Density functional theory (DFT) has been used to predict geometries, energies and spin states of a series of inorganic cobalt complexes containing the geometry-constraining Tp (tris(pyrazolyl berate)) ligand. For example, the carbonyl, TpCoCO, is a stable 16-electron compound with an unusual asymmetric geometry. This molecule is the structural and electronic analog of an unstable intermediate in C-H bond activation. DFT studies of TpCoCO permit detailed comparisons of theory to experiment, and a rationale for the structure that applies to unstable species as well. More recent work has focused on reactive molecules, including (TpCo)(2)O-2 complexes (with 36 heavy atoms) that may be useful in catalyzing alkane oxidation. DFT provides a useful guide to the feasibility of proposed oxidation mechanisms.