Synthesis and photoisomerization of divinyltetramethyldisiloxane and divinyltetramethyldisilazane complexes of (eta 5-C5R5)Rh (R=H, Me). Crystal and molecular structure of (n(5)-C(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 divinyldisiloxane 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 coordinated through the vinyl groups as for a diene. The structure of (eta(5)-C(5)Me(5))Rh[eta(4)-(CH2=CHSiMe(2))(2)O] was established crystallographically. The two coordinated double bonds are arranged with their Si substitutents in a cis orientation with respect to the metal. The strain in the structure is suggested by the scissor geometry of the vinyl C-C bonds relative to the Cg plane of the ring, combined with unequal 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) results in the formation of the complexes of type 1, together with an isomer in which the substituents on the coordinated C=C bonds are oriented trans at rhodium (2) and species containing two dangling divinyldisiloxane ligands, (eta(5)-C(5)R(5))Rh[eta(2)-(CH2=CHSiMe(2))(2)X](2), in both cis and trans configurations. Direct irradiation of species of type 1 converts them to the photoisomer 2. The 16-electron intermediate with 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 isomerization of type 2 species to type 1 species proceeds by first-order kinetics. For trans-(<eta(5)-C5H5)Rh[eta(4)-(CH2=CHSiMe(2))(2)O] (2a) the values 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-determining dissociation of one double bond. Replacement of C5H5 by C(5)Me(5) increases H-double dagger by 9 +/- 2 kJ mol(-1) The assignment of the photoisomers 2 as the trans species is supported by further observations that (i) a mixture of 1 and 2 can be synthesized by hydrolysis of chlorovinylsilane 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 conformationally locked species with a cyclic trivinyltrimethyltrisiloxane ligand, (eta(5)-C5H5)Rh[eta(4)-(CH2=CHSi(Me)O)(3)], does not undergo photoisomerization, and (iii) the species with the Si-O-Si linkage replaced by a C-O-Si linkage (eta(5)-C5H5)Rh[eta(4)-(CH2=CMe)CH(2)OSiMe(2)(CH=CH2)] also fails to undergo photoisomerization. These complexes represent the first examples of adoption of a trans configuration at a metal by eta(4)-divinyldisiloxane ligands and the first examples of this type of isomerization.