SYNTHESIS AND PHOTOISOMERIZATION OF DIVINYLTETRAMETHYLDISILOXANE AND DIVINYLTETRAMETHYLDISILAZANE COMPLEXES OF (ETA-5-C5R5)RH (R=H, ME) - CRYSTAL AND MOLECULAR-STRUCTURE OF 5)ME(5))RH[ETA(4)-(CH2=CH)ME(2)SIOSIME(2)(CH=CH2)]

The synthesis and photochemistry of a series of new rhodium complexes of 1,3-divinyl-1,1,3,3-tetramethyldisiloxane, (CH2=CHSiMe(2))(2)O, and its disilazane analogue (CH2=CHSiMe(2))(2)-NH are reported. The divin yldisiloxane or divinyldisilazane ligands in (eta(5)-C(5)R(5))Rh[eta(4 )-(CH2=CHSiMe(2))(2)X] (R = H, Me; X = O, NH; type 1 complexes) are co ordinated through the vinyl groups as for a diene. The structure of (e ta(5)-C(5)Me(5))Rh[eta(4)-(CH2=CHSiMe(2))(2)O] was established crystal lographically. The two coordinated double bonds are arranged with thei r Si substitutents in a cis orientation with respect to the metal. The strain in the structure is suggested by the scissor geometry of the v inyl C-C bonds relative to the Cg plane of the ring, combined with une qual Rh-C and C=C bond lengths. Photolysis of (eta(5)-C(5)R(5))Rh(C2H4 )(2) (R = H, Me) in the presence of (CH2=CHSiMe(2))(2)X (X O, NH) resu lts in the formation of the complexes of type 1, together with an isom er in which the substituents on the coordinated C=C bonds are oriented trans at rhodium (2) and species containing two dangling divinyldisil oxane ligands, eta(5)-C(5)R(5))Rh[eta(2)-(CH2=CHSiMe(2))(2)X](2), in b oth cis and trans configurations. Direct irradiation of species of typ e 1 converts them to the photoisomer 2. The 16-electron intermediate w ith one eta(2)-divinyldisiloxane ligand may be trapped by addition of CH2=CHSiMe(3) or excess (CH2=CHSiMe(2))(2)X (X = O, NH). The isomeriza tion of type 2 species to type 1 species proceeds by first-order kinet ics. For trans-(<eta(5)-C5H5)Rh[eta(4)-(CH2=CHSiMe(2))(2)O] (2a) the v alues of Delta H-double dagger and Delta S-double dagger are 114 +/- 1 kJ mol(-1) and 41 +/- 4 J mol(-1) K-l, consistent with rate-determini ng dissociation of one double bond. Replacement of C5H5 by C(5)Me(5) i ncreases H-double dagger by 9 +/- 2 kJ mol(-1) The assignment of the p hotoisomers 2 as the trans species is supported by further observation s that (i) a mixture of 1 and 2 can be synthesized by hydrolysis of ch lorovinylsilane precursors (eta(5)-C5H5)Rh(eta(2)-CH2=CHSiMe(2)Cl)(2), a method which generates the Si-O-Si linkage in situ, (ii) the confor mationally locked species with a cyclic trivinyltrimethyltrisiloxane l igand, (eta(5)-C5H5)Rh[eta(4)-(CH2=CHSi(Me)O)(3)], does not undergo ph otoisomerization, and (iii) the species with the Si-O-Si linkage repla ced by a C-O-Si linkage (5)-C5H5)Rh[eta(4)-(CH2=CMe)CH(2)OSiMe(2)(CH=C H2)] also fails to undergo photoisomerization. These complexes represe nt the first examples of adoption of a trans configuration at a metal by eta(4)-divinyldisiloxane ligands and the first examples of this typ e of isomerization.