The physical properties and structures of a series of six complexes of the type (NiN2S2)W(CO)(4) have been used to establish electronic and steric parameters for square planar NiN2S2 complexes as bidentate, S-donor ligands. According to the nu(CO) stretching frequencies and associated computed Cotton-Kraihanzel force constants of the tungsten carbonyl adducts, there is little difference in donor abilities of the five neutral NiN2S2 metallodithiolate ligands in the series. The dianionic Ni(ema)(2-) (ema = N,N-ethylenebis(2-mercaptoacetamide)) complex transfers more electron density onto the W(CO)(4) moiety. A ranking of donor abilities and a comparison with classical bidentate ligands is as follows: Ni(ema)(=) > {[NiN2S2)](0)} > bipy approximate to phen > Ph2PCH2CH2PPh2 > Ph2PCH2PPh2. Electrochemical data from cyclic voltammetry find that the reduction event in the (NiN2S2)W(CO)(4) derivatives is shifted to more positive potentials by ca. 0.5 V compared to the ca. -2 V Ni-II/I redox event in the free NiN2S2 ligand, consistent with the electron drain from the nickel-dithiolate ligands by the W(CO)(4) acceptor. Differences in Ni-II/I Delta E-1/2 values appear to have a ligand dependence which is related to a structural feature of the hinge angle imposed by the (mu-SR)(2) bridges. Thus the angle formed by the intersection of NiN2S2/WS2C2 planes has been established by X-ray diffraction analyses as a unique orientational feature of the nickel-dithiolate ligands in contrast to classical diphosphine or diimine ligands and ranges in value from 136 to 107 degrees. Variable-temperature C-13 NMR studies show that the spatial orientations of the ligands remained fixed with respect to the W(CO)(4) moiety to temperatures of 100 degrees C.