STRUCTURAL AND BONDING TRENDS IN RHENIUM CARBONYL CLUSTER CHEMISTRY -METAL-METAL AND METAL-LIGAND BOND LENGTHS AND CALCULATED STRENGTHS

The published Re-Re bond lengths, d(Re-Re), of neutral and anionic rhe nium carbonyl clusters, including those containing hydride ligands and core carbon atoms, [Re-x(CO)(y)H-z](0-) and [RexC(CO)(y)H-z](c-) have been converted into rhenium-rhenium bond enthalpies, E(Re-Re), using E(Re-Re) = A d(Re-Re)(-4.6) where A = 2.118 x 10(13) for d(Re-Re) in p icometres and E(Re-Re) in kJ mol(-1). Values for the total metal-metal bond enthalpies, Sigma E(Re-Re), have been calculated for each cluste r. Trends in Sigma E(Re-Re) are explored as a function of the cluster nuclearity, Jr, and the number of electrons donated by the ligands, Tr ends in CE(Re-Re) are also explored as a function of the number of ele ctron pairs involved in cluster bonding, determined by both localised (EAN) and delocalised (PSEPT) models. For the rhenium clusters contain ing core carbon atoms the total metal-metal bond enthalpy is consisten tly higher than for other species of similar electron counts, indicati ng that the metal atoms in such clusters are pulled closer together by the core carbon atoms, which have an effective radius of ca 0.622 Ang strom. Moreover, the addition and removal of ligands from core carbon clusters does not alter Sigma E(Re-Re) as greatly as in smaller cluste rs devoid of core atoms. The [ReH(CO)(4)](n) (n = 2,3,4) clusters are compared and contrasted with the isolobal cycloalkanes, (CH2)(n), and the requirements of octahedral Re(CO)(4)X-2 fragments and bent Re-H-Re 3c-2e bonds are shown to favour the 6-membered (ReH)(3) ring as the s train-free ring. (C) 1998 Elsevier Science Ltd. All rights reserved.