COMPUTATIONAL, SPECTROSCOPIC, AND CRYSTALLOGRAPHIC DETERMINATION OF THE MOLECULAR-STRUCTURES OF CO ADDUCTS OF VASKA-TYPE COMPLEXES TRANS-IRL2(CO)(2)X

The products of CO addition to Vaska-type complexes, trans-Ir(PH3)(2)( CO)(2)X (X = Cl, I), have been calculated by use of ab initio electron ic structure methods with effective core potentials on all non-hydroge n atoms and basis sets of double-zeta plus polarization function quali ty. The calculated trigonal-bipyramidal structure of Ir(PH3)(2)(CO)(2) Cl differs significantly from the results of a previous crystallograph ic structure determination of Ir(PPh(3))(2)(CO)(2)Cl for which signifi cant disorder and crystal decomposition. during data collection were r eported. In particular, the calculated OC-Ir-CO angle (133-138 degrees ) is significantly greater than the reported crystallographic value (1 21 degrees). Results of a crystallographic (X = I, 131 degrees) and IR spectroscopic (X = CI, 132 degrees; I, 133 degrees) reinvestigation a re in good agreement with the calculations. The origin of the distorti on from idealized trigonal-bipyramidal geometry is briefly discussed.