HYDROGEN-BONDS INVOLVING TRANSITION-METAL CENTERS - A BRIEF REVIEW

Hydrogen bonding is a topic which has received much attention over the years and continues to do so as the importance of such interactions is established in all areas of chemistry. The class of hydrogen bonds that directly involves electron-rich transition metal centres in the three-centre interaction has received little attention until quite recently. Such interactions are of importance in understanding intermolecular interactions between organometallic molecules and are particularly relevant to understanding proton transfer reactions that directly involve transition metal centres. Hydrogen bonds in which transition metal centres serve as the hydrogen-bond acceptor, i.e. X-H...M (X = C, N, O, S), can be identified on the basis of geometric and spectroscopic criteria which confirm: (i) that such interactions should be classified alongside conventional hydrogen bonds (X-H...X'), and (ii) that these three-centre four-electron interactions can be distinguished from the better known three-centre two-electron X-H...M (X = B, C, Si, P, S) interactions. In salts of the type R(3)NH(+)Co(CO)(3)L(-), where L = CO, P(OR)(3) or PR(3), it is shown that increasing the basicity of the hydrogen-bond acceptor [Co(CO)(3)L(-)] by changing L leads to strengthening of the N-H...Co hydrogen bond. Furthermore, in reactions where the products were dictated by a competition between N-H...N and N-H...Co hydrogen-bond formation, results suggested that N-H...N were formed preferentially, inferring that those hydrogen bonds involving metal centres are the weaker hydrogen bonds. Some initial results that point towards the construction of larger hydrogen-bonded assemblies involving X-H...M hydrogen bonds are also discussed.