The physical properties and structures of a series of six complexes of the type (NiN(2)S(2))W(CO)(4) have been used to establish electronic and steric parameters for square planar NiN(2)S(2) 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 NiN(2)S(2) 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)(=) > {[NiN(2)S(2)](0)} > bipy approximately phen > Ph(2)PCH(2)CH(2)PPh(2) > Ph(2)PCH(2)PPh(2). Electrochemical data from cyclic voltammetry find that the reduction event in the (NiN(2)S(2))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 NiN(2)S(2) ligand, consistent with the electron drain from the nickel-dithiolate ligands by the W(CO)(4) acceptor. Differences in Ni(II/I) DeltaE(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 NiN(2)S(2)/WS(2)C(2) 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 (13)C 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.

That Cys-X-Cys tripeptide linkages can serve as tetradentate N 2 S 2 ligands, utilizing carboxamido nitrogen and cysteinyl sulfur atoms as donors in metalloenzyme active sites, has recently been verified in several protein crystal structures. [1][2][3][4][5] It was further discovered that the nickel-bound Cys-Gly-Cys NiN 2 S 2 moiety of acetyl coA synthase binds through bidentate bridging thiolate groups to a second nickel center which mediates the organometallic reactions required of the biocatalyst (the assembly of CH 3 + , CO, and SR À into the acetyl coA thioester CH 3 C( = O)SR). [1,2] The (Cys-GlyCys)Ni unit joins a host of synthetic NiN 2 S 2 complexes that are known to form multimetal clusters through m-SR interactions. Natures control of binuclearity in the construction of an organometallic catalyst presents the intriguing possibility that the NiN 2 S 2 complexes might be suitable for development as a novel class of ligands for organometalllic chemistry and catalysis. To this end we have characterized a series of NiN 2 S 2 complexes, four of which are shown in Figure 1, according to their electron-donating ability and stereochemical fea- Figure 1. NiN 2 S 2 complexes used as S-donor ligands; bme-daco = 1,5-(1,5-diazacyclooctane)di(ethylthiolate), bme*-daco = di(2-methyl-2-propylthiolate, bme-Me 2 pda = N,N'-dimethyl-2,9-diazanonanedithiolate, ema 4À = 2,7-dioxo-3,6-diazaoctanedithiolate.

The molecular structure of the acetyl CoA synthase enzyme has clarified the role of individual nickel atoms in the dinickel active site which mediates C-C and C-S coupling reactions. The NiN2S2 portion of the biocatalyst (N2S2 = a cysteine-glycine-cysteine or CGC4- tripeptide ligand) serves as an S-donor ligand comparable to classical bidentate ligands operative in organometallic chemistry, ligating the second nickel which is redox and catalytically active. Inspired by this biological catalyst, the synthesis of NiN2S2 metalloligands, including the solid-phase synthesis of resin-bound Ni(CGC)2-, and sulfur-based derivatization with W(CO)5 and Rh(CO)2+ have been carried out. Through comparison to analogous well-characterized, solution-phase complexes, Attenuated Total Reflectance FTIR spectroscopy establishes the presence of unique heterobimetallic complexes, of the form [Ni(CGC)]M(CO)x, both in solution and immobilized on resin beads. This work provides the initial step toward exploitation of such an evolutionarily optimized nickel peptide as a solid support anchor for hybrid bioinorganic-organometallic catalysts.

In pursuit of an analogy between bidentate diphosphane ligands and cis-dithiolato nickel complexes, the easily prepared [bis(mercaptoethane)diazacycloheptane]nickel complex, Ni-1Ј, has been treated with Pd II and Ni II sources. With the latter, trinickel species are observed with typical slantchair structures; for palladium(ii) chloride, a basket-type structure is realized in which the floor of the basket is a PdS 4 unit and the sides are N 2 S 2 units. One chloride atom is held

For the first time, a NiN2S2 metallothiolate ligand is used as a bidentate bridging ligand for a multiply bonded dimetal unit.