Trans Element-Hydrogen Bonds: A Distinctive Difference Between Transition Metals and Main Group Elements

The change in sign of the interaction
force constant between element-hydrogen
stretching modes of trans-dihydrides of the d block
and p block elements is analyzed for the first time. As the transition
metal M approaches group 12, the higher energy symmetric trans-H–M–H vibration νsym approaches the
energy of the antisymmetric vibration νasym. Crossing
to group 13 elements E, the trans-H–E–H
vibration νsym increasingly drops below νasym. This reversal is attributed to the d orbital that participates
in the H–M–H bonding but is nonbonding in the H–E–H
compounds. DFT calculations are used to probe the energetics of isoelectronic
triatomic [H–M–H]n+ and
[H–E–H]n− to reveal
this trend and also to demonstrate that the magnitude of these interactions
(νgap) increases down groups 11, 12, and 14 but remains
fairly constant for group 13. They are also used to show that this
reversal is seen in the transition state for hydride transfer to CO2 from the model compounds trans-NiH2(porphyrin) and trans-EH2(porphyrin),
E = Si and Ge in their singlet states.