EXPLANATION OF THE UNEXPECTED DIFFERENCES IN THE GROUND-STATES OF DIMETHYL AND DICHLOROBIS(1,2-BIS(DIMETHYLPHOSPHINO)ETHANE)TITANIUM
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Experimentally, Ti(CH3)2(dmpe)2 has a diamagnetic singlet ground state while TiCl2(dmpe)2 has a paramagnetic triplet ground state. However, the stronger donor, Cl, is expected to cause a larger splitting of the t2g-like orbitals. Hence, if the ground states are different one would expect TiCl2(dmpe)2 to be diamagnetic and Ti(CH3)2(dmpe)2 to be paramagnetic. In agreement with this simple reasoning, approximate molecular orbital calculations also predict a result contrary to experiment. Ab initio calculations with reasonable basis sets are required to produce qualitative agreement with the experimental results, while quantitative agreement requires significant correlation energy. Results reported here include single-determinant, generalized-valence-bond, complete-active-space, and direct configuration-interaction calculations in several basis sets on the title compounds and several model compounds. The explanation of the differences in states lies in the electronegativity difference between CI and CH3. The more electronegative Cl withdraws enough charge from the Ti to contract the d orbital sufficiently to cause the increased d-d electron repulsions in the Cl complex to outweigh the orbital splitting. 1991, American Chemical Society. All rights reserved.