SYNTHESIS AND CRYSTAL-STRUCTURE OF [CO(CO)4(H7SI8O12)] - A NEW-TYPE OF MONOSUBSTITUTED OCTANUCLEAR SILASESQUIOXANE WITH A SILICON COBALT BOND

Reaction of octacarbonyldicobalt [Co2(CO)8] with octahydrosilasesquiox ane H8Si8O12 in toluene leads to [Co(CO)4(H7Si8O12)], a new monosubsti tuted, octanuclear silasesquioxane with a silicon-cobalt bond. The pro duct has been analysed by H-1, C-13, Si-29 NMR and IR spectroscopies, mass spectrometry and microanalysis. The crystal structure of [Co(CO)4 (H7Si8O12)] has been determined by single-crystal X-ray diffraction an d is compared with the structures of H8Si8O12, [Co(CO)4(SiCl3)] and [C o(CO)4(SiF3)]. The H7Si8O12 fragment exhibits structural distortions c onsistently around, and originating from the Si atom to which the Co(C O)4 fragment binds. The remaining part of the cage essentially retains C3 symmetry, but is, nevertheless, in principle closer to the ideal O (h) symmetry for this type of cage, than is crystalline H8Si8O12, for which a non-crystallographic molecular symmetry of T(h) is present. Th e comparison of the structures of the Co(CO)4 fragments shows that the se are consistently distorted and deviate from ideal C3v symmetry. The distortions are essentially of the same kind in the three compounds a nd differ only in magnitude. The Si-Co distance in [Co(CO)4(H7Si8O12)] is 2.285 angstrom. The experimental findings have been compared with extended-Huckel molecular orbital calculations. Interaction between th e Co(d(z2)) and the Si(p(z)) orbitals leads to the bond between the tw o fragments. Overlap population analysis indicates small but significa nt bonding interaction between Si and C(eq) and antibonding interactio n between the Co and the nearest O(Si). Fractional molecular orbital a nalysis indicates that the electronic structure of [Co(CO)4(H7Si8O12)] consists of three parts: orbitals belonging only to one of the two fr agments and orbitals shared by both fragments. The highest-occupied mo lecular orbital (HOMO) of [Co(CO)4(H7Si8O12)] consists of oxygen lone pairs localised on H7Si8O12. The lowest-unoccupied molecular orbital ( LUMO) is identical with the LUMO of Co(CO)4 and the first electronic t ransitions observed in the near UV are of H7Si8O12 (oxygen lone pair) to Co(CO)4 fragment charge-transfer type.