The nickel(II) complex of an N
2S
2 ligand, derived from a diazacycle,
N,
N'-bis(mercaptoethyl)-1,5-diazacycloheptane,(bme-dach)Ni,
Ni-1', serves as a metallodithiolate ligand to Ni
II, Cu
I, Zn
II, Ag
I, and Pb
II. The binding ability of theNiN
2S
2 ligand to the metal ions was established through spectrochemical titrations in aqueous media and comparedto classical S-donor ligands. For M = Ni, Zn, Pb, binding constants, log
K = ca. 2, were computed for 1:1
Ni-1'
/M(solvate) adducts; for Ag
+ and Cu
+, the 3:2 (
Ni-1'
)3M
2 adducts were the first formed products even in waterwith log
3,2 values of 26 and >30, respectively. In all cases, the binding ability of Ni-S-R is intermediate betweenthat of a free thiolate and a free thioether. The great specificity for copper over nickel and zinc by N
2S
2Ni, whichserves as a reasonable structural model for the distal nickel of the acetyl CoA synthase active site, relates tobiochemical studies of heterogeneity (metal content and type) in various preparations of acetyl CoA synthaseenzyme.