INTRAMOLECULAR AND INTERMOLECULAR BONDING IN BENZENE CLUSTER ISOMERS

The bonding in the prototypical high-nuclearity arene cluster complexes Ru5C(CO)12(C6H6) and Ru6C(CO)11-(C6H6)2 has been investigated using extended Huckel calculations. The relative stability of the known isomeric pairs Ru5C(CO)12(eta6-C6H6) and RusC(CO)12(mu3-eta2:eta2:eta2-C6H6), Ru6C(CO)11(eta6-C6H6)2 and Ru6C(CO)11(eta6-C6H6)-(mu3-eta2:eta2:eta2-C6H6) has been related to the chemically characterized interconversion process occurring in solution. Attention has been focused on the relationship between the apical (eta6) and facial (mu3-eta2:eta2:eta2) bonding modes of benzene with the central cluster unit. The calculations lead to the conclusion that the apical isomers are the more stable, although the local benzene-ruthenium interaction is stronger in the facial isomers. The molecular organization in the respective crystal structures as well as the relative cohesion of the solid materials has been investigated by empirical packing potential energy calculations. The relationship between stability of the individual arene cluster molecules and that of the same molecules in the solid state has been addressed in terms of the relative crystal cohesion. Hydrogen bonds of the C-H..O-C type have been detected in crystals of the apical isomers. In crystalline trans-Ru6C(CO)11(eta6-C6H6)2 molecular piles are formed by molecules joined by direct benzene-benzene interactions; a similar packing motif is also present in crystalline Ru6C(CO)11(eta6-C6H6)(mu3-eta2:eta2:eta2-C6H6).