J . Am. Chem. SOC.1988, 110, 108-112
108
therefore, it is important to realize the importance of the difference between the type A and type B structures.
Acknowledgment. E.C. and Y.M. thank the Institut de Chimie Molikulaire d'Orsay (ICMO) for a grant that made possible part of this work and the CIRCE for providing computing facilities.
Work a t North Carolina State University was in part supported by the U S . Department of Energy, Office of Basic Energy Sciences, Division of Materials Science under Grant DE-FGOS86-ER45259. Registry No. ZrTe,, 39294-10-5.
Sic Agostic Interaction with Ti: Origin of Alkenyl Group Distortion in Ti( C( SiH2CH3)=CH2)X2+.An ab Initio MO Study Nobuaki Koga and Keiji Morokuma* Contribution from ihe Institute for Molecular Science, Myodaiji, Okazaki 444, Japan. Received June 29, 1987
Abstract: We have optimized the structure of Ti(C(SiH2CH3)=CH2)(C1)2t by an ab initio MO method and found a distorted alkenyl group with a small TiCSi bond angle, a long Sicy bond, and a short Ti-Cr distance, all of which are in good agreement with the experiment on Ti(C(Si(CH3)3)=C(C,H5)(CH,))(Cp)2t. Evidence has been found showing that the alkenyl group distortion is a consequence of the donative interaction from the CSi u bond to a Ti vacant d orbital, similar to the CH-M agostic interaction, and thus it is proposed that this SiC-Ti interaction is called the @Sicagostic interaction. It is suggested that there exist various types of agostic interaction between varieties of u bonds and the electron-deficient metal with varying structural distortion and stability depending on the donative bonds, metal, ligands, and coordination unsaturation.
Recently the first isolated intermediate Ti(C(Si(CH,),)=C(C6H5)(CH3))(Cp)2+(1) of a Ziegler catalyst system has been reported by Eisch et al.' in the reaction of C6H5C=CSi(CH3), with Cp2TiCl2and CH3AIC12. The X-ray structure of 1 shows an interesting distortion in the alkenyl group: a small TiCSi angle of 89' and a short distance between Ti and CY of 2.52 A. They
Relative Energy (in kcal/mol) of Optimized Structures and Assumed Structures with Undistorted Alkenvl G r o w optimized assumed structure structure difference Table I.
5s 5b
4.0
5c
14.0
0.0"
18.1
1Oa 10b
17.6
14.1 11.6
"The total energy is -2136.42895 hartrees. (dmpe)(Cl)? ( 2 ) or theoretically in Ti(C2H,)(C1)2(PH3),(H)5a (3) and Pd(C2H5)(H)(PH3)6(4). The MCC bond angles of the
1
have ascribed this distortion to the hyperconjugation between the C'Si CT bond and a Ti 4p vacant orbital. Although the electron deficiency of the central metal would surely create a driving force for distortion, it is not certain that such a hyperconjugation is operative. The C"Si bond, which is expected to be longer upon hyperconjugation, is found experimentally not much longer than the standard CSi single bond length: 1.853 A in vinylsilanea and 1.867 8, in methylsilane.2b This is in contrast with the hyperconjugation between a C"H bond and a metal vacant orbital found in the distorted carbene c o m p l e x e ~ . The ~ STO-3G calculations have given the long CQHbond of 1.13-1.18 A in the model titanium carbene complexes.3 On the contrary, the Sic7 bond length in 1 is longer by about 0.1 A. It may be noted that the distortion of alkenyl group is similar to that of ethyl groups found experimentally in Ti(C2Hs)(1) Eisch, J. J.; Piotrowski, A. M.; Brownstein, S. J.; Gabe, E. J.; Lee, F. L.J . Am. Chem. SOC.1985, 107, 7219. (2) (a) OReilly, J. M.; Pierce, L J . Chem. Phys. 1%1,34, 1176. (b) Kilb, R. W.; Pierce, L. Ibid. 1957, 27, 108. (3) Francl, M. M.; Pietro, W. J.; Hout, R. F., Jr.; Hehre, W. J. Organomerallics 1983, 2, 281.
.'IP-J 2
PH3 3
PH3
4
ethyl groups are about 90°, and their C& bonds are much longer (by 0.3-0.5 A) than the normal C H bond. These structural features are signs of the intramolecular CH-M interaction, called the agostic interaction.& The origin of the agostic interaction has been found to be the electron donative interaction from a C H u bond to a metal low-lying vacant orbitaL5x6 In this paper, we report theoretical evidence on the origin of the alkenyl group distortion in 1, which is similar to the agostic (4) (a) Dawoodi, Z.; Green, M. L. H.; Mtetwa, V. S. B.; Prout, K. J . Chem. SOC.,Chem. Commun. 1982,802. (b) Dawoodi, 2.; Green, M. L. H.; Mtetwa, V. S . B.; Prout, K. Ibid.1982, 1410. (c) Brookhart, M.; Green, M. L. H. J . Organomet. Chem. 1983, 250, 395. (5) (a) Koga, N.; Obara, S.; Morokuma, K. J . Am. Chem. SOC.1984,106, 4625. (b) Obara, S.; Koga, N.; Morokuma, K. J . Organomer. Chem. 1984, 270, C33. (6) Koga, N.; Obara, S.; Kitaura, K.; Morokuma, K. J . A m . Chem. SOC. 1985, 107, 7109.
0002-7863/88/1510-0108~01.SO/O 0 1988 American Chemical Society
J . Am. Chem. Soc.. Vol. 110. No. 1, 1988 109
S i c Agostic Interaction with Ti
\
ti
\ 1.076
H\ 1 077
1
116 2 133.6
H l05.L
H
1901 LlOS 7
Figure 1. Optimized geometries (in A and deg) of 5a, 5b, and 5c. The numbers in parentheses are corresponding experimental values for 1.'
interaction. We have determined with the a b initio MO method the structure of Ti(C(SiH2CH3)
