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Metallation of (2-CH2RC6H4)2, with excess Li(tmen) Bun (tmen = NNN'N'-tetramethylethylenediamine) yields the crystalline compounds [{Li(tmen)}2{(2-CHRC6H4)2}] [R = H (7) or SiMe, (9)] whereas for 2-methyl-2'-trimethylsilylmethylbiphenyl, although selectivity is maintained, the crystalline product is a monolithiated species. This difference has been rationalised on the basis of the results of the crystal structure determination of the lithium complexes ( 7) and ( 9). The structure of ( 7) is polymeric (cf. dimeric in benzene by ebullioscopy) with a ' metallacycle ' lithium and a lithium bridging methylene groups of different ' metallacycles ' (electron-deficient bonding) : Li-C (mean) 2.30 A ; Li-C-Li (mean) 159.5, C-Li-C (mean) 106.5". Compound (9) is best described as a ' contact ion-pair ' structure; Li-C(H)SiMe3 contacts, 2.22(1) and 2.1 9(2) A. Lithiation of 2-[bis(trimethylsilyl)methyl]-2'-methylbiphenyl results in the formation of (9), via an anionic 1,6-silyl migration.

The crystal structure of tetraphenyltin(1v) has been redetermined at 295(1) K, full-matrix least-squares refinement yielding a residual of 0.031 for 557 independent 'observed' reflections. The tin-phenyl carbon distance is 2.143(5) A, with the angles subtended at the tin atom being 108.9(2) and 110.5(2)".In the course of a series of structural studies on tin(~v)/alkyl systems in progress in this laboratory, it became apparent that the standard tin(~v)-phenyl carbon distances available for tetraphenyltin(~v), being derived from film or limited diffractometer data, were of limited precision and inappropriate for comparative purposes in the present context. Accordingly, a redetermination of the crystal structure of tetraphenyltin(1v) has been carried out at 295 K; although the results differ only trivially from those previously r e p~r t e d , "~ they offer a significant improvement in precision. As described previously, the molecule has 3 symmetry, with only one phenyl ring being crystallographically independent. We find the carbon atom skeleton of the ring to be planar (a 0.004 A, X 2 1.15), with distances lying in the range 1.38(1)-1.41(1) and angles within the ring 11 8.5(5)-121.2(6)" ; Sn-C-C angles are 121 .7(4), 119.8(4)". About the tin atom, Sn-C is 2.143(5) A ; the two independent C-Sn-C angles are 110.5(2)" (about the 3 axis) and 108.9(2)". Structure determination.-A unique data set was measured to 2&,, = 60". A Syntex P 2, fourcircle diffractometer was used in conventional 2010 scan mode yielding 814 independent reflections, 557 of which with I > 3a(I) were considered 'observed' and used in the full-matrix least-squares refinement after analytical absorption correction. For the non-hydrogen atoms, (x, y, z, Uij (anisotropic)) were refined; for the hydrogen atoms (x, y, z, U(isotropic)) were refined. At convergence residuals were (R, R') 0.031, 0.038, reflection weights being (a2(Fo) + 0.0005(F0)2)-'. Neutral atom scattering factors were used, those from the non-hydrogen atoms being corrected for anomalous ' Chieh, P. C., and Trotter, J., J. Chem. Soc. A, 1970, 912. Akhmed, N. A., and Aleksandrov, G. G., Zh. Strukt. Khim., 1970, 11, 891.