Kinetic evidence of ring closure step in the substitution of cis-[Cr(bpy)(2)(OH2)(2)](3+) with [M(CN)(8)](4-) (M = Mo, W)

The substitution reaction of cis-[Cr(bpy)(2)(OH2)(2)](3+) with [M(CN)(8)](4 -) (M = Mo,W) results in the formation of {[(bpy)(2)Cr(mu -NC)(2)M(CN)(6)]} (-). The new species exhibit intense MMCT transitions and a high degree of electronic coupling revealing a borderline localized-to-delocalized class o f behavior. The complex formation reactions were studied as: a function of [M(CN)(8)](4-) concentration, pH and temperature. Two consecutive reaction steps were observed: the first exhibits dependence on the [M(CN)(8)](4-) co ncentration and the second is concentration-independent. The mechanism has been rationalized in terms of an ion-pair formation preceding two successiv e anation steps: initial attachment of the N-end of the cyano ligand of the octacyanometalate moiety followed by slower intramolecular binding of the other N-site leading to ring closure. The large positive DeltaH(not equal) and large and negative DeltaS(not equal) values for the second step suggest associative character of the substitution reaction. (C) 2000 Elsevier Scie nce B.V. All rights reserved.