Complexes of the type CoX[N(SiMe2CH2PPh2)(2)], where X = Cl, Br, or I, can be prepared via reaction of CoX2 with LiN(SiMe2CH2PPh2)(2); these derivatives are tetrahedral high-spin d(7) systems. Reaction of these halide complexes with organolithium, sodium, or potassium reagents generates square-planar, low-spin hydrocarbyl complexes of the formula CoR[N(SiMe2CH2PPh2)(2)] (R = Me, CH2Ph, CH2SiMe3, C5H5). One-electron oxidations have been carried out; only the product of halide abstraction is observed. For example, addition of PhCH2X to the halide derivatives CoX[N(SiMe2CH2PPh2)(2)] generates trivalent, paramagnetic complexes, CoX2[N(SiMe2CH2PPh2)(2)]; these derivatives show variable-temperature magnetic susceptibility data that are consistent with zero-field splitting of the S = 1 state. Addition of methyl bromide or methyl iodide to low-spin CoMe[N(SiMe2CH2PPh2)(2)] results in the formation of the Co(II) halide derivatives CoX[N(SiMe2CH2PPh2)(2)] along with methane and bibenzyl. It is proposed that the Co(III) methyl halide complex CoMe(X) [N(SiMe2CH2PPh2)(2)] is unstable and loses methyl radical homolytically to generate the Co(II) halide derivative; the methyl subsequently reacts with the toluene solvent to produce methane and bibenzyl. Addition of excess benzyl halides has also been found to generate the Co(II) halide complexes initially, followed by a one-electron oxidation to the Co(II) dihalide derivatives. In much of the one-electron chemistry of the Co(II) derivatives incorporating the amidodiphosphine ligand, the decomposition of the putative but unstable Cs(III) alkyl halide derivative CoRX[N(SiMe2CH2PPh2)(2)] is proposed as a recurring event.