7] Crystal structure analyses. colorless crystals of 3a and 15 were investieated on a lour-circle diffractometcr (CAD4, Enraf-Nonius) at -80 'c' with Cu,, radiation ( 2 =154.178 pm). 3a: C,,H,,B,Li,SiZ.2THF. crystal dimensions 0 6 x 0.3 x 0.2 inm'. monoclinic. space group P2,c. Z = 4. (I = 9 7 2 3 2 ) . h = 1173.5(2). <' = 2890.7(6) pm. / I = 98.45(3) . Vr7 = 3262.1 x lO-"m', pca,Ld = 0.99X gcm -'; 3994 measured reflections in the range up to 20 = 110 (w scans). 3903 of which +ere independent and were used for the further calculati~ns alter Loreiitz and polarization correction: no absorpt~on correction 111 = I 1 Ocm-') The structure was solved by direct methods and refined against the ti: data xith full matrix. A disorder showing up in the difference Fourlei-5yntheses was accounted for bq a "split atom model" where a C 5 -SiMe, group superposes the B3-iBu unit at 8 3 and. vice versa. a B3-/Bu group superposes the C5-SiMe3 unit at C 5 . Due to these superpositions. geometrrc restraints olihe B C (160 pm) and Si-C (187 pm) bond length wcrc used during refinement. and the temperature factors of atoms approaching each other were defined to be equal. In this way good conbergence and a maximal residual electron density 0.35 e k ' were achieved. The distribution of the alternative arrangements of the molecules was determincd to be 59.3(4) aiid 40.7%). The H a t o m u'ere localized iii part by dillerence Fourier syntheses: the others were calculated; they were included in the calculation riding on their bonding partners and with isotropic temperature factors coinmon by gsoups. Anisotropic teinpcratui-e factors %ese used foi-all the remaining atom& except C 7 . wRi = 0.2066 (SHELX 93 program). corresponding to R = 0.069 for 2951 reflections with &>4u(F,>). inanimal residual electron density 0.33 e k ' . -15: C,,H,,B,Si,, crystal dimcnsions 0.7 x 0.3 x 0.2 n1m3, monoclinic. space group P2,:n. Z = 4. ci = 928.9(2). h = 1655,2(3). c = 2154.4(4) pin. /) = 92.80(3) , VF7 = 3308.5 x I O -" in'. pc,,L6 = 1.009 gcni-'; 4461 reflections iii the range up to 211 =110 ( ( , I scans), 4157 of wliicli were independent 41 54 reflections +ere used foi-further calculations after Lorentr aiid polarizatioii correction. the structure solutioii and refinement a s for 3 a led to wR,=0.1955, corresponding to R = 0.064 for 2123 reflections uith ti) > 40 (&,). and ;I maximal residual electron densit) of 0 34 e k ' . Further details of the crystal structure investigations inay be obtained from the Fachiiiformationsreiitrum Karlsruhe. D-76344 Eegenstein-Leopoldslitifeii (FRG).