HYDROGEN BONDING IN METAL COMPLEXES: EMPIRICAL AND COMPUTATIONAL COMPARISONS WITH ORGANIC ANALOGUES A. Guy Orpen, Dena Bellamy, School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.

The ability of organic moieties to participate in hydrogen bonding is fundamental to life and to the burgeoning field of solid state engineering. Similar functionalities are frequently present in metal complexes albeit in different electronic and steric environments. In this paper we report comparative studies of hydrogen bonding for a series of metal-bound ligands and their organic analogues. Systems studied include: M-Cl, Cl^- and C-Cl; M-CN and C-CN; M₂CO and C₂CO. In each case the relative frequency and geometries of hydrogen bonds involving these fragments were established from the Cambridge Structural Database (CSD).¹ These provide structural indicators of the strength of the various units as hydrogen bond acceptors. To provide an alternative measure of these interactions, ab initio molecular orbital (MO) calculations were performed on representative examples of each system. Atomic charges (calculated by a variety of algorithms) electrostatic potentials and energies and geometries of interaction with a water molecule were computed. General conclusions from the CSD study may be summarised as follows: metal-bound chloride is a stronger H-bond acceptor than is C-Cl (although both are weaker than Cl^-); metal-bound carbonyl and cyanide groups are weaker hydrogen bond acceptors than their organic analogues. These observations will be discussed and shown to be in accord with the results of the MO studies.

1. F.H. Allen et al., Acc. Chem. Res., 1983, 16, 146; F.H. Allen et al., J. Chem. Inf. Comput. Sci., 1987, 31, 187.