Isotropic and frozen-solution EPR spectra are reported for several Cr(I) carbonyl complexes of the type cis-[Cr(CO) 4 L 2 ] + , mer-and fac-[Cr(CO) 3 L 3 ] + , and trans-[Cr(CO) 2 L 4 ] + , where L is a monodentate (or L 2 a bidentate) phosphine or phosphonite ligand. Extended Hü ckel molecular orbital theory calculations were performed for cis-and trans-[Cr(CO) 4 L 2 ], merand fac-[Cr(CO) 3 L 3 ], and cis-and trans-[Cr(CO) 2 L 4 ] (L ) PH 3 , 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 31 P hyperfine couplings can be understood in terms of spin densities in the Cr 3d, P 3s, and P 3p orbitals.

The preparation and characterization of [Cr(CO)2L(η-C6Me6)] (L = PEt3, PPh3, P(OEt)3 and P(OPh)3, are reported. One-electron oxidation affords unstable Cr(I) cations, [Cr(CO)2L(η-C6Me6)]+, EPR spectra of which are reported. Detailed analysis of the anisotropic 31P hyperfine interaction indicates that, in frozen CH2Cl2/THF, the phosphine and phosphite complexes have 2A‘ and 2A‘‘ ground states, respectively. The hyperfine anisotropy can be accounted for by dipolar interaction of the 31P nucleus with spin density on Cr and, in the case of the phosphite complexes, with ∼0.01 P 3p y spin density resulting from π-backbonding. Line width anomalies observed in EPR spectra of these and other Cr(I) and Mn(II) “piano stool” complexes can be understood in terms of molecular distortions resulting from solvation in frozen solutions.