Five-Coordination in Platinum(II) and Palladium(II) Chemistry

Abstract

In recent years a number of chelating ligands, when combined with a π-acid such as an alkene, have been found to stabilize five-coordination in palladium(II) and platinum(II) chemistry. The complexes have invariably a trigonal bipyramidal geometry with the bidentate ligand and the alkene in the equatorial plane. The π-acceptor capacity of the alkene and a small bite of the chelate ligand (between 70 and 85°) are both required for accommodating them in the equatorial plane where the d ² _z electron concentrations (which in the square planar d ⁸ complexes are located above and below the coordination plane) are also confined. Two mono-dentate ligands, in place of one chelate, do not stabilize the five-coordination since for entropic reasons the dissociation of one of them is strongly favored. A great bulk of the chelate ligand is also found to stabilize five-coordination since interligand steric interactions are smaller in the trigonal plane of a five-coordinate complex than in the square-plane of a four-coordinate species. Moreover the same steric factors that destabilize four- versus five-coordination can also destabilize four-versus three-coordination, and T-shaped ML₃ species appear to provide a low energy path for the interconversion between four- and five-coordinate complexes. The alkene is strongly bound to the metal, the metal-carbon bond distances are shorter, and the bending back of the alkene substituents is greater than those found in four-coordinate compounds. The activation energy for alkene rotation is also greater in five-coordinate species, and in a number of cases atropisomers are generated. The chelate dinitrogen ligand also lying in the equatorial plane is, on the other hand, loosely bound to the metal, and dissociation of one end of this ligand appears to be the preliminary step in all substitution processes involving the alkene, the axial ligands, and the bidentate ligand itself. These processes should therefore occur through a mechanism analogous to that operating in four-coordinate complexes. The dissociation of one end of the bidentate ligand also provides a low energy path to proton exchange and inversion of configuration at coordinated nitrogens. Other aspects related to the conformation of the alkene and of the chelate ligand as well as to the preparation and reactivity of this class of compounds are discussed.

Key Words:

• five-coordination
• alkene complexes
• π-acid ligands
• Pt(II)
• Pd(II)