Xenophilic transition metal clusters

Abstract A new subset of the class of compounds containing polar metalmetal bonds is described in which two transition metals with open d-shells form a direct, unbridged bond between a metal center attached to π-acceptor ligands and one which is bound only to donor ligands that have no π-acceptor capabilities. The heterogeneous character of the metalmetal bond in these compounds is generated by the nature of the metal–ligand interactions, either with or without additional contributions from the differing identities and formal oxidation states of the metals. The clusters are remarkable in that they often exhibit unusually short metalmetal distances, and a number of them are highly paramagnetic. The term ‘xenophilic’ metal cluster has been proposed to refer to transition metal compounds that contain this type of metal–metal interaction. The group includes both heteronuclear ‘early–late’ transition metal clusters as well as homonuclear compounds in which the two metal centers are the same element, but differ in the type of ligands attached.