Cp*M2+ complexes (M = Rh, Ir; CP* = C5Me5) are described for 6-(carboxymethyl)-4-methyl-2-hydroxypyridine (cmhpH(2)), an analogue of the guanylylpyridone cofactor in the hydrogenase Hmd.. Three findings indicate that Cp*M(Hcmhp)(+) stabilizes the binding of hydrogen-bond acceptors to the sixth coordination site: (i) water binds in preference to Cl-, (ii) the adduct Cp*Rh(cmhp)(2-hydroxypyridine) exhibits a very short intramolecular hydrogen bond (r(o-o) = 2.38 angstrom; H-1 NMR delta(H) 17.2), and (iii) Cp*lr(cmhpH)Cl efficiently catalyzes the dehydrogenation of PhCH(OH)Me to PhC(O)Me.