Stabilization of cobalt(I) by the tripodal ligands tris(2-pyridyl)methane and tris(2-pyridyl)phosphine. Structural, spectroscopic and ab initio studies of the [CoL(2)](n+) species

The nature of bonding in a series of complexes [CoL 2 ] n+ [L = the tripodal ligand tris(2-pyridyl)methane or tris(2-pyridyl)phosphine, n = 1-3] has been investigated by single-crystal X-ray diffraction, X-ray absorption and electronic spectroscopy and density functional theory ab initio calculations. The structural studies reveal that the cobalt ions each exist in a distorted octahedral geometry defined by six N-donor atoms; the cations are all centrosymmetric. In both series of complexes the bond lengths Co I -N ≈ Co II -N > Co III -N. Data from the various studies indicate that the 'cobalt(I)' state of the complex [Co{X(C 5 H 4 N-2) 3 } 2 ] + (X = CH or P) is better described by the d 8 cobalt(I)-ligand formulation rather than as d 7 cobalt(II)-ligand radical.