Theoretical Elucidation
of a Classic Reaction: Protonation
of the Quadruple Bond of the Octachlorodimolybdate(II,II) [Mo<sub>2</sub>Cl<sub>8</sub>]<sup>4–</sup> Anion
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Abstract
The protonation reaction of the unbridged quadruple metal–metal
bond of [Mo<sub>2</sub>Cl<sub>8</sub>]<sup>4–</sup> anion producing
the triply bonded hydride [Mo<sub>2</sub>(μ-H)(μ-Cl)<sub>2</sub>Cl<sub>6</sub>]<sup>3–</sup> is studied by accurate
Density Functional Theory computations. The reactant, product, stable
intermediates, and transition states are located on the potential
energy surface. The water solvent is explicitly included in the calculations.
Full reaction profiles are calculated and compared to experimental
data. The mechanism of the reaction is fully elucidated. This involves
two steps. The first is a proton transfer from an oxonium ion to the
quadruple bond, being rate determining. The second, involves the internal
rearrangement of chlorine atoms and is much faster. Activation energies
with a mean value of 19 kcal/mol are calculated, in excellent agreement
with experimental values