Comparative analysis of the electronic and EPR spectra of copper(II) and nickel(I) complexes; insights into nickel(I) electronic structure

Abstract

The nickel(I) complex with the ligand 1,2-bis(pyridine-2-carboxamido)benzene (H₂bphen), generated by reduction with sodium amalgam of the homologous nickel(II) complex, has been studied by EPR and electronic spectroscopy. The electronic, structural and bonding properties of the complex were elucidated based on a comparison of the EPR and UV/VIS data of isoelectronic complexes of ⁶³Cu^II and ⁶¹Ni^I of H₂bphen. Analysis of the spectroscopic data revealed that the complex exhibits low covalency in the σ in-plane bonding and high covalency in the out-of-plane π bonds, and has enabled the assignment of the electronic bands at ≈13 500 and 19 600 cm^–1 of [Ni^I(bphen)]^– to d_xz, d_yz ← d_xy and d_{x² – y²} ← d_xy transitions. Implications for the spectral characterization of cofactor F₄₃₀ systems are also presented.