Schrock, Fellmann
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3359
Preparation and Characterization of M(CH2CMe3)3(CHCMe3)
Multiple Metal-Carbon Bonds. 8. a Preparation, Characterization, and Mechanism of Formation of the Tantalum and Niobium Neopentylidene Complexes, M( CH2CMe3)3( CHCMe3) Richard R. Schrock*lb and Jere D. Fellmann Contribution from the Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39. Received October 31, 1977
Abstract: The reaction between Ta(CHzCMe3)3CI2 and 2 mol of LiCH2CMe3 in pentane gives thermally stable Ta(CHzCMe3)3(CHCMe3) (1) in quantitative yield. The rate-determining step is believed to be formation of thermally unstable Ta(CH2CMe3)4CI (7) (which can be prepared from 1 and HCI at -78 “C). 7 reacts very rapidly with LiCH2CMe3 cornpared to the rate at which Ta(CHzCMe3)3CIz reacts. In the absence of LiCH2CMe3 7 decomposes to thermally unstable Ta(CH2CMe3)2(CI)(CHCMe3) (8) above ca. -10 “C (in noncoordinating solvents); 8 can be trapped as (inter alia) TaCp(CHzCMe3)2(CHCMe3). We postulate that 1 forms from 7 via 8 (path A) and directly from 7 by formal dehydrohalogenation, possibly via short-lived Ta(CH2CMe3)5 (path B). All postulates are supported by deuterium labeling results. A deuterium isotope effect for the a-hydrogen abstraction reaction leading to 1 was shown to be 2.7 f 0.2. Thermally unstable Nb(CHzCMe3)3(CHCMe3) (2) can be prepared similarly. The reactions of 1 and 2 are characteristic of the neopentylidene ligand being nucleophilic but the final products suggest that Ta and N b prefer to bind to elements more electronegative than carbon; e.g., 1 and CH3CN give E and Z isomers of Ta(CH2CMe3)3[N(CH3)C=CHCMe3](6a). All five-coordinate complexes of formally Nb(V) and Ta(V) are believed to be trigonal bipyramids with the most electronegative substituents in the axial positions. This structure and steric crowding are both believed to be important factors in determining when a-hydrogen atom abstraction to give alkylidene complexes occurs; we postulate that a relatively more nucleophilic axial alkyl a-carbon atom will remove a relatively more acidic proton from an equatorial alkyl a-carbon atom in a trigonal bipyramidal intermediate.
LiCl is removed by filtration, 1 can be isolated in quantitative Introduction yield by removing pentane in vacuo. Diethyl ether is also a Transition metal alkyl complexes in which the alkyl contains suitable solvent for the above reaction; the rate in this case is one to three @-hydrogenatoms often decompose fairly readily comparatively rapid ( T 9, variable; here T 9.15). A 2-mmol sample of 6a in ether was poured into aqueous N a O H . ' H N M R (C6D6): E isomer, 7 3.95 (br s, I ) , 7.62 (d, 3, J 2 Hz), Two organic products were identified by GLC/mass spectroscopy and 8.69 (s, 27), 8.72 (s, 9). Z isomer, T 5.23 (br s, I ) , 7.69 (d, 3, J = 2 Hz), by preparative GLC isolation and identification by IR and N M R . The 8.50 (s, 9), 8.79 (s, 27). Only one broad singlet assignable to neopentyl minor product was neopentyl alcohol (identical with an authentic methylene protons in either isomer was seen at T 8.63. IR (Nujol): sample), the major methyl neopentyl ketone [IH N M R in CDC13 T 1600 cm-I br, m ( UC=C). 7.65 (s, 2), 7.86 (s, 3), 8.95 (s, 9): IR 1710 c m - l s ( u c = ~ ) ] . 18. Preparation of Nb(CHzCMe3)3[0(Me)C=CHCMe3] (12). ' H N M R (C6D6): E isomer, T 4.36 (m, 1 , J x 1 Hz), 7.76 (d, 3, J LiCH2CMe3 (0.3 12 g, 4 mmol) in 25 mL of pentane was added to a = 1 Hz), 8.74 (s, 9), 8.79 (s, 27), 9.18 (s, 6). Z isomer, T 5.38 (q, I , 50-mL pentane solution of Nb(CH2CMe3)3C12 (0.754 g, 2 mmol) at J = I . l H z ) , 7 . 8 0 ( d , 3 , J = l . I H z ) , 8 . 5 5 ( ~ , 9 ) , 8 . 8 5 ( ~ , 2 7 ) , 8 . 9 8 ( ~ ,-78 "C followed by warming to 25 " C for 1 5 min. The solution con6). IR (Z-6a, Nujol): 1610 cm-l m (uc-c). taining Nb(CH2CMe3)3(CHCMe3) was cooled to -78 " C and 0.1 56 13. Preparation of Ta(CHzCMe3)3[N(Ph)C=CH2CMe3] (6b). An g of acetyl chloride added. Lithium chloride was filtered off and most orange solution of Ta(CH2CMe3)3(CHCMe3) (1.0 g in 20 mL of of the pentane removed in vacuo (volume = 5 mL). The product ether), on addition of a solution of 0.22 g of benzonitrile in 3 m L of (greenish-yellow needles) was filtered off after standing at -30 "C ether, turned yellow. Removing all solvent left a yellow oil whose IH overnight and identified by comparison of its H N M R and infrared N M R spectrum showed it to be 290% the Z isomer. Recrystallization spectrum (virtually identical) with that of Ta(CH3CMe3)3[O(Me)from acetonitrile followed by sublimation at 7 1 "C and 1 fi for 1 h yield C=CHCMe3] (3a). It is 1 9 5 % the E isomer. 0.40 g of nitrile-free Z-6a as a yellow oil. Hydrolysis with 1 N HCI ' H N M R ( C ~ D ~ ) : r 4 . 6 O ( p o o r q , l , J1=Hz),7.43(brs,6),8.15 gave two organic products, neopentyl alcohol and neopentyl phenyl (poor d, 3, J = I Hz), 8.77 (s, 27), 8.98 (s, 9). IR (Nujol): 1645 cm-1 ketone, according to GLC/mass spectroscopy. m (UC=C)). ' H N M R (C6Ds): Z isomer, 7 2.2 (m, 2, H ortho), 2.8 (m, 3, H 19. On the Reaction of Ta(CHzCMe3)3(CHCMe3)with HBF4 and meta and para), 4.87 (s, I), 8.42 (s, 9), 8.83 (s, 27) merged with 8.85 Ta(CHzCMe3)4CI with TIBF4. Attempts to Prepare [Ta(CH*C(s, 6). E isomer, olefinic proton at T 4.01. IR (neat oil): 1590 cm-' Me&]+BF4-. (a)Reaction of Ta(CHzCMe3)3(CHCMe3)with HBF4. Ta(CHzCMe3)3(CHCMe3) (1.5 g, 3.2 mmol) was dissolved in 20 mL m (uc=c). 14. Preparation of Ta(CH2CMe3)4CI(7). A solution of 0.93 g of of CH2C12 and 0.5 mL (2X excess) of HBF4:OMez (Aldrich) added Ta(CHzCMe3)3(CHCMe3) in 15 mL of pentane was cooled to -78 to the stirred solution at -78 "C. After 20 min it was warmed to room " C and 45 mL of anhydrous HCI added slowly by syringe. The solutemperature. The solvent was removed in vacuo and the residue subtion turned brilliant yellow and on standing (especially when more limed at 60-70 "C ( 1 p ) for 4 h to yield 0.4 g (29%) of concentrated) deposited yellow crystals of pure Ta(CH2CMe3)4CI Ta(CH>CMe3)3F2. in 1-2 h at -78 "C. Ta(CH2CMe3)&I decomposes at > - I O " C in (b) Reaction of Ta(CH2CMe3)4CIwith TIBF4. Ta(CH2CMe3)dCI aromatic or aliphatic hydrocarbons but is more stable in the presence ( I mmol) was generated as in 14 in 20 mL of Et2O. TIBF4 (0.29 g, 1 of acetonitrile (see text). The solid decomposes at 25 "C under nimmol) was added as a solid to the solution at -78 "C followed by 3 trogen. It can also be prepared in situ similarly in, for example, toluene m L of CH3CN. The solution was warmed to room temperature and or diethyl ether. stirred for 1 h. The solution was filtered to yield 0.21 g of a white solid (theory for TIC1 0.24 g) and the solution was stripped to a yellow solid. ' H N M R (toluene-d8, -20 "C): T 7.9 (br s, 2), 8.75 (br s, 9). The The solid was extracted with pentane and the mixture filtered. The ' H N M R spectrum is temperature dependent but this behavior has solvent was removed in vacuo and the residue sublimed at 60-70 "C not been investigated fully as yet. 13C N M R (toluene-dg, -20 "C, gated decoupled, 67.89 MHz): 144.6 (axial neopentyl C,, J C H = 105 ( 1 p ) for 4 h to yield 0.15 g (35%) of Ta(CHzCMe3)3F2. Hz), 116.9 (equatorial neopentyl C,, J C H = 112 Hz), 40.3 (axial Anal. Calcd for TaCl5H33F2: C , 41.67; H , 7.69. Found: C, 41.13; neopentyl Co), 35.2 (equatorial neopentyl C,, J C H = 123 Hz), 35.5 H, 7.91. ' H N M R (C6D6): T 8.26 (t. 6, J H ~ F 6.5 Hz), 8.87 (s, (equatorial neopentyl Co), 34.4 ppm (axial neopentyl C,, JCH = 121 27). Hz). 20. Reaction of Ta(CHzCMe3)4CI with Ph3P=CHz. 15. Trapping 8 as T ~ ( $ - C S H ~ ) ( C H ~ C M ~ ~ )(9). ~ ( C HTa(CHzCMe3)3(CHCMe3) CM~~) (0.46 g, 1 mmol) was dissolved in 50 m L Ta(CH2CMe3)4CI ( 1 rnmol) was generated as in 14 in 50 mL of tolof ether. Ta(CH2CMe3)dCI was generated by the addition of 0.78 rnL uene a t -78 "C. TIC5H5 (0.30 g, l l % excess) was added at -78 "C of a 2.1 M solution of HCI in ether at -78 "C; 0.28 g ( I mrnol) of
Schrock, Fellmann
Preparation and Characterization of M(CHzCMe3)3(CHCMe3)
3369
Table 111. tert-Butyl Ions in the Mass Spectra of Neopentanes" mle
55
56
57
58
Neopentane-do Neopentane-dl Neopentaned2 N eopentaned3 Neopentaned3 (96.8%) Neopentane-d2 (97.6%) Neopentane-dl (96.7%) Neopentaned3 (hexane) Expt A
3.01b 1.38
4.17? 4.48 2.14 1.39
100.00 35.07 34.60 32.66 32.2 33.3 38.4 33.8 76.6
0.13 100.00 3.29 3.30 2.4 5.7 100.00 3.3 10.9
1 .oo 1 .oo
4.1
59 0.13 100.00 2.71 6.0 100.00 0.1 17.7 100.00
60
0.13 100.00 100.00 0.3 100.00 12.5
61
Ref
0.13 0.3
43,44d Calcd Calcd Calcd e
f g
0.1
Table I Table I1
All corrected for 13C. An average of 2.8544 and 3.15,43corrected. An average of 4.3844 and 4.20,43corrected. m / e 54 (0.22) and 59 (0.07) are considered negligible. e Prepared by treating LiCDzCMe3 with excess CH30D. ./ Prepared by treating LiCDzCMe3 with excess C H 3 0 H . g Prepared by treating LiCHzCMe3 with excess C H 3 0 D .
Table IV. Results of the Kinetic ExDeriments Trial"
Time. minb
Filtering time. minC
mmol LiCld
1
33.1 23.8 43. I 65.5 35.7 23.6 11.5
6.2 7.7 6.3 11.0 11.3 7.2 3.0
1.11 0.82 1.39 1.94 1.31 1.31 0.99
2 3 4 5 6 7
" Runs 1-6 are Ta(CH2CMe3)3CI2 and 2LiCH2CMe3. Run 7 is Ta(CH2CMe3)4CI and LiCH2CMe3. Time is the average time between the start and the end of the filtration. Filtration time varied from run to run and probably is the greatest source of error. LiCl titrated with 35.4 f 0.1 mM AgN03. Ph3P=CHz dissolved in 20 m L of ether was added dropwise at -78 OC. After stirring for 2 h at -78 "C it was warmed to room temperature. The brown solution was filtered leaving a light brown solid (0.22 g, theory for Ph3P+MeCI- 0.3 1 g) identified as Ph3P+MeC1- by IR comparison with an authentic sample. The filtrate's solvent was removed in vacuo and the tarry residue was extracted with 3 X 10 m L of pentane. The pentane was removed to yield 0.35 g of Ta(CHzCMe3)3(CHCMe3) (76%), identified by IH N M R . 21. Determination of Neopentane-d, Mix By Mass Spectroscopy. The cracking pattern of neopentane is well known. The parent ion is too weak to be observed and the highest molecular weight ion is due to C4H9+. The tert-butyl ion appears to be stable with no evidence for carbon skeletal r e g r o ~ p i n gWe . ~ ~have assumed during all of these studies that neopentane-dz is solely DzHCC(CH3)3, neopentaned3 solely D3CC(CH3)3, etc. The spectrum for n e ~ p e n t a n eis~shown ~ . ~ ~in Table 111. The spectrum of neopentane-dl, -dz, and -d3 calculated from this data (assuming no isotope effect for C-D vs. C-H cleavage and a 57 peak 32% the size of the higher molecular weight fragment; cf. 76% CMe3(13CH3), 24% CMe3 from Me3C(i3CH3)42)is normalized to 100.0 for m/e 58, 59, and 60, respectively. The spectra for experimental samples of neopentane-di, -dz, and -d3 are similar; clearly there is not significant H / D scrambling. The isotopic purity is calculated to be approximately 97% in each case. The errors are clearly greatest for do, which is obtained by difference; in this case they amount to less than &3%; this is a reasonable experimental error which we have assumed throughout this study. The isotopic distribution in the neopentanes obtained from the reaction of Ta(CDzCMe3)3C12 and 2LiCD2CMe3 (Table I) is calculated similarly (e.g., Table 111, eighth entry). More complex mixtures are not significantly more difficult to evaluate (see last entry, Table 11). One assumes that the highest mass peak in the 13C corrected data set corresponds to the amount of that isotope (arbitrary units, unnormalized), then subtracts the appropriate theoretical amount from all lower mass peak totals; three such operations give the amounts of neopentaned3, -dz, and - d l .The amount remaining in the m/e 57 column is due to neopentane-do; however, this must be normalized by multiplying by 0.75 since an m/e 57 fragment from neopentane-do is generated four times out of four but neopentane-dl, -dz, or -d3 gives m/e 58, 59, and 60 fragments, respectively, only three times out of four. For example, the last entry in Table 111
reduces to 22.9 (57), 6.3 ( 5 8 ) , 100.0 (59), and 12.5 (60) or 16% do, 4% d l , 71% d l , and 9% d3 (Table 11). The mass spectrum of neopentane was determined by GC/mass spectroscopy under conditions where the G C peak was sharp. Therefore, differently labeled neopentanes did not separate to any significant extent. In several experiments the labeled neopentane was collected and examined by conventional mass spectroscopy on a different spectrometer; the results agreed within &2%. 22. Rate of Reaction of Ta(CHzCMe3)3ClZ vs. Ta(CH2CMe3)4CI with LiCHzCMe3 in Heptane. The heptane used in these experiments was washed with 5% HNO3 in H2S04, rinsed with water, distilled, dried over 4A series, and purged with nitrogen. A g N 0 3 (Mallinckrodt) and Bacteriological Dextrin (Eastman) were used as received. Reagent grade KCI (Baker) was dried at 150 "C for 24 h and stored in a desiccator. Anhydrous LiCl was obtained from the reaction of Ta(CHzCMe3)3CI2 and LiCHzCMe3 in pentane. Following the procedure45 for halide analyses (adsorption method) a stock solution of AgNO3 was prepared using both LiCl and KCI as primary standards. All titrations were buffered with 10 drops of 1 M acetate buffer (pH 5.5). Dichlorofluorescein in ethyl alcohol was the adsorption indicator. Ta(CH2CMe3)$12 (0.465 g, 1 mmol) was dissolved in 30 mL of heptane in a three-necked 100-mL flask fitted with a N2 inlet, dropping funnel, and a fine fritted Schlenck filter. LiCHzCMe3 (0.160 g) was dissolved in 20 mL of heptane and placed in the dropping funnel. The reaction vessel was immersed in an insulated water bath maintained at 22 "C. The neopentyllithium solution was added rapidly and after the required time interval the solution was filtered. The isolated LiCl was washed once with 20 mL of pentane and dried in vacuo. The LiCl was dissolved in 50 m L of H2O and titrated with standardized A g N 0 3 using the above procedure. The results are outlined in Table IV (runs 1-6). Ta(CHzCMe3)3(CHCMe,) (1,0.465 g, 1.0 mmol) was dissolved in 20 mL of ether and the solution cooled to -78 "C; 0.78 mL of a 1.29 m solution of HCI in ether was added to the stirred solution. After 20 min all volatiles were removed in vacuo and the residue dissolved in 20 mL of heptane. After warming to -20 "C a solution of 80 mg of LiCH2CMe3 in 30 mL of heptane at 25 OC was added rapidly as the vessel was placed in the bath (at 23 OC; final T 22 "C). Upon addition of the LiCHzCMe3 the solution became cloudly immediately and did not change throughout the run. The LiCl was isolated and titrated using the above procedures. The result is listed in Table IV (run 7).
Acknowledgment. We thank the National Science Foundation for financial support and the Francis N. Bitter National Magnet Laboratory for use of their high-field N M R facilities. We also thank D. Seyferth for helpful discussions and G. Whitesides for u s e of his GC/mass spectral facilities. References and Notes (1) (a) Part 7: R . R. Schrock and P. R. Sharp, J. Am. Chem. Soc., 100,2389 (1978); (b) Alfred P. Sloan Foundation Fellow, 1976-1978. (2) (a) R . R. Schrock and G. W. Parshall, Chem. Rev., 76, 243 (1976), and references cited therein; (b) P. J. Davidson, M. F. Lappert, and R. Pearce, Chem. Rev., 76, 219 (1976), and references cited therein. (3) By analogy with "&hydride elimination", this description implies that an a-hydrogen atom migrates to the metal to give a complex containing a hydride and an alkylidene ligand. In contrast, the term "a-hydrogen atom
3370
Journal of the American Chemical Society / 1OO:l I / May 24, 1978
abstraction" has been used to describe a situation in which the ahydrogen atom could be removed by some other atom, e.g., the a-carbon atom of another alkyl ligand. These two processes are virtually indistinguishable in all cases known to date. (4) N. J. Cooper and M. L. H. &een, J. Chem. Soc.,Chem. Commun., 208,761 (1974). (5) A primary alkylidene (derived from a primary alkyl) is defined to be a monosubstituted methylene ligand. Methylene itself (cf. methyl) is a unique member of this family. A secondary alkylidene is a disubstituted methylene ligand. (6) R. R. Schrock, J. Am. Chem. SOC.,96, 6796 (1974). 96, 5399 (1976). (7) R. R. Schrock, J. Am. Chem. SOC., (8) Unpublished observations. (9) S.J. McLain, C. D. Wood, and R. R. Schrock, J. Am. Cbem. SOC.,99,3519 (1977). (10) R. R. Schrock, J. Am. Chem. SOC.,97, 6577 (1975). ( 1 1) The postulate that the neopentylidene a carbon is K bonded to the metal so far rests on x-ray structures of other alkylidene complexes such as T ~ C P ~ C H Z X C HTaCpz(CHPhMCH2Ph), ~),~~ and TaCpn(CHCMe3)CI.13b Crystals of I were well formed but did not diffract well. (12) L. J. Guggenberger and R. R. Schrock. J. Am. Chem. SOC., 97, 6578 (1975). (13) (a) Part 9: R. R. Schrock, C. D. Wood, L. W. Messerle, and L. J. Guggenberger, J. Am. Chem. SOC.,in press. ( b ) M.R. Churchill, f . J. Hollander, andR. R. Schrock, ibid., 100, 647 (1978). (14) M. H. Chisholm and S.Gcdleski, Prog. lnorg. Chem., 20, 299 (1976). (15) D. M. Graham and C. E. Hoiloway, Can. J. Chem., 41, 2114 (1963). (16) V. M. S.Gil and C. F. G. C. Geraldes in "Nuclear Magnetic Resonance Spectroscopy of Nuclei Other Than Protons", T. Axenrod and G. A. Webb, Ed., Wiley, New York, N.Y., 1974, Chapter 14. 97, 2935 (17) L. J. Guggenberger and R. R. Schrock, J. Am. Chem. SOC., (1975). (18) For olefins of the type Me3CCH=CR1R2 where R1 = Hand R2 = CH3 (2 isomer) 'JCH= 149.95 Hz; 'JCH = 147.38 for the €isomer: P. P. Nicholas, C. J. Carman, A. R. Tarpley, Jr., and J. H. Goldstein, J. Phys. Chem., 76, 2877 (1972). (19) (a) R. M. Silverstein, G. C. Bassler, and T. C. Morrill, "Spectroscopic Identification of Organic Compounds", 3rd ed, Wiley, New York, N.Y., 1974. (b) The U C intensity ~ is greater when a nitrogen atom is attached to the olefinic carbon atom (as in enamines: cf. vinyl etherslSa). (20) R. R. Schrock, J. Organomet. Chem., 122, 209 (1976). (21) H. Schmidbaur and W. Tronich, Chem.Ber., 101,604 (1968). We assumed Au, = 120 Hz and J = 13 Hz at 373 K (T,) to givez2 kc = 276 s-l, and, by the Eyring equation, A e 3 , 3 = 17.9 kcal mol-'. (22) D. Kost. E. H. Carlson, and M. Raban, Chem. Commun., 656 (1971). (23) H. Schmidbaur, private communication. (24) Only Vycor flasks, flamed out and evacuated, gave reproducible results (see Experimental Section). Pyrex flasks, even those flamed out similarly, gave up to 3 0 % neopentane-d2.Neopentane-d2was not found in a blank run (omitting Ta(CD2CMe3)3CI2)in toluene-d8 or hexane which had been passed through activated alumina. (25) The relative rates of abstracting H from hexane vs. D from C~HSCDJ are available for the methyl radical. At 25 OC the rate of abstracting H from
hexane is ca. 50 times the rate of abstracting D' from C6H5CD3.26 The relative rates of abstraction by the neopentyl radical should not differ greatly from those for the methyl radical. (26) A. F. Trotman-Dickenson and G. S. Milne, "Tables of Bimolecular Gas Reactions", National Bureau of Standards, US. Government Printing Office, 1967. (27) The major species in solution, assuming that 97% of the neopentyl groups are CD2CMe3 and 3 % CDHCMe3, would be Ta(CD2CMe& (85.9%), Ta(CD&Me3)4(CHoCMe3)(13.3%), and T ~ ( C D Z C M ~ ~ ) ~ ( C H (0.8 D C %). M~~)~ The first gives all neopentane-d3, the second a mixture of 5 6 % -4and 4 4 % -d2, and the third a mixture of 30% -4,58% -d2, and 12% -dl, all assuming kn/kD = 3.28 (28) k ~ l = k 8~ f 1 for formation of butane in U(q5-C5H5)(q5-C5D5)2((+H9!zs while kHlk0 = 3.4 f 0.3 for deprotonation of [TaCp2Me2] with M e 3 M H 2 . 1 ak ~ l for k ~abstraction of H or D from solvent (e.g., hexane) by CH3' is often about 6.30 (29) T. J. Marks, Acc. Chem. Res., 9, 223 (1976). (30) J. Evans, S.J. Okrasinski, A. J. Pribula. and J. R. Norton, J. Am. Chem. SOC., 99, 5835 (1977). (31) (a) G. 0. Doak and L. D. Freedman, "Organometallic Compounds of Arsenic, Antimony, and Bismuth", Wiley-lnterscience, New York, N.Y., 1970; (b) R. Hoffmann, J. M. Howell, and E. L. Muetterties, J. Am. Chem. Soc., 94, 3047 (1972). and references cited therein; (c) A. R. Rossi and R. Hoffmann, lnofg. Chem., 14, 365 (1975), references cited therein; (d) H. Schmidbaur, Adv. Organomet. Chem., 14, 205 (1976). (32) (a) H. C. Brown, J. Chem. SOC.,1248 (1956); (b) D. Seyferth and G. Singh, J. Am. Chem. SOC.,87,4156 (1965). (33) D. Seyferth. W. B. Hughes, and J. K. Heeren, J. Am. Chem. SOC.,87,2847 (1965). (34) The a, values employed at this stage were obtained using known Ta-ligand distances and angles or reasonable guesses. They are q5-C5H5(1327, CI (97O), $45Me5(178'), CH2CMe3(117'), =CHCMe3(117'), CH2Ph(10I0), and =CHPh (108'). (35) R. B. Calvert and J. R. Shapley, J. Am. Chem. SOC.,99, 5225 (1977). (36) D. Seyferth, W. B. Hughes, and J. K. Heeren, J. Am. Chem. SOC.,87,3467 (1965). (37) In general, an accurate determination of k H / k D for a rate-determining aabstraction step has so far been difficult since if the precursor to an alkylidene complex is stable, the alkylidene complex is not under the conditions needed to cause abstraction, and vice versa. (38) C. P. Casey, T. J. Burkhardt, C. A. Bunnell, and J. C. Calabrese, J. Am. Chem. Soc., 99, 2127 (1977). (39) M. Brookhart and G. 0. Nelson, J. Am. Chem. SOC., 99,6099 (1977). 140) . . G. A. Wilev. B. M. Rein. and R. L. Hershkowitz. Tetrahedron Lett.. No. 36. 2509 (1964). (41) S. W. Benson, "The Foundations of Chemical Kinetics", McGraw-Hill. New York, N.Y.. 1960, p 27. (42) A. Langer and C. P. Johnson, J. Phys. Chem., 61, 891 (1957). (43) C. P. Johnson and A. Langer, J. Phys. Chem., 61, 1010 (1957). (44) API "Selected Mass Spectral Data", Vol. 1. Texas A 8 M University, Serial No. 8. (45) J. S. Fritz and G. A. Shenk, Jr., in "Quantitative Analytical Chemistry", 2nd ed, Allyn and Bacon, Boston, Mass., 1969, pp 205-208, 534-537.
