The molecular structure of a nickel(0) complex with P,S-donor atom ligands has been characterized by X-ray crystallography. Complex 1, (Ph(2)P(o-C6H4)SCH3)(2)Ni-0, prepared by Na/Hg amalgam reduction of the Ni-II complex [(Ph(2)P(o-C6H4)SCH3)(2)Ni](BF4)(2) ([2](BF4)(2)), degrades photochemically with loss of CH3 radicals to yield complex 3, (Ph(2)P(o-C6H4)S)(2)Ni-II. Crystallographic parameters for the three compounds are as follows : 1, monoclinic space group C2/c with a = 11.467(2) Angstrom, b = 17.613(3) Angstrom, c = 15.733(2) Angstrom, beta = 96.450(10)degrees, V = 3157.5(9) Angstrom(3), and Z = 4; [2](BF4)(2), monoclinic space group P2(1)/c with a = 9.417(4) Angstrom, b = 14.822(9) Angstrom, c = 13.773(2) Angstrom, beta = 98.55(3)degrees, V = 9101(14) Angstrom(3), and Z = 2; and 3, monoclinic space group P2(1)/c with a 9.651(2) Angstrom, b = 12.971(8) Angstrom, c = 12.540(2) Angstrom, beta = 110.46(2)degrees, V = 1470.7(11) Angstrom(3), and Z = 2. While complex 1 has a distorted tetrahedral geometry, complexes [2](BF4)(2) and 3 are square planar with trans stereochemistry. The cyclic voltammogram of [2](BF4)(2) in CH3CN shows two redox events assigned to Ni-II/I and Ni-I/0, whereas the thiolate 3 reveals only one reversible wave assigned to Ni-II/III. The chemical reduction of [2](BF4)(2) with Cp(2)Co provided a Ni-I species, [(Ph(2)P(o-C6H4)SCH3)(2)Ni-I](+), characterized at 100 K by an axial EPR signal with g(x) = g(y) = 2.10 and g(z) = 1.96. Hyperfine spectral features resulting from coupling to two P-31 nuclei suggests a retention of substantially square planar geometry. In contrast the isotropic character of the EPR signal of [(Ph(2)P(o-C6H4)SCH3)(Ph(2)P(o-C6H4)S)Ni-I], presumed to be the first product of the photochemical demethylation of 1 ultimately yielding the doubly demethylated complex 3, suggested the intervening thioether/thiolate Ni-I species to be pseudotetrahedral. Protonation of the nickel(0) species 1 produced a five-coordinate nickel hydride complex, [(H)(arom-PSMe)(2)Ni]BF4, 4.