Kinetic studies of metal-metal bonded species: protonation of octachlorodimolybdate(4-) ion and hydrogen evolution from octachlorohydrodimolybdate(3-) ion

Theoretical Elucidation of a Classic Reaction: Protonation of the Quadruple Bond of the Octachlorodimolybdate(II,II) [Mo2Cl8]4– Anion

“…This H bridged intermediate structure with no bridging chlorine atoms and an overall charge of 3–, is found to be stable local minimum of the potential energy surface. This intermediate was first postulated by Miller and Haim but has never been, to our knowledge, isolated or theoretically studied. This therefore is the first observation of [Mo 2 (μ-H)­Cl 8 ] 3– ; it was therefore fully optimized and its structure and IR spectrum were calculated.…”

“…An activation energy of 17.6 and 19.9 kcal/mol is found, with B3LYP at the v = 0 and equilibrium levels respectively. This activation energy is now much more promising compared to the experimental data, 9,27 showing that the oxonium approach is in principle correct. It is therefore investigated further in the next section.…”

“…This, however, is not found to be the case from the experimental data obtained in solution. 9,27 It is therefore absolutely necessary to simulate the true reaction environment as closely as possible in order to explain the measured reaction barrier.…”

“…8 The kinetics of the reaction in aqueous HCl has been shown to obey a linear dependence with respect to the acidity function. 9 The theoretical study of metal−metal multiple bonds (order four or higher) has become of increasing interest in recent years because of the advances of Density Functional Theory and of wave function methods that include electron correlation. A recent benchmark study on quadruple metal−metal bonds 10 covers a number of compounds involving Tc, Re, Ru, Os, Rh, and Ir but not [Mo 2 Cl 8 ] 4− .…”

“…The anion undergoes slow protonation with concentrated hydrochloric acid, giving the bridging hydride [Mo 2 (μ-H)­(μ-Cl) 2 Cl 6 ] 3– (eq ) with the reaction constituting the first example of an oxidative addition involving a well-defined metal–metal bond . The kinetics of the reaction in aqueous HCl has been shown to obey a linear dependence with respect to the acidity function false[ Mo 2 Cl 8 false] 4 − + normalH + → false[ Mo 2 Cl 8 normalH false] 3 − …”