Electronic Structure and Orbital Interactions in Linear PtCO

The electronic and geometric structural properties of linear PtCO have been investigated by using an ab initio l-dependent relativistic effective core potential for Pt within the framework of the multiconfiguration self-consistent field and configuration interaction methods. The energy ordering of electronic states is found to be [formula omitted], where [formula omitted] correlates with Pt 5d¹⁰(¹S) and the triplet states arise from the Pt 5d⁹6s¹(³D) atomic multiple in the dissociated limits. Upon complexation with a Pt atom the loss of C=O antibonding character in the carbon lone-pair orbital dominates the decrease in Φ bonding due to the Pt(5d^T)-CO(tr*) interaction. Charge transfer between the Pt and CO fragments is dominated by the CO → Pt Σ dative bond at larger Pt-CO distances and by Pt→CO Φ-back-bonding at shorter bond lengths. The extra stability and shorter equilibrium Pt-CO bond distance of the ground [formula omitted] state is due to nonoccupancy of the strongly antibonding Σ orbital which in the triplet states has mainly Pt 6s character. This, in turn, allows a closer approach of CO to Pt and enhanced Φ bonding. Both ³Σ and ³Δ are calculated to be bound on the SCF level while ³II has a repulsive Pt-CO energy interaction curve even with configuration interaction, apparently due to reduced Φ bonding in its primary electronic configuration. The [formula omitted] state in its equilibrium geometry has a Pt atom configuration that is much closer to 5d⁹6s¹ than to 5d¹⁰ while the 5d orbital population in the ³Δ state is close to 8 e.