EPR Spectroscopic and Theoretical Study of Chromium(I) Carbonyl Phosphine and Phosphonite Complexes

Isotropic and frozen-solution EPR spectra are reported for several Cr(I) carbonyl complexes of the type cis-[Cr(CO)4L2]+, mer- and fac-[Cr(CO)3L3]+, and trans-[Cr(CO)2L4]+, where L is a monodentate (or L2 a bidentate) phosphine or phosphonite ligand. Extended Hückel molecular orbital theory calculations were performed for cis- and trans-[Cr(CO)4L2], mer- and fac-[Cr(CO)3L3], and cis- and trans-[Cr(CO)2L4] (L = PH3, P(OH)3). The EPR results are discussed in light of the electronic structure predictions, which also provide some insights on the oxidation potentials and the isomerization reaction energetics of these complexes. Isotropic 31P hyperfine couplings can be understood in terms of spin densities in the Cr 3d, P 3s, and P 3p orbitals.