Directionality of dihydrogen bonds: The role of transition metal atoms

A theoretical study on two series of electron-rich group 8 hydrides is carried out to evaluate involvement of the transition metal in dihydrogen bonding. To this end, the structural and electronic parameters are computed at the DFT/B3PW91 level for hydrogen-bonded adducts of [(PP 3)MH 2] and [Cp*MH(dppe)] (M=Fe, Ru, Os; PP 3= κ 4-P(CH 2CH 2PPh 2) 3, dppe= κ 2-Ph 2PCH 2CH 2PPh 2) with CF 3CH 2OH (TFE) as proton donor. The results are compared with those of adduct [Cp 2NbH 3] ·TFE featuring a "pure" dihydrogen bond, and classical hydrogen bonds in pyridine·TFE and Me 3N·TFE. Deviation of the H⋯H-A fragment from linearity is shown to originate from the metal participation in dihydrogen bonding. The latter is confirmed by the electronic parameters obtained by NBO and AIM analysis. Considered together, orbital interaction energies and hydrogen bond ellipticity are salient indicators of this effect and allow the MH⋯HA interaction to be described as a bifurcate hydrogen bond. The impact of the M⋯HA interaction is shown to increase on descending the group, and this explains the experimental trends in mechanisms of proton-transfer reactions via MH⋯HA intermediates. Strengthening of the M⋯H interaction in the case of electron-rich 5d metal hydrides leads to direct proton transfer to the metal atom. Classical or bifurcate? Metal involvement in dihydrogen bonding (DHB) is studied theoretically for two series of group 8 metal hydride complexes in comparison to hydride-only DHB of [Cp 2NbH 3] and classical H-bonds. The M⋯HA interaction increases on descending the group, and so the MH⋯HA system can be regarded as a bifurcate H-bond (see picture). Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.