The reactivity of organotransition metal complexes is dependent on the ligand environment of the metal. This Account describes the development and application of new diphosphine ligands, designed to induce large P-M-P angles in transition metal complexes. Aided by computational chemistry, a homologous range of diphosphines based on rigid heterocyclic aromatic backbones of the xanthene-type with natural bite angles of approximately 100-134 degrees have been developed. The special structure of the ligands has an enormous impact on stability and reactivity of various transition metal complexes. Highly active and selective catalysts have been obtained by influencing this reactivity.

The synthesis and characterization of [or Ph (3a)} are described. A comparative study of the catalytic activity of (a) the mixtures containing [Pd(η 3 -C 3 H 5 )(µ-Cl)] 2 and its parent ligand (1a) or H 2 N-C 6 H 4 -2SMe (4) and (b) the palladacycles [Pd-{[(η 5 -C 5 H 3 )-CHdN-(C 6 H 4 -2SMe)]Fe(η 5 -C 5 H 5 )}Cl] (5a), [Pd{(2-CH 2 -4,6-Me 2 C 6 H 2 )-CHdN-(C 6 H 4 -2SMe)}Cl] (5b) in the allylic alkylation of (E)-3-phenyl-2-propenyl (cinnamyl) acetate with sodium diethyl 2-methylmalonate is also reported.