AN ABINITIO CASSCF STUDY OF DELTA-BONDING IN THE MO2CL4(PH3)4 MOLECULE

The properties of the delta bond in the Mo2Cl4(PH3)4 species have been studied by calculations using ab initio CASSCF wave functions in conjunction with effective core potentials. The energies of the ground state and the first two excited states of the delta manifold have been calculated as a function of the angle of internal rotation. The results can be compared with experimental results for a series of molecules of M2X4(LL)2 type in which the steric requirements of the LL ligands introduce various angles of internal rotation away from the eclipsed conformation. For the separation between the singlet ground state and the triplet first excited state the results are in good agreement with those experimentally obtained. The singlet delta-delta* transition energies given by our calculations display a nearly constant discrepancy over the full range of the internal rotation when compared with the spectroscopic results, but this is consistent with the expected atomic intrapair correlation energy for the highly ionic singlet excited state. The delta-bond energy is estimated to be 16.8 kcal mol-1 and the calculated electronic contribution to the barrier of internal rotation is 12.4 kcal mol-1.