J. CHEM. soc. DALTON TRANS. 1995 394 1Carbon-Hydrogen Bond Properties and Alkyl GroupGeometries in Dic h loro( q5-cyclopentad ienyl) -methyltitanium( iv) and Di~hloro(~~-cyclopentadienyl) -ethyltitanium(iv) [TiR(+C,H,)CI,] (R = Me or Et)tA. H. Jean Robertson, Geoffrey P. McQuillan" and Donald C. McKeanDepartment of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen AB9 2UE, UKInfrared spectra have been recorded for various isotopomers of [TiMe(cp) CI,] (cp = cyclopentadienyl,qs-C,H; Me = CH,, CD, or CHD,) and [TiEt(cp)CI,] (Et = CH,CH,, CD,CH,, CH,CD, or CHD,CD,)and assignments proposed for the alkyl group vibrations. A method was elaborated for the calculationof Fermi-resonance shifts on CH and CD stretching modes in methyl groups with C,, and C, symmetry.Estimates of CH bond lengths, bond strengths and HCH angles were derived from the resonance-corrected frequency data.The results show the methyl group in [TiMe(cp)CI ] to be markedlyasymmetric, with the CH bond trans to the cyclopentadienyl ligand being ca. 0.005 h longer and 15 k Jmold' weaker than those trans to chlorine. In the ethyl compound, the terminal methyl group issimilarly asymmetric, again with one weak bond and two stronger bonds. The vCH, and vCD,frequencies for the methylene group are anomalous and can only be satisfactorily interpreted in termsof a model in which the two methylene CH bonds are inequivalent. The estimated bond lengths are1.1 00, and 1.1 05, A, and the corresponding bond dissociation energies are 403 and 383 kJ mol-l,respectively.These results appear to point to a direct a-interaction between at least one of themethylene CH bonds and the titanium atom. A similar effect may also occur in [TiMe(cp)CI,].Information on the properties of CH bonds in alkyl-metalcompounds is most often obtained by means of nuclearmagnetic resonance spectroscopy. The NMR data yield muchuseful information, but do not lead directly to the accurateestimates of CH bond lengths, bond dissociation energies orHCH angles which are particularly desirable in studies relatingto CH bond activation, hydride transfer or the identification of'agostic' C-H M interactions. '*' Diffraction techniques,apart from neutron diffraction in a few isolated cases, do notlocate hydrogen atoms with sufficient precision to provideuseful bond length or bond angle data.In principle, the required information is contained in thevibrational spectrum but the interpretative problems are severeand in the past have restricted quantitative studies to verysimple molecules.In practical terms, vibrational spectroscopyoffers two impdrtant potential advantages: it can be used inall phases, and the very short time-scale of the vibrationaltransition makes it possible to characterise near-instantaneousconformations in molecules undergoing almost unhinderedinternal rotation. The problems of time-averaging whichaffect the NMR specta of non-rigid molecules only arise inthe vibrational experiment at very much lower barriers( g 4 kJ mol-' for internal rotation).The difficulties associated with vibrational spectroscopy arisefrom several sources.Vibrational assignments are not alwaysobvious, even in very simple molecules (for example, a definitiveassignqent for chloroethane has only just become available).'The relationships between the observed vibrational modes andindividual bond properties are often complex and can bedifficult to establish in precise quantitative terms. Properallowances must be made for the effects of anharmonicity, andalso, most importantly for CH and CD vibrations, for F e e -resonance perturbations. In extreme cases, these resonancescan create serious problems in assignment, and even whereassignments are secure it is often difficult to make the accurateestimates of the resonance shifts which are needed to establishthe unperturbed frequencies of the fundamental modes.In earlier paperss-' we have shown that in methyl-metalcompounds many of these problems may be overcome bymaking use of frequency data obtained from the CH3, CD3 andCHD, isotopomers.The 'isolated' CH stretching frequency,viSCH, observed in the CHD2 species, is essentially a localmode involving the stretching of a single CH bond, virtuallyuncoupled from other molecular motions and unaffected byFermi resonance as the bending overtones which lie close to theCH stretching region in CH,X compounds are moved to muchlower frequencies in the CHD2 isotopomers. There is now awealth of data, from both experimental and ab initio. ~ o u r c e s , ~ .~ ~ * ~ ~ to show that vi"CH is linearly and very preciselyrelated to the CH bond length, and in certain circumstances tothe bond dissociation energy.12 Estimates of HCH angles canbe derived from the vasymCH3/vasymCD3 ratio, and checked byforce constant calculations for the CH and CD stretchingmodes in the CH,, CD, and CHDz compounds.In this paper, we report our first attempt to extend the partialdeuteriation technique from methyl- to ethyl-metal systems.Information on ethylmetal compounds is particularly desirablein view of their importance in hydride transfer processes, and ofthe involvement of p-CH bonds in agostic interactions.'V2 Wedescribe here the vibrational spectra of various isotopomers ofDiMe(cp)C12] and FiEt(cp)Cl,] (cp q5-CsH,): these twocompounds illustrate the utility of partial deutenation studies,and also some of their limitations, and are important in theirown right given the current interest in alkyltitaniumsystems.3-16ExperimentalpiR(cp)Cl,J (R = Me or Et).-These compounds wereprepared using an adaptation of the method of Erskine et 7 Non-SI unit employed: dyn = lo-' N3942 J. CHEM. soc. DALTON TRANS. 1995Scheme 1al. 1 7 * 1 8 The compound Fi(cp)Cl,] was obtained from thereaction of [Ti(cp),Cl,] with TiCl, in refluxing toluene. l 9Dialkylzinc compounds were prepared from the direct reactionsof CH,I, CD,I, CHD,I, CH,CD,Br, CD,CH,Br andCHD,CD,I with copper-activated zinc. (Ethyl halides withpartially deuteriated methylene groups, e.g. CD,CHDX, werenot available.) All reactions were carried out using rigorouslydried solvents in an atmosphere of dry nitrogen, or in a vacuumsystem.A solution of the dialkylzinc (0.5 g) in benzene (10 an3) wasadded slowly, with stirring, to a solution of Fi(cp)CI,] (1 g) inbenzene (25 cm3) at room temperature.Stirring was continuedfor a further 10-15 min, during which the solution becamedeeper in colour and ZnC1, precipitated. The solvent wasremoved under vacuum and the product sublimed by gentlewarming (R = Me, 55 “C; R = Et, 35 “C) onto an ice-cooledprobe. The methyl compound was obtained as orange crystalsand the ethyl compound as dark red crystals.IR Spectra.-Spectra were measured for freshly preparedCCl, solutions at various pathlengths using a Nicolet 7199FTIR spectrophotometer at 1 cm-’ resolution.Solventabsorptions tend to obscure weak sample peaks in the region810-700 cm-’: this region is not directly relevant to the presentstudy. Additional very weak peaks arising from traces ofdecomposition products {e.g. ~i(OMe)(cp)Cl,], [{Ti(cp)-Cl,) ,O] 7, were observed in some samples.Theoretical TreatmentIn methyl groups with C,, symmetry, only one V’TH band isobserved in the spectrum of the CHD, isotopomer. If thesymmetry is reduced to C,, two bands appear, arising fromstretching of the in-plane (CH’) and out-of-plane (CHa) bonds.In the CH, and CD3 isotopomers, the reduction in symmetrysplits the initially degenerate vasymCH3 (e) or vasymCD3 (e) levelsinto a‘ and a” components.The a‘‘ component lies above the a‘ ifthere are two strong bonds and one weaker one, and vice uersa.For the vCH vibrations in the CH, and CHD, species, theapproximate frequency-sum rule, equations (1) or (2), provides aC~,V,,,CH~ + ~v,,,,CH, = 3v’”CH (1)check on assignments and makes it possible in the case of agroup with C, symmetry to determine whether the group hasvi”CHa > vi”CHS (i.e. two strong bonds and one weak one), orv’”CH” c visCHs.Fermi Resonance.-In methyl groups with C,, symmetry,the most obvious and well documented interaction involvesv,,,CH, (a,) and the bending overtone 26,,,,CH3 (Al). If thebending fundamental, 6,,,,CH3 (e) is reliably identified, theunperturbed overtone frequency can be calculated, with anappropriate correction for anharmonicity (typically - 10cm-’), giving an estimate of the Fermi-resonance shift on theobserved band, and hence of the corresponding shift onv,,,CH,.However, the reliability of shifts calculated in this wayis variable, and can leave significant uncertainties in theunperturbed frequencies vz,,CH,. *In earlier work, resonances affecting vaSy,CH3, or involving26,,,CH3, were assumed to be negligible and were ignored, orat the most covered by small arbitrary adjustments to theobserved frequencies. Recent studies on methyl halides ” andCH,CD,,l have shown that these resonances can be significantand should be taken into account in serious quantitative work.Further possibilities arise in groups with C, symmetry, wherethe vaSymCH3 mode splits into a’ and a” components.To calculate these resonances, we have extended thelocal/normal mode model used by Duncan and Law2’ andTullini et al.,, for Fermi resonances in the A, species of asymmetrical methyl group to cover the case of an asymmetric(C,) methyl group, as described below.For a symmetrical group, the above model diagonalises a5 x 5 matrix whose diagonal elements are the three identicallocal mode frequencies (OM + 2xM) (i.e.v’”CH in the CHD,isotopomer) plus the unperturbed overtone levels 26,,,,CH3(A,) and 26,,,CH3 (Al);* o, is the harmonic local modefrequency and X, the associated anharmonicity constant. 2oThe off-diagonal term between the local mode levels is thequantity h, defined by equation (3), wherey,f, g’, g are the off-diagonal and diagonal elements of the f and g matrices; g’ =mc-’ cos HCH, g = mC-’ + mH-l, as for a diatomic molecule.The off-diagonal terms for the overtone levels incorporatethe Fermi-resonance parameters 20*22 W 155 and W,,, [e.g.forCH,X, W, refers to the resonance between v1 (vasymCH3, a,)and 2v, (26,,,,CH3, A,)] multiplied by the appropriate elementof the symmetry internal coordinate involved.Our extension for a C, methyl group involves adding furtherresonances with the 26,,,,CH3 (A’) and (A”) levels derived from26,,,,CH3 (E). To do this, we use the parameter W,,, whichconnects v4 (vasymCH3, e) with 2v5 (26,,,,CH3, E) transferredfrom a symmetrical methyl group? and reduced by 2-* becausethe resonance is now with the non-degenerate A’ and A” levels,and we introduce different local mode levels for the in-plane(CH”) and out of plane (CHa) stretching motions.The (aM +2x,) values for these modes are given by (vi’CHS - 2) and(visCHa - 2) measured in the spectrum of the CHD,isotopomer; the reduction of 2 cm-’ is made to allow forthe very small coupling between the CH and CD stretchingmotions in the CHD, group.The 7 x 7 matrix to be diagonalised is as in Scheme 1.Two parameters ha,,, ha,, are needed because the interactionconstants f’.,, and the HaCHa and HaCW angles maydiffer. Geometnc means of the quantities aM,f’ andfshould beused in equation (3) for ha,,. Standard values of the interactionparameters Wijk are taken from work on methylW155 = 32cm-’; W,,, = 20m-’; W,,, = 40m-’.Thelatter* Frequencies corrected for Fermi resonance are denoted vo,experimentally measured ones unaffected or uncorrected by resonanceas v.?The model assumes that 6,,,,CH3 is not significantly split into a’and a” components.This is the case, or almost so, in the titaniumcompounds. Our approximation could not be used for molecules likechloroethane, in which 6,,,,CH3 is more obviously splitJ. CHEM. soc. DALTON TRANS. 1995 3943is well-determined in CH3F (ref. 22) but not in the chloride,bromide or iodide.,' In CD, groups, Wvalues appear to bereduced by a factor of 1.2-1 .3.,' The unperturbed frequency ofthe first overtone of the non-degenerate symmetric bendingmode, 6,,,CH, is given by 2(6,,, + xsy3 where xsym is thecorresponding anharmonicity constant.For degenerate modes,two anharmonicity constants, x and g, are required.,' In a C,,methyl group, the first overtone levels of 6,,,,CH3 (e) are(E).* In a C, group, the E level will split into A' and Acomponents. If the parent fundamental, 6,,,,CH,, is notsignificantly split, these components will have identical, or verysimilar, unperturbed frequencies and any observed separationwill arise from Fermi-resonance effects.For the methyl group in fliEt(cp)Cl,], our procedureinvolved refining five parameters: A,,,, A,,, and the three2(13asym + xaSyh + gaSyd,t to fit six observed transitions (seediscussion of assignments and Table 7, below). The calculationwas repeated with the refined values of A,,s and A,,, butwith all the W values set to zero (i.e.with no allowance forFermi resonances). The changes in the computed values thengave the Fermi-resonance shifts on the normal CH, stretchingmodes. These shifts were applied to the observed frequencies.2(&asym + Xasym - g a s y 3 (A,) and 2(6asym + Xasym + gasym)overtone levels, 2(6sym + ~ s y d , 2(6asym + Xasym - g y y m ) andHarmonic Local Mode (HLM) Force Constant Calculation.-This is a so-called energy-factored force field which treats theCH and CD stretching motions in isolation from all others.Comparisons with molecules for which complete force fieldsare a ~ a i l a b l e ~ . ~ . ~ ~ confirm the validity of this approach. For agiven HCH angle, the calculation refines the CH stretching andstretch-stretch interaction force constants to values which mostaccurately reproduce the observed frequencies, corrected whereappropriate for Fermi resonance.Differences in the effects ofanharmonicity on the CH and CD stretching levels are takeninto account by decreasing all CD frequencies by a factor of1.01 1 .3,24 The experimental frequencies employed are takenfrom the CH,, CD, and CHD, isotopomers; in the refinement,the calculation is required to reproduce exactly thosefrequencies which are accurately known, but is less severelyconstrained, or unconstrained, for vibrations which may beaffected by resonances which are not reliably determined.A successful refinement reproduces the observed frequencieswith realistic values for the force constants.Failure to refine tothe required frequencies, or the appearance of unacceptableforce constants (e.g. large or very different values for theinteraction constants) indicates an incorrect HCH angle, faultyassignments or the presence of undetected resonances.A C,, methyl group generates a two-parameter force fieldcff') based on a single HCH angle. For a C, group, the forcefield may involve up to four parameters, f,, fa, fa,, f',,, witheither an average HCH angle, as before, or two angles, HaCHsand HaCHa.HCH Angles.-The G-matrix elements for vasymCH3 andvasymCD3 in a symmetric methyl group are given by equations(4) and (5).GasYmCH3 = mH-' + rn;' (1 - cos a) (4)The ratio of the observed frequencies, vasymCH3/vaSymCD3 isthen given by (GasYmCH,/GasymCD,)~ + 1.01 1, the factor 1.01 1being again introduced to allow for the differing effects ofanharm~nicity.~~ The value of the HCH angle a can thus bedirectly deduced from the ratio of the experimentally measuredfrequencies, although the sensitivity to angle is not great.For aC, methyl group, the ratio v,,,CH, (aR)/vaSymCD3 (a") similarlyyields the H'CH" angle. Angles obtained in this way can betested in the HLM force-field refinement-failure to fit the datasuggests that the experimental frequencies must be perturbed byunsuspected resonances. In cases where reliable frequency dataare not available, the force field calculation may be carried outwith a series of trial HCH values to determine which one leadsto the most satisfactory fit.Results and AssignmentsInfrared spectra for the isotopomers of the two compounds areillustrated in Figs.1 and 2, and wavenumbers are given inTables 1, 2, 4 and 5. Table 1 contains all the bands obviouslydue to the cyclopentadienyl moiety, which are clearly identical,or nearly so, in the methyl and ethyl derivatives. The strongG,,CH(e,, al) bands near 800 cm-' are poorly defined becauseof the intense solvent absorption in this region. Ourassignments follow the definitive work of Aleksanyan andLokshin.,'viMe(cp)Cl,].-The bands in Table 2 all move significantlyupon deuteriation and hence must be associated with the MeTimoiety. There is some uncertainty below 550 cm-' because ofhigh noise levels: analysis in this region must await a moredetailed study.Assignments for UiMeCl,] are included in theTable for comparison.The spectrum of pi(CHD2)(cp)C12] contains two viSCHbands, separated by 40 cm-', and the vasymCH3 and vaSymCD3bands in the CH, and CD, isotopomers are split into well-resolved a' and a" components. The methyl group thus hasclearly defined C, symmetry, like those in UiMe,(cp), J (wherehowever the viSCH bands are separated by only 17 an-') andunlike DiMeCl,], in which the methyl group retains its three-fold symmetry.The CH deformations are readily assigned by comparisonwith FiMeCl,], being found in both compounds at lowerfrequencies than is usually the case for these vibrations. Theantisymmetric mode 6,,,,CH3 is about 15 cm-' higher inr]Ti(CH3)(cp)C1, J than in vi(CH,)Cl,] and 8,,,CD, isinferred (see below) to be about the same amount higher inI VI* In this discussion, we have used the general descriptive terms tjsym,tjasym, 26,,, etc.In specific cases, these modes and the correspondinganharmonicity constants will follow the relevant numbering system forthe molecule concerned. Thus for C,, CH,X, 6,,,CH3 is v2, 6,,,,CH3 isrespectively .7 The combination level 6,,,,CH, + SSymCH3 is ignored." 5 , x s y p becomes ~ 2 . 2 and Xasym and gasym become ~ 5 . 5 and g 5 . 5 7Y I v32k=-2800 * 24'00 ' 2000 %K * 12i%-+T% ' $0ikm-fFig. 1 Infrared spectra of isotopomers of uiMe(cp)Cl,] (CCl,solutions, highest feasible concentrations, various pathlengths 0.014.1mm); Me = (a) CH,, (6) CHD,, (c) CD,.The region around 800 cm-'is obscured by solvent absorption3944tc0v)v).-.- s c El-J. CHEM. SOC. DALTON TRANS. 1995I(d )I , - . , ) 1'3200 2800 2400 2000f600 1200 800 400Gtcrn-'Fig. 2 Infrared spectra of isotopomers of niEt(cp)Cl,] (CC14solutions, highest feasible concentrations, various pathlengths 0.014.1mm); Et = (a)C,H,, (b)CH,CD,, (c)CD,CH,, (d)CHD,CD,. Theregion around 800 cm-' is obscured by solvent absorptionsPi(CD,)(cp)Cl,] than in [Ti(CD,)Cl,]. The symmetric modes,6,,,CH3 and 6,,,CD3, in contrast, are lower in [TiMe(cp)Cl,]than in [TiMeCl,], the difference being greater for the CD,mode. We attribute this to the TiC stretching motion present ina &,,Me mode and to a lower Ti-C stretching force constant inthe cyclopentadienyl compound.The 6,,,,CH3 band has a weak shoulder at 1388 cm-', about4 cm-' below the absorption maximum; apart from this it showsno sign of splitting into a' and a" components, Only one bandappears in the 26,,,,CH, (or 26,,,,CD,) overtone region.Bands due to 26,,,CH3 and 26,,,CD3 are not observed.At lower frequencies, a moderately intense band at 538 cm-'in Pi(CH,)(cp)Cl,] appears to rise to 550 cm-' in the CHD,species.This can only occur if a methyl rocking mode initiallyhigher than 538 cm-' has crossed over a less deuterium-sensitivemode as a result of the substitution, and indicates that at leastone methyl rocking frequency must be higher in [TiMe(cp)Cl,]than in [TiMeCl, 1, in which the highest methyl rock is found at464 C I T - ' .~Fermi resonances. As both the 6,,,,CH3 and 6,,,CH3 modeshave low frequencies, yielding overtones near 2770 and 2200cm-' respectively, the only significant resonance is likely to bewith the vSymCH3Table 3 shows results of some trial calculations made with thelocal/normal mode model described above. As the transitions to26,,, and the split 2tiasym (E) levels are not observed, the onlyparameters permitted to refine were A,,,, and the 26,,, (A,)level, the latter given by 2(6asym + x,,,, - gasym). The resultsshown under column 1, where conventional W values wereemployed, reveal an anomaly similar to that previously found inpiMeC1,],9 in that the value of - 1 1.2 cm-' for (xasym - gasym)is numerically much larger than the value of - 5.8 cm-' foundin CH,Cl.'*As the results in column 3 show, W,,, would need to be over47 cm-' for (xasym - gas,,) to approach - 5.8 cm-'.As we have= 1 level.Table 1moietyaInfrared bands (an-') attributed to the cyclopentadienylniMe(cP)CI,l3948w31 15m2507vw2435vw2288w2208 (sh)2 189vw2 139vw208%1 9 9 2 ~1 9 1 3 ~1844w1761w1666w144oms1366vw1 130 (sh)1118m1074w1021s928w846 (sh)z 835s71Owz 600wCTiEt(cp)C1213955w3115m2506vw2435vw2288w2089w1 9 1 3 ~1840wI757mw1665w1439ms1363w1 1 15mw1070w1021s923mw844 (sh)x 830ms708mwx 68Ovwx 603wAssignmentvCH + 846,835vCH a,, e l1440 + 1074E21366 + 1074A1, El1366 + 928 A,, El1440 + 846 A,, E,1260 + 1021 E,1366 + 846 E,1118 + 1021 El1074 + 1021 El1074 + 928 A,, E,1074 + 846 E,2 x 928 A,, El928 + 846E2928 + 835 E,846 + 835 El2 x 835A1vCC elvCC e21118 + 1074E2a Observed in all species.For convenience, combination sums quotefrequencies for the methyl compound only.no means of knowing whether WlS5, or (xasym - gas,,), orboth, are anomalous, we adopt as a compromise the resultsunder column 2, for W, 5 5 = 43 cm-'. The resonance shift onvsymCH3 is then 18.2 m-' placing vrYmCH, at =2858 cm-'A similar treatment for the CD, goup is inhibited by anabsence of W values from model compounds, and because6,,,,CD3 is obscured by a strong cyclopentadienyl band. Using6,,,,CH3 = 1391.8 cm-' and the ratio 6,,,,CD,/6,,,,CH3 =0.7343 found in [TiMeCl,], we predict that 6,,,,CD3 should lienear 1022 cm-'.Its overtone is observed at 2022 cm-' from whicha resonance shift, with vsymCD3, of about 12 cm-' seems likely,assuming (x,,,, - gas,,) to be - 5 cm-'. With a little extra shiftderived from 26,,,CD3, vsymCD3 is estimated at 2076 cm-' .Using v~,,CH, = 2858 cm-' in the frequency sum rule[equation (2)], we obtain hCH, = 8792 cm-'. The values ofZv'"CH are 8803 and 8763 cm-', respectively, for vis-CH" > v'"CH" and viSCHs > viSCHa. Accordingly, we concludethat the methy€ group contains two strong bonds, (CHa)roughly comparable with the CH bonds in PiMeCl,](v'"CH = 2952 cm-') and one weaker bond, CH".HLM forcefield. We now combine the CH,, CD, and CHD,frequencies corrected for Fermi resonance, in a full harmoniclocal mode refinement of the force field. The ratio vasymCH3(a")/vasymCD3 (a") indicates an HCH angle of 107.5".In Table 4,we show under columns 1 and 2 the force constants andfrequency fit for three- and four-parameter force fields using anaverage HCH angle of 107.5", under columns 3 and 4 similarforce fields for HCH = 110", and under column 5 amodification of column 2 in which the HaCH" angle is adjustedso that the off-diagonal constants fa," and f a , , become equal.Throughout, no weight has been placed on the frequencies mostaffected by Fermi resonance, v~,,CH, and v;,,CD,. It ispleasing to find their calculated values close to the correctedobserved onesJ.CHEM. SOC. DALTON TRANS. 1995 3945Table 2 Infrared bands (a-') in isotopomers of fliMe(cp)C12] assigned to the TiMeC12 moietyCHD, 2947.8~2907.6~2219.0~21 13vw1241.2m1084 (sh)927w587w549.9m481s455vsx 428vsCD3 2231 (sh)221 5 . 3 ~ - 2090~'2022vw(1021)f867 (sh)669w516 (sh)452vsviSCHaviSCHsVasymCDzvsymCD26CH a"6CH a'6CD2 a'Vobs{CTiMeC~311 a298 128942727137511124844642952223 1212512301080935223821001009880a Relevant methyl group modes only. Impurity bands at 2919 (sh) and 2024vw cm-' (these also remain in a decomposed sample). Impurity bands at2871 (sh), 2856 (sh) and 2930 (sh) an-'. Impurity bands at 206Ovw and 578 (sh) an-'. Coincident with cp band. Value estimated from overtonewith an allowance for Fermi resonance.Table 3 Fermi-resonance calculations for the CH3 group inrWH3)(CP)C12113240201.01.410.7-31.62- 30.032761.2-11.22773.62200.824340201 .o1.618.2-34.16- 32.442768.52773.62200.8- 7.634740201 .o1.721.6- 35.27- 33.472771.6- 6.02773.62200.845040201 .o1.724.4-36.15- 34.292774.1- 4.92773.62200.8a Units: a-'.W values used in initial refinement of ha,s, La,a and2(6prym + xmY? - gas,,& Calculated Fermi-resonance shifts. 26,,,,A'(A,) frequenaes calculated with refined values of La,.,, la,a and allWill = 0. 26,,,,A", frequency constrained. f 2tisym, A', frequencycons trained.Overall there is a clear preference for an angle close to 107.5",and either for ya," < f",,,, or for H'CH" > H"CH".In eithercase, at angles close to 10S0, they values are markedly less thanthose in compounds previously studied, including LTiMeC13 J(0.025 mdyn Ag-1).9 The effects off' and the HCH angle arecombined in the parameter h so that all the experiment directlyshows is that ha," is more negative than ha,,, as seen in Table 3.However, given the clear difference between the CH" and CH"bonds, a modest difference between the HTH' and HTH"angles is reasonable and likely. In effect, the calculation sets anupper limit of about 2" on this difference.~iEt(cp)Cl,].-Wavenumbers and assignments for the ethylgroup vibrations in ~i(CH,CH,)(cp)C1z]([2H0J), pi(CD,-(CD,CHD,)(C~)C~,J([~H,J) in Table 5.The methyl group isclearly asymmetric: two v'TH modes, separated by 35 cm-', areobserved in the C2H4] species, and vasrmCH3 in the ['H2]species is split into a' and a" components.In assigning the bands below 1500 cm-', we have to beguided by the recent assignments for various isotopic speciesof ~hloroethane.~ Experience with [TiMeCl,]' and withFiMe(cp)Cl,] suggests that the force constants associatedwith the bending motions of the methylene group,.in particular,will be significantly lower in the EtTi moiety than in EtCl, sothat these modes are likely to be shifted to substantially lowerwavenumbers in the titanium compound.The potentialenergy distributions for the relevant modes ofEtCl isotopomers are summarised in Table 6.As an aid to thecomparison, we number the ethyl modes in Table 5 in the sameway as the EtCl modes in Table 6, ignoring the TiCl, and Ti(cp)contributions to the overall vibrations of the molecule.*CH3)(cP)C121(C2HzI), CTi(CHzCD,)(cP)C121(ZH,) and CTi-Me thy2 group: p i ( CD, CH ,)(cp)CI ,]( [ H ,I) .-In the vCHregion we observe a pattern of five major bands between 2960* Certain bands 'cross over' from EtCl to uiEt(cp)Cl,]. Thus in theC2H3] species v5 in uiEt(cp)CI,] corresponds to vg in EtCI, and oicewrsa; and in the ['H2] species v, FiEt(cp)Cl,] corresponds to v6 inEtCl, and oice uersa3946 J. CHEM. SOC. DALTON TRANS. 1995Table 4 Harmonic local-mode treatments for FiMe(cp)Cl,]CH3CD3CHaD,CHsDD,AnglesForceconstants fvobsn2976.82957.12858.0d2231.12215.32076.0d2947.82219.021 13.22907.6I-{ ;::;:$:::: l-f,f aCw.s.e.#1c , n .b E , n&1 - 1.41 1.6100 0.010 - 1.110 1.5100 2.01 0.610 - 0.4100 - 3.51 - 0.8(2232.2)(2132.7)} 107.54.6226(30)4.7477(34)- 0.0005(40)5.732E , a,c0.00.0- 0.40.00.6- 2.50.0-0.8-3.10.0(223 1.1)(21 34.8)107.54.6 198(2)4.7502(2)- 0.001 5(2) { 0.0066(2) }0.0123- 1.41.70.7- 3.9- 1.25.00.60.3- 3.0- 0.8(2235.0)(2 128.7)110.04.6227(32)4.7476(36)0.0 142(43)6.354E, a,c0.00.00.2- 2.8-2.14.30.0- 3.40.60.0(2233.9)(2 130.8)110.04.61 98(7)4.7503(8)0.0132(9) { 0.0216(11)0.2485E,0.00.0- 0.20.0- 1.60.20.0-2.3- 1.20.0(21 31.1)(2 1 34.4)107.5108.884.6 198(5)4.7502(5)0.0067(6)0.0067(7)0.080a Units: cm-'.Uncertainty in vobs. vobs - vcalc; vCalc in parentheses. Fermi-resonance correction applied. HCH angles in *. Valence forceconstants in mdyn A-'. Sum of weighted squares of errors.and 2850 cm-', plus two minor bands just below (Fig. 2). Thetwo highest peaks, at 2959.8 and 2940.1 cm-', are the a" and a'components of vasymCH3, and the lowest, at 2859.4 cm-',vSymCH3. Using these assignments, we obtain ZvCH, = 8759.3cm-', in very good agreement with Zv"CH = 8760.6 cm-'(from the C2H4] spectrum) for the case of a methyl group withtwo strong bonds and one weak one, but not with thealternative value of Zv'"CH = 8725.8 cm-' for two weak bondsand one strong one.The two remaining bands in the vCHregion must derive from the bending overtones, 26,,,,CH3. Inprinciple, three such overtones are to be expected: the A' and A"levels derived from 26,,,,CH, (E) and a further A' levelcorresponding to 26,,,,CH3 (A,) in a symmetrical group. Ofthe two experimentally observed peaks the lesser one, at 2888.4cm-', is close to the expected unperturbed value of 26,,,,CH,and hence is assigned to the A" 26,,,CH3 (E) level. The secondband (2919.6 cm-') is then the A' (A,) component of26,,,,CH3, displaced upwards by xevonance with vsymCH3 (a')(2859.4 cm-'). The A' 26,,,,CH3 (E) level is not observed as aseparate band.The 6,,,CH3 band is readily assigned at 1372.4 cm-', closeto its value of 1370.6 cm-' in the C2H0] species, and in thecorresponding chloroethanes (1 384 cm-').The weak band at2724 cm-' is then clearly 26,,,CH3 (A') and the second weakband, at 2818 cm-', is 6,,CH3 + 6,,,,CH3 (A').Table 7 shows the results of a local/norrnal mode Fermi-resonance calculation, with W values taken as before from themethyl halides. Five parameters were refined: the two gfcoupling terms and A,,, and the unperturbed values ofxsym). As one of the two 26,,,,CH3 levels denved from the26,,,,CH3 (E) pair was not observed, the calculation wascarried out with the 2888.4 cm-' band assigned first as the A'26,,,,CH, (E) level, then as the A" level. Switching theassignment of the 2888.4 cm-' band in this way affected thecalculated Fermi-resonance shifts by only 0.1 cm-' .The resultsshown in Table 7 are with the 2888.4 cm-' band assigned as theA' mode; the A level is then computed to lie 1.7 cm-' above this.The shifts on vaSymCH3 (a"), vasymCH3 (a') and v,,,CH, (a')(28asym + Xasym - gasym), 2(6asym + Xasym + g a s y d and 2(6syrn +are calculated to be 3.1, 7.6 and -9.8 cm-' respectively. Usingthe resonance-corrected frequencies, ZvCH, is altered by only0.9 cm-' and confirms the initial assumption that the methylgroup has two strong bonds and one weak one. The assignmentof vasymCH3 (a") above vasymCH3 (a') is an immediateconsequence of this structural arrangement.The value of xSy, for 6,,,CH3 is found to be -4.7 cm-',which is reasonable for a mode of this kind.20 The value ofgasymfor 6asymCH3, -2.3 cm-', is not out of line with those found inthe methyl halides, which range from -0.1 to -3.0 cm-' butxasym, at - 1.1 cm-', is distinctly smaller than the typical valuesof -5.9 cm-' (CH,Cl) and -5.74 cm-' (CH,I).18 Thedifference may arise from uncertainties about the exact positionof the bending fundamental, coupled with a likely smallsplitting in this mode.Methyl group: [T~(CH,CD,)(C~)C~,]([~H,]).In the vCD,region (Fig. 2) instead of the expected split vasymCD3 (a', a")band, we observe a prominent band at 2236 cm-' and a complexone at 2208 cm-', with shoulders at 2202 and 2183 cm-'. Theband due to vSymCD3 is readily identified at 2066 cm-' . Analysisof the v,, ,CD3 levels entails a prior study of the region below 1 1200 cm- in order to identify fundamentals whose overtones orcombinations could appear near 2200 cm-' .On the basis of the data for CD,CH,Cl (Table 6) we expectfour A' modes (v5, v6, v7, v8) and three A modes (v,,, ~ 1 5 , v16)between 1200 and 850 cm-'.Two of these, v7 and ~ 1 5 , are thesplit components of tjasyrn CD, and are clearly the 1054.5/1049.3cm-' pair, with overtones at 21 14.2 and 2097 cm-', just abovevsymCD3. The wCH, mode v5 in CD,CH2C1 (1290 cm-') isexpected to fall appreciably in the titanium compound and isassigned to the strong band at 1075.6 cm-', where it becomes v6.The weak band at 930.5 cm-' is assigned to v8, which beingessentially a vCC/6,,,CD3 motion should lie close to its valueof 937 cm-' in CD3CH,C1.The moderately strong band at 1 131.9 cm-' is at the upperlimit of the possible range for v5 (6,,,CD,/vCC) given its valueof 1135 cm-' in CD,CH2C1 (where it is v6), but its intensity isvariable relative to other bands in different samples and must bedue at least in part to impurity.If this band does arise from v5CTi(CH,CH,)(cP)C1zI a CTi(CD 2 CH 3)(CP)C1Zl I3 r]Ti(CH,CDdq)C12lb2961.7m vaSymCH3 a'' 12 29 59.8m vasyrnCH3 a" 12 291 1.5m ~asymCH2 a"Table 5 Infrared bands attributed to the TiEtCl, moietyv,&m-' Assignment Mode vobs/cm-' Assignment Mode v,,,/cm-' Assignment2940.4m vaSymCH3 a' 1 2940. lm vasymCH3 a' 1 2837.6ms VsymCHz a'2915.3m vaSymCHZ a" 13 2919.6mw 26,,,CH3 A' 2749.3~ 26,,,CHz A2888.6mw 26,,,,CH3 A' 2888.4mw 26,,,,CH3 A 2235.9m2841 (sh) vSy,CH, a' 3 2818vw 1448 + 1372 A' 2202 (sh)2208ms 2750vw 26,,,CH, A' 2724.0~2730vw 26,,,CH3 A1452 (sh) 6,,,,CH3 4, 14 2 169vw VasymCD, a" 2183ms1385 (sh) 6,,,CH2 a' 5 2 1 2 0 w 1081 + 1042A' 21 14.2m1370.6m 6,,,CH3 a' 6 2090.3~ ~symCD2 a' 3 2097 (sh)1199.5~ tCH, a" 15 2 0 6 9 ~ 2 x 1042 4,14 2066.6s1 1 15vs wCH, a' 7 E 1448 (sh) 6,,,,CH, a', a" 5 1385.3m1074.5ms pCHJv,, 8 1372.4m 6symCH3 a' 1 13 1.9msvasymCD3 a"/1132 + 1088 A1120 + 1098 A'~asymCD3 a'/1132 + 1052A'or1120 + 1076A'26asymCD3 A'26,,,,CD3 A , A'vsymCD3 a'SsymCH2 a'Impurity +? 6symCD31098 (sh)1088 (sh) } { :':;::A'~ ~ ~ : } 6,,,,CD3 a', a"x865 (sh) ? 930.5m VCCAymCD~ a'652vw tCD, ? 16 865 (sh) pCH,/pCD, a''550vw pCDz ? 17 557m pCD3/pCH2 a"487vsx435 100% } {i:+icl2I 2861.6ms vS,,CH3 a' 2 2859.4m vsymCH3 a' 22 2 0 4 .0 ~ (br) } {%;(??&; A"/ 13933.8mw pCH,/v,, 9 1 134. lm Impurity ? 15575w (br) pCH3/pCHz 17 1081m PCH3 71032 (sh) Impunty . 895ovw 6symCD2 /VCC600w cp + ? or pCD, a'876 (sh) Mixed 16 1098 (sh) PCH3 6< 500 (Not observed) 1041.9ms 6symCD2/? 1075.6~ wCH, a'a Impurity band also seen: 181 1 cm-'. Impurity bands also seen: 2969,2882 and 181 1 cm-'. Impurity band also seen: 2855 cm-'. Mode numberin3948 J. CHEM. SOC. DALTON TRANS. 1995Table 6 Some fundamental bands and PE distributions in chloroethane species (ref. 4) *A' vobsCH3CH,Cl v4 1467vs 1459v7 1289v6 1384vg 1073v9 973CH3CD2CI v, 1459VS 1384v6 1126v7 1099vg 851CD3CH2C1 v, 1461v, 1290v6 1135v7 1062v g 1008vg 937~9 856vcc VCCl109540 1147 941001145252153111643 2512 224855102 101393698 5028 1343 35A"CH3CH2C1 ~ 1 4 1447~ 1 5 1251v16 1082~ 1 7 785CH3CD2CI ~ 1 4 1449V I S 1122V16 815~ 1 7 651CD3CH2Cl ~ 1 4 1203~ 1 5 1053v16 1028~ 1 7 663* Units: cm-'.5537153363101378801587328611 13615937 10Table 7 Local/normal mode Fermi-resonance calculations for theCH, group in [Ti(CD,CH,)(cp)CI,]"cm-' are likely to arise from a close resonance betweenvasymCD3 a" and an A combination 1120 + 1098 cm-'(assuming vz to be 1120 cm-') or 1132 + 1088 cm-' (if v5 is notsignificantly perturbed by Fermi resonance). The value ofv~,,,CD, (a") would then be about 2215 cm-'.The bands at2208 and 2183 cm-' are attributed to the A' resonance betweenvasymCD3 (a') and 1120 + 1076 crr-', or possibly 1132 + 1056cm-', suggesting a value of x2200 cm-' for V:~,,CD, (a').Table 8 shows the results of a trial local/normal modecalculation analogous to that for the CH, group. The diagonalelements for the CD" and CD' bonds were those for CH" andCHs divided by 1.347 15,'f the coupling parameters and La,awere calculated using a common HCH angle of 108" and thefvalues associated with it (see below), the Wvalues were 2-&times those for the CH, problem, and sensible values of 26:,,,(2213 cm-'), 26~sy, (2097 cm-') were adopted. The computedvalues of the higher 26,,,,CD, (A') level, 2112 cm-', and thelower A', A" pair, 2096 crr-', are very close to the observedbands at 2114.2 and 2097 cm-', suggesting that the Wvaluesassumed here are appropriate.The calculation predicts thevasymCD3 levels at 2215 cm-' (a") and 2199 cm-' (a') very closeto our estimated frequencies for the unperturbed modes andlending support to the assumption that these modes in the experi-mental spectrum are shifted by resonances with combinationmodes and not with the bending overtones. (The local/normalmode calculation only takes account of the latter interactions:effects involving combination bands are not included.)The main point of interest arising from the calculation is theresonance shift of 21.2 cm-' predicted in vsymCD3, which whenapplied to the observed vsymCD3 frequency yields vZY,CD, at2088 cm-'.HLM forcefield for the methyl group, As we do not have anvobs Vcalc(1)2959.8 2958.62940.1 2939.02919.6 2920.7- 2890.22888.4 2888.52859.4 2858.02724.0 2724.1Assignment vCalc(2)Vasym a" 2955.6Vasym a' 2931.428asym A'26asym A"26asym A'Vsym a' 2867.7 -26sym'9.76Parameters refined in (1) Parameters constrained in (1)ha,s - 22.27 * viSCHa 2929.8 d,eL , a -25.75d viSCHs 2895.0 d*e2@asym + Xasym - gasym) 2902-23 w135 32W a s y m + Xasym + gasym) 2893.26 W122 402(6sym + Xsym) 2735.47 W,,, 20" v In ad.' Calculated unperturbed frequencies. ' Fermi-resonanceshifts. Parameters constrained in (2) (no Fermi resonance present).Experimental frequencies - 2.we would expect to find an overtone near 2260 cm-', wherenothing is visible.Instead, if the prominent band at 2235.9 cm-'arises from a vaSy,CDj mode affected by resonance with anovertone or combination mode, we might look for anunperturbed fundamental v5 between 1 1 10 and 1 120 ern-.'. Thenearest absorptions are the 1132 cm-' band and two shoulders,at 1098 and 1088 an-', on v6. These two bands are reasonablyassigned to 2 x 557 (probably ~ 1 7 , pCD3) and v14, tCH,. Aresonance between v5 and 2v17 would then move the formerfrom an unperturbed frequency of x 1 120 cm-' to the observedfrequency of 1 132 cm-' .Given these assignments, we feel that the bands at 2236/2202 t The ratio (gCH/gCD)* - 1.01 1J. CHEM. soc. DALTON TRANS.1995 3949unequivocal value for v&,&D,, we have no means of makingan initial estimate of the HCH angle, and therefore have carriedout trial force-field calculations for HCH = 106, 108 and 110"(Table 9, columns 1, 3 and 5 ) and for two further situations,columns 2 and 4, in which HTH" is 2" less than HTH". Incarrying out the refinement, we attach the most weight tov~~,,CH, (a"), V~,~,ICH~ (a'), $,,,,CH,, v;,,CD,, v'"CH" andvLSCHS, implying confidence, in particular, in the Fermi-resonance correction calculated for v,,CD,. The vnsymCD3modes were allowed to refine without constraint.For the 110" calculation (column 5), the error in v;,,CD, isbecoming uncomfortably large. At the other end, the 106"calculation (column 1) producesf' values of 0.016 and 0.027mdyn A-', which are rather small when compared with theexpected values for a substituted ethane.In chloroethane, forexample, fa,, and fa,, are 0.0411 and 0.0416 mdyn A-'respectively, from the scaled a6 initio force field.4 We thereforeprefer 108" as the most likely mean angle. Setting HaCHs to 1 lo",with HTH" kept at 108", has the effect of makingf',,, andf',,,converge at values close to those found in chloroethane. Thecalculation here depends critically on the reliability of the Fermi-resonance corrections applied to V,,~,CH, .(a") and vasymCH3(a'). We conclude that our results are consistent with a widerH"CHs angle, but do not conclusively prove this to be the case.Methyhe group: ~i(CH,CD,)(cp)Cl,]. As seen in Fig.2,the 26,,,CH2 band at 2749.3 cm-' is clearly in resonance withvSymCH2 at 2837.6 cm-'. Assuming a normal anharmonicity(xsym x -5 cm-') for the 6,,,CH2 mode, the resonance shiftwould be 11.3 an-', putting v;,,CH, at 2826.3 cm-'. WithTable 8 Local/normal mode Fermi-resonance estimates for the CD3group in ~i(CHzCD3)(cp)C1,]. Assumed parameters (cm-I):26:yrn 2213, W,,, 32.2-*, W,,, 40.2-*,' W,,, 20.2-', HCH = 108'.Output frequencies and assignment: 1, Was above; 2, all W = 0trcD 2174.8, v'CD 2149.0, La,, -39.65, La, -39.31, 26",,, (A,E) 2097,1 2 Av *26syrn A' 2219.6 2213.0 6.6Varyrn a" 2215.3 2214.5 0.8Vssyrn a' 2198.7 2198.1 0.626,s- A' 21 12.0 2097.0 15.026ssym A" 2096.2 2097.0 -0.826asym A' 2096.0 2097.0 - 1.0Vsym a' 2064.9 2086.1 -21.2* Fermi-resonance shifts.vasymCH2 at 291 1.5 cm-', the average vCH value is 2869(3) cm-'and the vmymCH2 - v;,,CH, separation is 85(5) cm-', theuncertainties ansing from our ignorance of the exact value ofx,,, for 6,,CH,.Both of these results are anomalous and willbe further discussed further below.Methylene group: ~i(CD,CH,)(cp)Cl,]. In the vCD, regionwe encounter similar difficulties to those in the vCD, region inthe C2H3] species. The v,,CD, band is obvious at 2090.3 cm-',although a small part of its intensity derives from an underlyingcp band, but where V,,~~CD, is expected there is a broad regionof absorption between 2169 cm-' and ca. 2204 cm-'. Between1300 and 900 cm-', we expect three A' modes (vg, v7 and v8) andv1 (A").Of these, v7 and v1 in CH3CD,Cl are substantiallypCH3 modes (Table 6) and should fall just a little on passingto the titanium compound. Mode vg in CH,CD2Cl (1 126 cm-')is mostly CD, motion and should fall substantially inpi(CD,CH,)(cp)Cl,]. The highest band observed in thisregion (1 134 cm-') thus appears to be too high to be due to vg,v7 and v15 and we attribute it to an impurity. The moderateband at 1081 cm-', with a shoulder at 1098 cm-', isappropriately placed for the two pCH, modes, renumbered inthe titanium compound as v6 and v15. The strong band at1041.9 cm-' is then assigned as v7, displaced from 1 126 cm-' inCH,CD,Cl where it is vg. Mode v8 in CH3CD,Cl (1008 cm-')should fall by a similar amount to v6 and the only candidate isa very weak band at 950 cm-'.There remains a prominentshoulder at 1032 cm-', which may represent a combination orovertone or resonance with v,, or else arise from impurity.Placing v7, a predominantly 6,,,CD, mode, at 1042 cm-'provides a satisfactory explanation for the weak band at 2069cm-' as 26,,,CD2 in resonance with vsymCD2 at 2082 f 3 cm-'.The antisymmetric stretch, vasymCD2, is expected in theregion 2150-2200 cm-': the broad band of absorption in thisregion could also contain overtone contributions (2 x 1098 =2196, A', and 2 x 1081 = 2162 cm-', A') as well as acombination 1098 + 1081 = 2179 cm-' (A). Of these, only theA" combination could be in resonance with v99,,CD2.From the average vCH value of 2869 cm' deduced for theCH, group in pi(CH,CD,)(cp)Cl,], we predict a correspond-ing v,,CD value of 2130 cm-' for the CD2 group in~i(CD,CH,)(cp)Cl,], using the factor (gCH/gCD)* i 1.01 1 =1.347 15, as bef0re.t With V;~,CD, placed at 2082 cm-' andf This treatment works very well in the chloroethanes, where anindependent check is po~sible.~Table 9 Harmonic local-mode calculations for the methyl group in FiEt(cp)Cl,]GroupCH3CD3CHsDD,CHaD,HaCHa/"HaCHS/Of af,Y a , af ' a , sZw.s.e.1v0,,/cm-' QV a E V2956.7' 1 0.22932.5' 2 0.72869.2' 4 2.4- - (22 14.2)- - (2194.2)2088.0' 5 -0.52897.0 1 -0.2- - (22 1 4.2)- - (2132.5)- - (2200.5)293 1.8 1 -0.3- - (2120.1)1061064.59334.70430.01620.02730.632EV0.20.82.2(22 16.4)(2193.1)0.9- 0.2(22 16.4)(2 130.0)(2200.3)(2 120.2)108106- 0.44.59344.70470.02860.02790.703EV0.31 .11.6(22 16.4)(2196.1)4.2- 0.2(22 16.4)(21 30.0)(2202.3)(21 17.9)108108- 0.64.59414.70560.02970.04121.514EV0.31 . 11.3(22 1 8.6)(2 1 95 .O)5.4- 0.2(22 18.6)(2 1 27.6)- 0.6(2202.2)(2177.9)1101084.59424.70610.04190.04182.165&V *0.41.40.7(22 18.5)(2 1 97.9)8.6(22 18.5)(2 127.6)(2204.2)(21 15.6)1101104.59504.70700.04820.05504.32-0.3-0.8a Uncertainties in wavenumbers. * E , = vOh - vCaIC; v,,,~ in parentheses. ' Corrected for Fermi resonance3950 J. CHEM. SOC. DALTON TRANS. 1995v:,,CD calculated to be 2130 cm-', v:~,,CD, is estimated at2178 k 5 cm-'.We therefore provisionally assign the weakpeak observed at 2169 cm-' to vasymCD2, in resonance with thecombination (1098 + 1081 = 2179 cm-') (A").The estimated separation V:~,,CD, - v;,,CD, is therefore2178 - 2082 = 96 k 10 cm-', which leads to the unexpectedconclusion that the methylene group is asymmetric, as discussedbelow.HLM Calculations for the methylene group. The vasymCH2 -V;~,CH, separation in the titanium compound is about twicethat in chloroethane (Table 10). The vasymCD2 - v;,,CD,separation in chloroethane is twice as much as the vasymCH2 -vsymCH2 separation, but in the titanium compound it isonly 13% more than vaSymCH2 - v;~,CH,, albeit with anuncertainty of about 20%. Together, these two results castdoubt on the symmetry of the methylene group.The difficulty is highlighted in a quantitative fashion bycalculating the force constantsfand7 required to reproduce thev~~,,CH, and v:~,CD, frequencies, for various HCH angles,and comparing the values of v:,,CD, and v:~,CD,, whichthen result, with the experimental frequencies.Table 11 showsthe results for HCH angles from 104 to 114". Values offcompatible with those found in [TiMe(cp)Cl,] appear onlyat the top end of the range, around 113", and for thesethe v:~,,CD, -v:~,,CD, separation of 116 cm-' is muchgreater than that deduced from the spectrum, and welloutside the uncertainty imposed by the Fermi-resonancecorrections. It is not possible to find a solution, for a CH,group with two equivalent CH bonds, which will reproducethe vz,,CD, - vz,,CD, separation with a realistic value forthe interaction constant.The simplest explanation is that theTable 10 vCH and vCD data (in cm-') for the methylene groups"in EtCl and FiEt(cp)Cl,]CTiEt(CP)Cl,I291 1.52826(3)85(5)2869(3)[2 1 78( 5)] '2083(3)96( 10)[2130(4)]1.347 15'"Based on the partially deuteriated species CD,CH, and CH,CD,. * Data from ref. 4. ' Derived from vaVCD and vSymCD,. Derived fromvavCH/1.347 15. ' Assumed.methylene CH bonds are inequivalent and have differentv'"CH values.In part (b) of Table 11 we explore the range of isolated CHstretching frequencies and HCH angles which togetherreproduce the vasym - vsym splittings of 85.2 and 96.0 cm-I forthe CH, and CD, groups respectively, as a function of theinteraction force constantf.Once the latter is fixed, then boththe v'TH values and the HCH angle are determined. We choosevalues off' of - 0.01 and + 0.01 as spanning the likely range off for the methyl group in FiMe(cp)Cl,] (see above), while thatof 0.025 mdyn A-', is similar to those found in [TiMeCl,] .9 TheHCH angles found using these interaction constants are in therange 106-log", which is compatible with the angle of 107.5'observed in FiMeCl,], and a difference of 50-60 cm-' isrequired in the two v'TH values.A further calculation with vasymCH2 - vz,,CH, kept at 85.2cm-' but V:~,,CD, - v;,,CD, increased to 101 cm-' showsthat the splitting v'TH (1) - v'TH (2) falls by about 6 cm-'and the angle increases by 1.6".The main factor determining thesplitting is in fact the choice of thef value. Our final choice ofthe two v'TH values is 2900(5) and 2847(5) cm-', implying amarked degree of asymmetry in the methylene group.[Ti(CH,CH,)(cp)Cl,]. The collection of bands in the vCHregion is very close to being the sum of those in the ['H,] and['H,] species. Small differences in frequencies are as likely toarise from variations in Fermi resonance as from couplingbetween the CH, and CH, stretching motions, the interactionforce constants for which are very The relativeconstancy of the tiasymCH3, 6,,,CH, and 6,,-;;1CH, frequencieshas already been mentioned. There remain the modes v7, Vg, vg,v1 5 , v16 and vl, to be assigned.In CH,CH,Cl, v7 (1289 cm-') isprimarily a wCH, motion which in the titanium compound willhave a lower force constant, and must mix with CC motion. Thevery strong band at 11 15 cm-' is suitably placed. Bands v8 andv9 at 1073 and 973 cm-' in CH,CH,Cl are a pair of coupledpCH,/vCC modes which should remain little changed in[Ti(CH,CH,)(cp)Cl,]: the bands at 1075 and 934 cm-' in thetitanium compound are plausibly assigned to these modes. Theweakband at 1199.5 cm-', if a fundamental, can only be due tovI5, tCH,, perhaps raised in frequency by coupling to pCH3motion. A shoulder at 876 cm-' could arise from v16 and theweak band at 575 cm-' is probably v1 7, a coupled pCH3/pCH2mode.DiscussionIn Table 12, we list the predicted CH bond lengths, bonddissociation energies and HCH angles for the two compounds.Table 11(a) Symmetrical CH, groups.Force constants (mdyn A-') and HCH angles fitting v$,CH, = 291 1.5, V:~,CH, = 2826.3 cm-'Harmonic local-mode calculations for the methylene group in piEt(cp)Cl,]HCH/O 104 106 108 110 112 1144.5087 4.5088 4.5090 4.5092 4.5095 4.5099- 0.0494 - 0.0376 - 0.0259 -0.0144 - 0.0030 0.0082vasymCD2 * 2131.0 2133.3 2135.5 2137.7 2139.8 2141.9VSYrnCDZ* 2035.7 2033.3 2030.9 2028.6 2026.2 2023.8AWD,) 95.3 100.0 104.6 109.1 113.6 118.1ff'(b) Asymmetric CH, group. vis values and HCH angles fitting Av(CH,) = 85.2 cm-' for several values off' and Av(CD,) = 96.0 or 101.0 cm-'f' -0.01 0.01 0.025AvCD, = 96.0cm-' visCH( 1) 2897 2902 2904v"CH(2) 2845 2840 2837HCH/O 105.75 106.9 107.8AvCD, = 101.0 cm-' v'"CH(1) 2894 2899 2902v'"CH(2) 2849 2843 2840HCH/O 107.2 108.5 109.4* Computed values x 1.01 1J.CHEM. soc. DALTON TRANS. 1995 395 1Table 12 Predicted CH bond lengths, bond angles and bond dissociation energies in FiR(cp)Cl,] (R = Me or Et)Compound Group viSCH"/cm-' r,CH/A HCH/O Do298 (C-H)/W mol-'viMe(cp)Cl,] Methyl CH" 2958 1 .0956 107.5(10) (HaCHa) 42 1CHs 2918 1.1000 108.9(10) (H'CH") 406PiEt(cp)Cl2] Methyl CH" 2942 1.097, 1 lO(2) (H'CH") 41 5CHs 2907 1.100~ 108(2) (HBCH' 402Methylene CH( 1) 2910 1.100, 106.3( 15) 403CH(2)' 2857 1.10S9 383" v,, (CCl, solution) + 10 cm-'. Calculated frequencies.Table 13 Isolated CH stretching wavenumbers" (cm-') in methyltitanium compoundsCompound viSCHa v"CH' Avis v"CH (av.) Ref.CTiMe(cpF321 2958 2918 40 2945 This workCTiMe,(CP)q 2932 2915 17 2926 6CriMe,(cpll 2938 290 1 37 2926 14CTiMe,(cp)l 2905 2948 - 43 29 19 14viMe,Cl,] 2948 14CTiMeCl,] 2952 9" Corrected to gas phase.' Crystal. Matrix.The CH bond lengths, r,CH, are related to viSCH by equation(6) 27 and are accurate to at least 0.001 A. The bond dissociationenergies D0298(cH) are obtained from equation (7).12r,CH (A) = 1.3982 -0.000 102 3 v'TH (cm-') (6)D0298CH (kJ mol-') = 0.375 vi"CH (cm-') - 688 (7)The experimental data used to derive equation (7) refer tosmall organic molecules rather than organometallic compoundsand overall the relationship is less precise than that betweenviSCH and r,CH. Uncertainties in the absolute values ofDO,,,(CH) may be of the order of 3-4 kJ mol-', but areconsistent over the whole calibration, so that the differencesbetween the CH bond dissociation energies, particularly forbonds within the same alkyl group, should be reliable forprocesses leading to radicals having conformations appropriateto the particular bond broken.The viSCH frequencies upon which the v'"CH-r,CHrelationship is based were all obtained from gas-phasemeasurement^.^ Typically, v'TH rises by x 10 cm-' fromsolution to the gas phase.,, The viSCH frequencies quoted inTable 12 are therefore our experimental values (CCl, solution),plus 10 cm-', and the r,CH values are notional 'gas phase'bond lengths.Isolated CH stretching frequency data for allthe methyl titanium compounds for which this informationis available are listed in Table 13.The separation viSCH" - viSCHs (40 cm-') observed in themethyl group in FiMe(cp)Cl,] is comparable with the largestyet found in methylmetal compounds and correlates with adifference of 15 kJ mol-' ( ~ 4 % ) between the in-plane and out-of-plane CH bonds.Assuming a staggered conformation [Fig.3(a)], the CHs bond will be trans to the cyclopentadienyl ligand,and the CH" ones trans to chlorine. In aliphatic systems, thereis increasing evidence that P-substituent effects tend to act ongauche CH (or SiH) bonds, rather than trans ones,29 suggestingthat the CH" bond in FiMe(cp)Cl,] being gauche to twochlorine atoms should be similar to the CH bonds inDiMeCl,] [Fig. 3(b)] whereas the CH" bonds may be different,depending on the @effect of the cyclopentadienyl ligand, aboutwhich we have as yet no information.In the event, the v'"CH"frequency is very close to viSCH in PiMeCl,], whereas viSCHss markedly lower. Conceivably, this could be indicative ofL weak agostic ' v 2 CH' Ti interaction, although other:xplanations may also be possible.The data for the other methyltitanium compounds listed in(a 1IH'(2918)(b 1IH(2952)( c 1H'(2915) I(e 1H'(2901)I( f )IH'(2948)Table 13 present a confusing picture. In F i M e , ( ~ ) , l , ~ theCH" bonds will be gauche to methyl and cyclopentadienyl [Fig.3(c)]. Methyl substituents in aliphatic systems tend to have amodest weakening effect on the gauche P-CH bonds,' and theviSCH" frequency of 2932 cm-I in DiMe2(cp),] would becompatible with such an effect.However, if we interpret thelow value of viSCHs (2915 cm-') in the same way, we have toconclude that the P-effkct of the cyclopentadienyl ligand mustbe very much stronger than that of methyl. There is nosuch evidence in the spectrum of ~iMe(cp)Cl,] wher3952 J. CHEM. SOC. DALTON TRANS. 1995Table 14 Isolated CH stretching wavenumbers (cm-') for ~i(CD,CHD,)(cp)Cl,], * CHD,CD, and CHD,CD,X (X = F, C1, Br or I)Compound viSCHa v'TH' AviSCH v'TH (av.) Ref.~i(CD,CHD,)(cp)Cl,] 2942 2907CHD,CD, 2950CHD,CD,F 2973 2957CHD,CD,CI 2972 2945CHD,CD,Br 297 1 2936CHD,CD,I 2968 2928* Bands in CCI, solution, + 10 cm-'.351627354029302967296329592954This work331313131viSCHa, gauche to cyclopentadienyl, is normal or even slightlyhigh.Further problems arise in the case of FiMe,Cl,] [Fig.3(d)] where only one v'TH band is observed, despite thevery different P-substituent effects of methyl and chlorine inaliphatic systems.Finally, in the case of [TiMe,(cp)] [Fig. 3(e)and cf)] the v'TH frequencies invert on change of phase,with v'"CHa > viSCHs in the crystal, by 37 cm-', butvi'CHS > viSCHa in a N, matrix, by 43 cm-'. The origin of thisinversion, or its relationship to the data for the othercompounds, is far from obvious.Overall, it is clear that substituent effects observed inrelatively simple aliphatic systems cannot be extrapolateddirectly to the more complicated transition-metal compounds.Furthermore, there is no clear pattern even amongst the fivemethyltitanium compounds discussed here.It is necessary tobear in mind that these compounds are not necessarily directlycomparable: the formal electron count for the titanium atom iseight for FiMeCl,] and niMe,Cl,], twelve for FiMe(cp)Cl,]and DiMe,(cp)] and sixteen for [TiMe,(cp),]. Moreover,although all five compounds are formally tetrahedral,[TiMe(cp)Cl,] and [TiMe,(cp)] are perhaps better regarded aspseudo-octahedral, with the cyclopentadienyl ligand occupyingthree co-ordination sites, and piMe,(cp),] by the same tokenwould be eight-co-ordinate. There is no good a priori reason toexpect all of them to behave in the same way.We can note, inpassing, that piMe(cp)Cl,] is the only one of these moleculesin which a unique CH bond is in a position to interact with anappropriately oriented titanium orbital, and to this extent isperhaps the most likely compound to exhibit an agostic effect.The aoerage viSCH values for the various methyl titaniumcompounds show a progressive reduction as chlorine ligandsare replaced by methyl or cyclopentadienyl, the latter twoligands appearing to have rather similar effects, as far as can bejudged and bearing in mind the caveats mentioned above. Thetwo very different sets of results for niMe,(cp)], in the matrixand in the crystal, nevertheless yield very similar v'THaverages. In other series of methylmetal compounds [e.g.MMe, (M = Zn, Cd or Hg);5 MMe, (M = C, Si, Ge, Sn orPb);5 [MMe(CO),] (M = Mn or Re)*] we have shown thatv'TH is inversely related to the mean M-CH, bond energy,D M < H 3 .Very roughly, a fall of 1 cm-' in vi'CH correlates withan increase of 4.2 k 1.2 kJ mol-' in DMCH3. These results refer toisostructural compounds in which only the central metal atomis changed; we have no direct evidence to confirm that a similarrelationship exists between v'TH and DM<H3 for a given metalas the ligands in the co-ordination sphere are changed.Nevertheless it is not entirely unlikely that some suchrelationship will exist. If so, our results indicate that replacingthe chlorine ligands in the FiMeCl,] with methyl orcyclopentadienyl leads to a measurable strengthening of theTi-CH, bond.Table 14 includes v'TH frequencies, bond length and bondenergy data for r]TiEt(cp)Cl,], adjusted as before to notional'gas-phase' values.Data for ethane and ethyl halides are alsoincluded in the Table. There is no evidence in the spectra tosuggest the presence of more than one conformer of the ethylgroup: we observe only one band for 6,,,CH3 whereascompounds which contain two conformers (i.e. arising fromrotations around the M-C bond) typically display a clearly split6,,,CH, band, and two overtone^.^^ As already discussed, themethyl group has well defined local C, symmetry but there is noneed to assume that this indicates, or derives from, overall C,symmetry for the molecule; on the contrary, it is most unlikelythat the methyl CH bonds experience significant y-effects fromgroups beyond the titanium atom.We therefore draw noconclusions as to the ooerall symmetry of the molecule from ourobservations of the behaviour of the methyl group vibrations.Comparison of the v'"CH values for [Ti(CD,CHD,)(cp)Cl,]with v'"CH in ethane (2950 cm-') shows that the introductionof the p-titanium atom weakens all three methyl CH bonds, butparticularly CH', which is trans to the titanium atom. Whetherthis is a general effect in ethylmetal .systems, or one peculiarto titanium, remains to be seen. At the moment, ourunderstanding of the trans and gauche effects of substituents islimited to the effects of halogen and methyl substitution inhalogenoethanes and related molecules.373The most striking feature of the FiEt(cp)Cl,] spectra,however, arises from the analysis of the stretching vibrations ofthe methylene group. It is clear that the vCH, and vCD,frequencies cannot be reconciled with a 'normal' methylenegroup containing two equivalent CH bonds. Our predictedv'TH values, even allowing for the inevitable uncertainties inthe analysis, indicate a major difference in r,CH between thetwo bonds; both are relatively weak and one of them (viSCH =2857 crr-I), is unusually so.In a molecule with overall C, symmetry, both methylene CHbonds would be trans to chlorine atoms and there would beno reason to expect them to differ from one another. Bycomparison with the CH" bonds in FiMe(cp)Cl,], we wouldexpect the P C H values for the CH bonds in an unperturbedmethylene group to be found up to x42 cm-I lower, as a resultof the usual a-effect of a substituent methyl group.The shiftsrequired to produce the calculated v'TH frequencies of 2910and 2857 cm-' are 48 and 61 cm-', respectively. The formeris only marginally outside the range expected for an a-Meeffect, but the latter is substantially so. Given that the twomethylene CH bonds are inequivalent, it is difficult to escapethe conclusion that the ethyl ligand must be rotated around theTi-CH, bond in such a way that one methylene CH bondinteracts more strongly than the other with the metal atom.Such an interaction would also account for the observationof only one conformer of the compound. Alkyltitaniumcompounds have of course been a fertile source of postulatedagostic interactions, 1.2.32*33 involving both a- and P-CH bonds.While not all of these have survived closer e~amination,~*'~-'~some certainly have, and the surprising differences between themethyl groups in [TiMe,(cp)] in the matrix and in the crystal l4provide us with a timely reminder of the complexity ofalkyltitanium systems.Finally, we note that if the methylene group is not in factasymmetric, as we have inferred, the only other way in which wecould account for the vCH, and vCD, frequencies wouldbe by accepting a highly improbable stretch-stretch interactioJ.CHEM. soc. DALTON TRANS. 1995 3953constant f, or an HCH angle well outside the normallyacceptable range, or both. Such a result would in itself bean indication of a major anomaly in the methylene group andwould equally forcibly point to a strong interaction between themetal atom and the a-CH bonds.At this stage, we can onlyconclude that there are features of both the [TiMe(cp)CI,] and[TiEt(cp)Cl,] spectra which can be interpreted in terms of aweak CH Ti interaction involving the a-carbon atom, andthat this effect appears to be stronger in the ethyl than in themethyl compound. For both compounds, the need for ab initioand neutron diffraction studies is obvious.AcknowledgementsWe thank Dr. J. L. Duncan for advice on Fermi-resonancecorrections, Dr. G. S. McGrady for providing us with data on[TiMe,(cp),] and [TiMe,(cp)] in advance of publication andthe SERC for a research assistantship (A.J. H. R.) and for thepurchase of the spectrometer. D. C. M. thanks the ChemistryDepartment, University of Edinburgh, for hospitality andcomputing facilities.References1 M. Brookhart and M. L. H. Green, J. Organomet. 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