A series of dicationic bis-chelated palladium(II) complexes [Pd(N-N)2][X]2 (N-N = 2,2‘-bipyridine (bipy), 1,10-phenanthroline (phen), and their substituted derivatives; X = PF6 -, BF4 -, OTf-, OTs-) has been synthesized and completely characterized both in the solid state and in solution. The synthetic procedure involves a simple one-pot reaction between Pd(MeCOO)2 and [(N-N)H][X]. These compounds are very active precatalysts for the CO/styrene copolymerization yielding perfectly alternating polyketones. The crystal structures of some complexes of the series provide evidence that a distorsion from the ideal square planar geometry toward a twist conformation occurs. In DMSO solution, one of the two nitrogen-donor ligands is involved in a dissociative equilibrium yielding a monochelated complex with two cis coordination sites available for the copolymerization catalytic process. The catalytically active species is very stable in 2,2,2-trifluoroethanol, where its activity was found unaltered for at least 48 h of reaction without apparent decomposition to palladium metal. The addition of 1,4-benzoquinone (BQ) to the catalytic system has a strong influence on the yield and, above all, on the molecular weight of polyketones. The zerovalent palladium complexes [Pd(N-N)(BQ)], which might be formed during the copolymerization process, have been synthesized and characterized. The crystal structure of [Pd(bipy)(BQ)] shows that benzoquinone acts as a mono-olefinic ligand to Pd. In the presence of protons, the Pd(0) complexes are readily oxidized to Pd(II) with the reduction of benzoquinone to hydroquinone. When [(N-N)H][X] is used as the source of protons, the resulting Pd(II) species is the precatalyst and can immediately re-enter the catalytic cycle.

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A topological analysis of bis-chelate disulfoxide metal complexes is presented together with the results of molecular mechanics calculations on dichloro-1,2-bis(methylsulfinyl)ethane Ru(II) complexes. The goal of the work is to study the influence of the nonbonded atom interactions arising from linkage and cis-trans isomerism, combined with different arrangements of the sulfur chiral centers, on the strain energy of the compounds. The calculated strain energies show that the van der Waals and electrostatic interactions largely favor the S-bonding with respect to a mixed S,O-bonding and the cis isomers with respect to the trans isomers. Different arrangements of the chiral sulfur centers yield markedly different molecular energies; in general, meso ligands produce strain energies lower than racemic ligands. A "conformation discriminator" DeltaC has been introduced for the description of the ring conformations in the case of cycles characterized by marked inequalities of bond lengths and angles.