Density functional theoretical study of the structure, bonding and electrochemistry of N₂S₂ nitrosyl complexes of iron and cobalt

Density functional calculations are used to study the structure, bonding and properties of the N₂S₂ nitrosyl complexes (bme-pda)M(NO) and (bme-dach)M(NO) M = Fe, Co. The complexes studied in this work have been previously shown to exhibit unusual structural and electrochemical properties. Calculated optimal geometries and vibrational frequencies are compared to experimental data and found to be in excellent agreement. A very small energy barrier for rotation of the NO ligand relative to the S-donor atoms of the N₂S₂ ligand is calculated. The similarity in reduction potentials of Fe and Co complexes is investigated using molecular orbital analysis and calculation of absolute reduction potentials. It is concluded that reduction of the Fe complexes may produce triplet-state products. Such products are determined to be of lower energy than singlet-state products, and also consistent with the observed reduction potentials of the Fe and Co complexes.