The electrochemistry of the nickel(II) tris(2-diphenylphosphinoethyl)phosphine (P3P’) halide cations [Ni(eta 4(l)-P3P’)X](+) (X = Cl, Br, I) has been studied in acetonitrile and dichloromethane. The cations undergo reduction to the nickel(0) [Ni(eta(4)-(PP)-P-3’)X](-) species. For the bromo and iodo complexes, reduction is followed by loss of halide and the formation of a P3P’-bridged nickel(0) dimer. In acetrnitrile [Ni(eta(4)-P3P’)I](+) also loses iodide via a solvent exchange equilibrium. All the complexes are reversibly oxidized to the analogous nickel(III) species, and irreversibly oxidized to the corresponding nickel(IV) species. The structure of [Ni(eta(4)-P3P’)Br][PF6].1/2(MeCN)-(H2O) was determined by single-crystal X-ray crystallography. The complex crystallizes as dark purple plates in space group P $($) over bar$$ 1, with two molecules in the unit cell, with a 10.171(7) Angstrom, b = 10.672(4) Angstrom, c = 21.040(8) Angstrom, alpha = 87.27(3)degrees, beta = 85.11(4)degrees, gamma = 76.69(4)degrees, and R = 0.058. X-ray photography indicates that although crystals are disordered, [Ni(eta(4)-P3P’)I][PF6] is isostructural with the bromo complex.