THEORETICAL-STUDY OF THE NATURE OF THE BONDING IN [CP2M(MU-X)]2, WHERE M = ZR (X = I, PH2, AND NH) AND M = TI (X = CL)

We have completed Hartree-Fock-Slater quantum chemical studies on the title molecules in order to examine the nature of the metal-metal and metal-ligand bonding in these complexes. Our results indicate that the highest occupied molecular orbital produces a Zr(III)-Zr(III) bond for X = I, PH2. This contradicts the interpretation of the X-ray structure: namely, that there is no Zr-Zr bond since the intermetallic distance of approximately 3.65 angstrom is 0.4-0.5 angstrom longer than for ''typical' Zr-Zr single bonds. But for a number of reasons we are loathe to give this bond the status of a ''single'' bond and instead assign it a bond order of about 0.5. The Zr-Zr distance for the imido complex is approximately 3.20 angstrom, even though it is formally Zr(IV) and there can be no formal Zr-Zr bond. Our results indicate that the Zr-Zr distance results from the interplay between the Zr-X bridge bonds, the metal-metal bond, and the steric interaction between the four Cp rings. The last point is corroborated by selected molecular mechanics calculations on various configurations of the four Cp ligands. Studies on model dication and dianion systems illustrate how the Cp rings function as electron ''buffers'' by keeping the effective electronic charge in the M2X2 portion of the molecule roughly constant. The magnetic properties and the absence of a Ti(III)-Ti(III) bond for the Ti compound are briefly discussed.