Hartree-Fock-Slater calculations on cation-induced changes in the adsorption of carbon monoxide on iridium tetrameric clusters

The adsorption of CO on various sites of a tetrahedral Ir4 cluster was studied by using the Hartree-Fock-Slater method. The changes in the adsorption when a small metal cluster is deposited in a pore of a zeolite are modeled with a Mg2+ ion at the opposite side of the Ir4 cluster. The formation of strong bonds of metal d and CO 2p* orbitals favors the 2- and 3-fold adsorption geometry. In the presence of a Mg2+ ion, the distance between CO and the metal cluster increases, and the frequency of the CO stretch vibration is shifted upwards. The changes can be ascribed to polarization of the Ir4 clusters in the electrostatic field generated by the Mg2+ ion. As a result, the repulsive interaction between the CO 5s orbital and the Ir4 cluster is reduced, and back-donation into the unoccupied CO 2p* orbital decreases. Implications of these results for catalysis are discusse