Sulfur bridging interactions between three cis-planar Ni-II-S2N2 complexes and Ni-II, Cu-I,Cu-II, Zn-II, and Hg-II reactants were investigated by synthesis and X-ray crystal structures of some 24 complexes. This work was stimulated by recent crystallographic structures of the A-cluster of carbon monoxide dehydrogenase/acetylcoenzyme A synthase. This bridged biological assembly has the minimal formulation [Fe4S4]-(mu(2)-S-Cys)-[M((mu(2)-S-Cys)(2)GIy)Ni] with M = Ni-II, Cu-I, and Zn-II at sites distal and proximal, respectively, to the iron-sulfur cluster. Bridges supported by representations of the distal nickel site were sought by reactions of the complexes [Ni-II(L-H-S2N2)](2-) and [Ni-II(L-R-S2N2)], with 5-5-5 chelate ring patterns. Reaction products implicate the bridges Ni-(mu(2)-S)(1,2)-M in a variety of molecular structures, some with previously unknown connectivities of bridge atoms. The most frequently encountered bridge units are the nonplanar rhombs Ni(mu(2)-S)(2)M involving both sulfur atoms of a given complex. Those with M = Ni-II are biologically relevant inasmuch as the catalytic metal at the proximal site is nickel. The complex [Ni(L-655)](2-), containing the 6-5-5 ring pattern and coordination sphere of the distal nickel site, was prepared and structurally characterized. It was shown to sustain Ni-2(mu(2)-S)(2) rhombic interactions in the form of trinuclear [{Ni(L-655)}(2)Ni](2-) and [{Ni(L-655)}Ni(R2PCH2CH2PR2)] (R = Et, Ph) in which the second Ni-II simulates the proximal site. Bridging interactions of Ni-II-S2N2 complexes are summarized, and geometrical features of Ni-2(mu(2)-S)(2) rhombs in these complexes, as dependent on ring patterns, are considered (L-H-S2N2 = N,N'-ethylenebis(2-mercaptoisobutyramide)(4-); L-R-S2N2 = trans-rac-N,N'-bis(2-mercapto-2-methylprop-1-yl)-1,2-cyclohexanediamine (2-); L-655 = N-(2-mercaptopropyl)N'-(2'-mercaptoethyl)glycinamide(4-)).