Hydrogen bonding is a topic which has received much attention over the
years and continues to do so as the importance of such interactions i
s established in all areas of chemistry. The class of hydrogen bonds t
hat directly involves electron-rich transition metal centres in the th
ree-centre interaction has received little attention until quite recen
tly. Such interactions are of importance in understanding intermolecul
ar interactions between organometallic molecules and are particularly
relevant to understanding proton transfer reactions that directly invo
lve 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 c
riteria 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) i
nteractions. 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 h
ydrogen-bond acceptor [Co(CO)(3)L(-)] by changing L leads to strengthe
ning of the N-H...Co hydrogen bond. Furthermore, in reactions where th
e 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 p
referentially, inferring that those hydrogen bonds involving metal cen
tres are the weaker hydrogen bonds. Some initial results that point to
wards the construction of larger hydrogen-bonded assemblies involving
X-H...M hydrogen bonds are also discussed.