PALLADIUM(II) AND PLATINUM(II) COMPLEXES WITH 1,5-DITHIACYCLOOCTANE (DTCO) - STRUCTURES OF [PD(DTCO)CL2] AND [PD(DTCO)2](NO3)2 AND KINETICS OF LIGAND SUBSTITUTION IN [PD(DTCO)2]2+ BY BIDENTATE LIGANDS

Complex formation of divalent transition metals with the cyclic dithioether dtco = 1,5-dithiacyclooctane was studied, and the complexes [Pd(dtco)Cl2], [Pt(dtco)Cl2], and [Pd(dtco)2](NO3)2 were prepared and characterized. It follows from conductivity measurements that in water [Pd(dtco)Cl2] dissociates partially and [Pd(dtco)2](NO3)2 dissociates completely. Both complexes are diamagnetic. X-ray structure analysis confirms square-planar coordination geometry around the palladium for both [Pd(dtco)Cl2] (monoclinic; space group P2l/c; a = 6.583 (2) angstrom, b = 12.489 (3) angstrom, c = 13.429 (3) angstrom, beta = 113.32 (1)-degrees; Z = 4; R(F(o)) = 0.0274) with a S2Cl2 set of donor atoms and [Pd(dtco)2](NO3)2 (monoclinic; space group 2/m; a = 9.970 (4) angstrom, b = 12.793 (5) angstrom, c = 7.350 (3) angstrom, beta = 91.49 (1)-degrees; Z = 2; R(F(o)) = 0.0253) with a S4 set of donor atoms, the dtco ligands being coordianted in a folded, "butterfly-like" form (boat-chair conformation). The two nitrate ions in the (dtco)2 complex are located perpendicularly to the PdS4 coordination plane, each one bridging two Pd atoms. Stopped-flow spectrophotometry was used to study the kinetics of ligand substitution in [Pd(dtco)2]2+ by bidentate ligands L-L = bpy (2,2'-bipyridine) and phen (1,10-phenanthroline) in MeOH/H2O (9:1) under pseudo-first-order conditions ([L-L] greater-than-or-equal-to 10[Pd(dtco)2(2+]) at 293 K and I = 0.1 M (NaNO3). The reactions lead to the mixed ligand species [Pd(dtco)(L-L)]2+ and follow a second-order rate law, rate = k[L-L][Pd(dtco)2(2+)], with k(bpy) = 20.0 +/- 0.2 and k(phen) = 70.2 +/- 2.1 M-1 s-1. For L-L = bpy, the activation parameters are DELTA-H dagger = 9.46 +/- 0.44 kcal mol-1 and DELTA-S dagger = -32.2 +/- 1.40 cal K-1 mol-1. The kinetic results support the operation of an associative (A) mechanism. The reaction of [Pd(dtco)2]2+ with L-L = mnt2- (dianion of dimercaptomaleonitrile) is a two-step process leading to the intermediate [Pd(dtco)(mnt)] (k > 2 x 10(5) M-1 s-1), which is converted to the product [Pd(mnt)2]2- in a consecutive second-order reaction (k = 14.5 +/- 0.2 M-1 s-1). The structural and kinetic results are discussed in comparison to corresponding complexes with the diaza ligand daco = 1,5-diazacyclooctane.