Analogue reaction systems of selenate reductase, which reduces substrate in the overall enzymatic reaction SeO42- + 2H(+) + 2e(-) -> SeO32- + H2O, have been developed using bis(dithiolene) complexes of Mo-IV and W-IV, On the basis of the results of EXAFS analysis of the oxidized and reduced enzyme, the minimal reaction (MoOH)-O-IV + SeO42--> (MoO)-O-VI(OH) + SeO32- is probable. The square pyramidal complexes [M(OMe)(S2C2Me2)(2)](1-) (M = MO, W) were prepared as structural analogues of the reduced enzyme site. The systems, [ML(S2C2Me2)(2)](1-)/SeO42- (L = OMe, OPh, SC6H2-2,4,6-Pr-3(i)) in acetonitrile, cleanly reduce selenate to selenite in second-order reactions whose negative entropies of activation implicate associative transition states. Rate constants at 298 K are in the 10(-2)-10(-4) M-1 s(-1) range with Delta S-double dagger = -12 to -34 eu. When rate constants are compared with previous data for the reduction of (CH2)(4)SO, Ph3AsO, and nitrate by oxygen atom transfer, reactivity trends dependent on the metal, axial ligand L, and substrate are identified. As in all other cases of substrate reduction by oxo transfer, the kinetic metal effect k(2)(w) > k(2)(Mo) holds. A proposal from primary sequence alignments suggesting that a conserved Asp residue is a likely ligand in the type 11 enzymes in the DMSO reductase family has been pursued by synthesis of the [Mo-IV(O2CR)(S2C2Me2)(2)](1-) (R = Ph, Bu') complexes. The species display symmetrical eta(2)-carboxylate binding and distorted trigonal prismatic stereochemistry. They serve as possible structural analogues of the reduced sites of nitrate, selenate, and perchlorate reductases under the proposed aspartate coordination. Carboxylate binding has been crystallographically demonstrated for one nitrate reductase, but not for the other two enzymes.