New insights into the dihydrogen bonds (MHδ-•••Hδ+X) in CpM(PMe3)(L)2H•••HX (M=Cr, Mo, W; L=PMe3, CO; X=F, OH, NH2)

Dihydrogen bonds (DHBs) play a fundamental role in catalytic processes, organometallic reaction mechanisms, and potential hydrogen storage materials. In this work, we analyzed the interactions of transition metal (TM) hydrides CpM(PMe3)(L)(2)H (M=Cr, Mo, W; L=PMe3, CO) with poor, moderate, and strong proton donors HX (NH3, H2O, and HF), and focus on the DHBs in these complexes. All important factors that can affect the DHBs have been considered: transition metal, proton donor, and substituent group. Electrostatic potential (ESP) analysis, topological (atoms in molecules) analysis and noncovalent interactions index (NCI) analysis of the electron density, energy decomposition analysis (EDA) and the effect of electric field were applied to better understand the nature of the DHBs (MH delta-center dot center dot center dot H delta+X) in CpM(PMe3)(L)(2)H center dot center dot center dot HX (M=Cr, Mo, W; L=PMe3, CO; X=F, OH, NH2). The calculated results showed that both the MH delta-center dot center dot center dot H delta+X bonds and M center dot center dot center dot H delta+X bonds can form in CpM(PMe3)(L)(2)H center dot center dot center dot HX complexes. Electron-rich 5d metal (W in this case) hydrides have a greater chance of forming M center dot center dot center dot H delta+X bonds rather than MH delta-center dot center dot center dot H delta+X bonds. Cr is more likely to form DHBs than Mo and W. This is a very inspiring finding because this may indicate that the first-row transition metal, which shows low cost, low toxicity, and exceptional synthetic versatility, is more suitable for catalytic hydrogenation. The type of proton donor and the substituting of PMe3 by CO can alter the strength of DHBs. The stronger proton donor involves the stronger DHBs form. The substitution of CO decreases the strength of the dihydrogen bond. Both the electrostatic interaction and the orbital interaction play important roles in DHBs, R (H delta-center dot center dot center dot H delta+) = 1.6 angstrom seems to be the boundary between these two kinds of interactions. The addition of electric field is conducive to H-2 formation for strong DHB complexes, while it has no effect on the weak DHB complexes.