Spectra of synthetic zeolites containing transition metal ions. V.* Π complexes of olefins and acetylene with Co(II)A molecular sieve

Abstract

The electronic spectra of the cobalt(II) ion in anhydrous cobalt(II) zeolite A in the presence of adsorbed unsaturated hydrocarbons demonstrate that olefins and acetylene are π bonded to cobalt in well defined surface complexes. Gravimetric adsorption data show that one ethylene molecule is bound by each cobalt ion with an energy of about −68 kJ mol−1. Theoretical analysis treating the unsaturated hydrocarbon as a low symmetry electrostatic perturbation to a tetrahedral Co(II) ion gives a quantitative interpretation of the band splitting in the electronic spectra. This splitting is distinguished from that occurring through the dynamic Jahn-Teller effect in purely tetrahedral Co(II) complexes. The low symmetry field contributions to the ligand field stabilization energies have been determined from the fine structure in the experimental spectra and have been found to decrease from ethylene to trans-2-butene in accord with increasing steric hindrance from substitutional methyl groups. These contributions are small, however, and do not exceed 5 kJ mol−1. The spectrum of the Co(II) zeolite A acetylene complex is similar to that of the ethylene complex but shows in addition features characteristic of charge transfer processes.