# Tuning metal-metal distances through ligands

Abraham, Santosh P and Samuelson, Ashoka G (1996) Tuning metal-metal distances through ligands. In: Proceeding of the Indian Academy of Science Chemical Sciences, 108 (3). pp. 131-142.

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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 gold $d^1^0—d^1^0$ interactions to be attractive. Among the latter class $Cu_3P_6X_2$ 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 of $\pi$-donation or acceptance of the ligand.