A method has been devised that creates a planar Ni-II site from a tetrahedral site in a NiFe3S4 cubane-type cluster. Reaction of [(Ph3P)NiFe3S4(LS3)](2-) (2) with 1,2-bis(dimethylphosphino)ethane affords [(dmpe)NiFe3S4(LS3)](2-) (3), isolated in ca. 45% yield as (Et4N)(2)[3a]-2.5MeCN and (Et4N)(2)[3b](.)0.25MeCN, both of which occur in triclinic space group P (1) over bar. Each crystalline form contains two crystallographically inequivalent clusters with the same overall structure but slightly different dimensions. The cluster is bound by three thiolate terminal ligands to semirigid cavitand ligand LS3. The NiFe3S4 core contains three tetrahedral sites, one Fe(mu(3)-S)(3)(SR) and two Fe(mu(3)-S)(2)(mu(2)-S)(SR) with normal metric features, and one distorted square planar Ni(mu(3)-S)(2)P-2 site in a Ni(mu(3)-S)(2)Fe face with mean bond lengths Ni-P = 2.147(9) Angstrom and Ni-S = 2.29(2) Angstrom. The opposite Fe-2(mu(3)-S)(mu(2)-S) face places the mu(2)-S atom at nonbonding and variable distances (2.60-2.90 Angstrom) above the nickel atom. Binding of the strong-field ligand dmpe results in a planar Ni-II site and deconstruction of the full cubane geometry. The structure approximates that established crystallographically in the C-cluster of C. hydrogenoformans carbon monoxide dehydrogenase whose NiFe4S4 core contains a planar NiS4 site and three tetrahedral FeS4 sites in a fragment that is bridged by sulfide atoms to an exo iron atom. Mossbauer studies of polycrystalline samples containing both clusters 3a and 3b reveal the presence of at least two cluster types. The spectroscopically best defined cluster accounts for ca. 54% of total iron and exhibits hyperfine interactions quite similar to those reported for the S = (5)/(2) state of the protein-bound cubane-type cluster [ZnFe3S4](1+), whose Mossbauer spectrum revealed the presence of a high-spin Fe2+ site and a delocalized Fe2.5+Fe2.5+ pair. Development of reactions leading to a planar nickel and a sulfide-bridged iron atom is requisite to attainment of a synthetic analogue of this complex protein-bound cluster. This work demonstrates a tetrahedral (2) --> planar (3) Ni-II stereochemical conversion can be effected by binding of ligands that generate a sufficiently strong in-plane ligand field (dmpe = 1,2-bis(dimethylphosphino)ethane, LS3 = 1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(p-tolylthio)b enzene(3-)).