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

Abstract

The protonation reaction of the unbridged quadruple metal-metal bond of [Mo(2)Cl(8)](4-) anion producing the triply bonded hydride [Mo(2)(μ-H)(μ-Cl)(2)Cl(6)](3-) 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.

Experimental and Computational Studies on Quadruply Bonded Dimolybdenum Complexes with Terminal and Bridging Hydride Ligands

“…Some tetranuclear complexes comprising two Mo≣Mo units bridged by hydride ligands and stabilized by coordination to p ‐aryl substituted formamidinate groups were reported by Cotton, Murillo, and co‐workers [18–21] . A few dimolybdenum hydride complexes with lower Mo−Mo bond orders have also been described [22–25] …”

Mechanisms in solution

“…Activation energies with a mean value of 19 kcal mol À1 are calculated, in excellent agreement with experimental values. 31 Reaction between ferrocenylthiosemicarbazone and [CuCl 2 (PPh 3 ) 2 ] resulted in copper ion mediated, unprecedented selective cleavage of the carbon-sulfur bond of the ferrocenylthiosemicarbazone, leading to the formation of a mixed geometrical bis-m 2 -sulfido bridged bicopper complex [(PPh 3 )Cu(m-S) 2 Cu(PPh 3 ) 2 ]. The reaction is proposed to go via an intermediate in which a PPh 3 ligand is ejected from one of two four-coordinate copper centres.…”