Tuning metal-metal distances through ligands

Metal-metal distances in polynuclear metal complexes are often controlled by factors other than favourable metal-metal (M-M) interactions. Recently metal cluster topology has been identified as a major factor in controlling Cu-Cu distances in tetrahedral copper clusters. The more obvious ligand control in M-M distances was shown to play a secondary role. In order to probe ways to tune metal-metal distances in clusters, a study of clusters with similar ligands around the metal has been taken up. Two broad classes were identified: Clusters in which there are two-electron two-centre bonds, and a second class where multicentre multielectron bridging is involved. Clusters held together by two-electron two-centre bonds can be analysed fairly effectively using a test electrophile like the proton. This is illustrated by analysing structures available in the literature. The sensitivity of the method to detect M-M interactions will be illustrated with examples from gold clusters where relativistic interactions cause goldd 10–d 10 interactions to be attractive. Among the latter class Cu3P6X2 core makes a specially interesting case study. The capping ligand X determines the extent of Cu-Cu interaction present in the cluster. Simple EHT calculations on the cluster reveal that Cu-Cu reduced overlap populations change significantly with the extent ofπ-donation or acceptance of the ligand.