Quadrupole Coupling in Dicobalt Octacarbonyl

Abstract

The pure nuclear quadrupole resonance spectrum of 59Co in dicobaltoctacarbonyl was studied in the temperature range of − 196 to − 30°C. At liquid-nitrogen temperature, the quadrupole coupling parameters for the two independent crystallographic sites are (A)e2qQ / h = 90.18 ± 0.15 MHz, η = 0.3149 ± 0.001 and (B)e2qQ / h = 89.30 ± 0.15 MHz, η = 0.4837 ± 0.001. The asymmetry parameters for the two sites approach each other with increasing temperature. To understand the observed parameters, the environment of the cobalt atom in Co2(CO)8 is described in terms of an octahedron with distortions along its three and fourfold axes. Defining two corresponding axially symmetric field gradient tensors with z components qa and qb, a quadratic equation relating the ratio qa / qb to the observed asymmetry parameter η is derived. The calculated field gradient with the z axis along the Co–Co direction is in reasonable agreement with the observed quadrupole splitting in Fe2(CO)9. The difference in the η values for the two sites and the temperature dependence is explained assuming additional distortions of the molecule in the solid. The observed quadrupole coupling constant is shown to be almost equal to the contribution of the tetragonal distortion of the molecule. This part of the field gradient is attributed to a difference in occupation number between those bonding orbitals directed towards the position not occupied by a bridging carbonyl and other bonding orbitals. Values of 0.16–0.18 are obtained for the population difference for a single molecular orbital, depending on the degree of 4p participation in the bonding.