ABINITIO CONFIGURATION-INTERACTION STUDY OF THE METAL METAL COUPLING IN SOME ZR(III) AND TI(III) DIMERS - EVIDENCE FOR A SUPERLONG ZR-ZR BOND IN [CP2ZR(MU-PR2)]2 AND RELATED COMPLEXES

The metal-metal distance in [Cp2Zr(mu-PH2)]2 (1) and [Cp2Ti(mu-Cl)]2 (2) has been optimized by means of ab initio configuration interaction (CI) calculations, all other distances being kept constant at their experimental values. The computed equilibrium distances are d(Zr-Zr) = 3.77 angstrom and d(Ti-Ti) = 3.965 angstrom, in good agreement with the observed structures. The singlet-triplet (S-T) splitting computed for 1 at the observed geometry is 10 065 cm-1. The computed S-T splitting of the Zr(III) dimer steadily decreases as the Zr-Zr distance increases, thus supporting the interpretation of the diamagnetism of 1 in terms of a through-space metal-metal coupling. In the hypothetical, unsupported Zr(III) dimer [CP2Zr]22+, the S-T splitting is computed to be close to 14000 cm-1 at d(Zr-Zr) = 3.65 angstrom, thus ruling out the hypothesis of a superexchange origin for the observed diamagnetism. The Zr-Zr distances of approximately 3.6 angstrom observed in [Cp2Zr(mu-X)]2 dimers (X = I, PR2) should be interpreted as a compromise between the steric repulsion originating in the Cp rings and the restoring force associated with the metal-metal bond. The computed S-T splitting of the titanium dimer 2 appears much lower, due to the relatively weak 3d-3d overlap. Beyond 3.3-3.4 angstrom, the Ti-Ti interaction can be assimilated to an antiferromagnetic coupling, in agreement with the reported magnetic behavior of the [Cp2Ti(mu-X)]2 complexes. The weakness of the Ti-Ti coupling explains why the metal-metal distances reported for those Ti(III) dimers are consistently longer than those reported for equivalent Zr(III) complexes. Direct metal-metal bonds can exist, however, in Ti(III) dimers lacking steric strain. The case of [Cp(mu-eta-1:eta-5-C5H4)Ti(PMe3)]2 (d(Ti-Ti) = 3.223 angstrom) has been investigated using ab initio CI calculations. The dimer is described in terms of two Cp2ML3 fragments coupled by means of a bent metal-metal bond. In spite of the bond bending (alpha = approximately 20-degrees), the overlap of the metal hybrids is better than for [Cp2Ti(mu-Cl)]2 at a similar Ti-Ti distance. The computed S-T splitting is 4607 cm-1, accounting for the reported diamagnetism.