Tl+ [317]. A later study was of the water solution Raman spectrum of the Cl− salt and the solid‐state infrared spectra of the NO3− and Cl− salts [318]. These two studies agree except in the assignment of one band. In the original study [317], a very weak intensity band at 569 cm−1 was assigned to the ρr(CH3) mode, which is a relatively low frequency for this mode. Using later infrared data, it seemed more reasonable to assign the 569 cm−1 band to the νa(TlC) mode and a very strong intensity band at 803 cm−1 to the ρr(CH3) mode. Although (CH3)2TlBr is a molecular unit as a gas [262], solid (CH3)2TlX consists of (CH3)2Tl+ and X− ions and is better formulated as [(CH3)2Tl]X (X = Cl, Br [262], I) [319]. Infrared and Raman data for solid [(CH3)2Tl]X (X = Cl, Br, I) at low temperatures and high pressures show phase transitions that include a change in the conformation of the (CH3)2Tl+ anion from linear to bent at just below 25 kbar [319]. The infrared and Raman spectra of solid [(CH3)2In]InI4 [331] and water solutions of (CH3)2InCl [316], (CH3)2InClO4 [316], and (CH3)2SnX2 (X = ClO4, NO3) [320, 332] have all been interpreted in terms of linear metal–carbon skeletons. A comparison has been made of the Raman spectra of (CH3)2Tl+ and (CH3)2Sn²+ as single crystals and in water solutions [333]. Raman data for water solutions of (CH3)2PbX2 (X = ClO4, NO3) show (CH3)2Pb²+ to be isostructural with (CH3)2Sn²+ with a weak intensity band at 425 cm−1 assigned to the ν(PbO) mode of coordinated water [321]. No ν(MO) assignments could be made for water solutions of any other (CH3)mMn+ complexes.

Low‐frequency vibrational data are available for the two isostructural (CH3)2Mn+ series (CH3)2Cd, (CH3)2In+, and (CH3)2Sn²+ and (CH3)2Hg, (CH3)2Tl+, and (CH3)2Pb²+. Complete normal coordinate calculations have been reported for all six compounds using a Urey–Bradley force field [316, 320]. Figure 4.5 illustrates the variation of the kMC values for each series. While the increase in the kMC value for the former series is expected as the metal charge increases, the opposite trend is found for the latter series. Solvation effects alone cannot explain these data since they are found in both series. It has been suggested that stabilization of the metal–carbon bonds depends on metal–carbon orbital overlap. The largest kMC values are for the Hg─C, Tl─C, and Sn─C bonds that are most inert to attack by acids and bases [316].

Graph of kMC (mdyn Å−1) for the isoelectronic series (CH3)2Cd, (CH3)2In+, and (CH3)2Sn2+ (squares linked by a dashed line) and (CH3)2Hg, (CH3)2Ti+, and (CH3)2Pb2+ (circles linked by a solid line).

Figure 4.5 Variation of the Urey–Bradley metal–carbon bond stretching force constant (kMC) for the isoelectronic series (CH3)2Cd, (CH3)2In+, and (CH3)2Sn²+ and (CH3)2Hg, (CH3)2Tl+, and (CH3)2Pb²+.

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