Effect of Magnesium Bond on the Competition Between Hydrogen and Halogen Bonds and the Induction of Proton and Halogen Transfer.

HOX (X=Cl, Br, I, and At) can engage in either a H-bond (HB) or halogen bond (XB) with a base-like HCN, NH3 , and imidazole. Although the former is energetically preferred for X=Cl and Br, it is the XB that is more stable for At, with I showing little preference. MgY2 forms a Mg-bond with the O atom of HOX, which grows stronger in the order X=Cl<Br<I<At and Y=F<Cl<Br. When all three molecules are combined, both the Mg and the H/X bonds are cooperatively strengthened to a large degree. Rather than causing a reversal in the HB/XB competition, the Mg-bond acts primarily to amplify the natural preference within the dimer. The Mg-bond induces a certain degree of transfer from O to N of the bridging atom in the H/X bond. Comparison is also made with the effects of a Be-bond.