The compounds (PNP)CoX, where PNP is ((Bu2PCH2SiMe2)-Bu-t)(2)N- and X is Cl, l, N-3, OAr, OSO2CF3 and N(H)Ar, are reported. Some of these show magnetic susceptibility, color, and H-1 NMR evidence of being in equilibrium between a blue, tetrahedral S = 3/2 state and a red, planar S = 1/2 state; the equilibrium populations are influenced by subtle solvent effects (e.g., benzene and cyclohexane are different), as well as by temperature. Attempted oxidation to Co(III) with O-2 occurs instead at phosphorus, giving [P(O)NP(O)]CoX species. The single O-atom transfer reagent Phl=O likewise oxidizes P. Even l(2) oxidizes P to give the pendant phosphonium species ((t)Bu2P(l)CH2SiMe2-(NSiMe2CH2PBU2)-B-t)Col(2) with a tetrahedral S = 3/2 cobalt; the solid-state structure shows intermolecular Pl center dot center dot center dot lCo interactions. Attempted alkyl metathesis of PNPCoX inevitably results in reduction, forming PNPCo, which is a spin triplet with planar T-shaped coordination geometry with no agostic interaction. Triplet PNPCo binds N-2(weakly) and CO (whose low CO stretching frequency indicates strong PNP -> Co donor power), but not ethene or MeCCMe.