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.