Metal-Metal Bonded Compounds of the Group IX Elements

Abstract

This article describes the compounds containing CoCo, RhRh, or IrIr bonds that have been reported since 2005, focusing on structural data and bonding. For Co, a variety of geometries and oxidation states have been reported for metal-metal bonded compounds leading to a variety of spin states and new bimetallic reactivity. In addition to discrete bimetallic Co2 complexes, higher-order multi-Co compounds can be found with chain or cluster geometries. Dirhodium complexes are the most numerous metal-metal bonded compounds known; most employ the paddlewheel-type geometry having four bridging ligands (e.g., carboxylate ligands) that hold the two Rh atoms in close proximity. Half-paddlewheel compounds are known that only have two bridging ligands, and there are some new examples of compounds with unsupported RhRh bonds. The Rh complexes have important applications in catalysis and in biology. Compounds with IrIr bonds are not as well represented as Co or Rh complexes. Nevertheless, IrIr bonded compounds supported by phosphine ligands have been shown to undergo a variety of bimetallic transformations involving the activation of small molecules.