In the present study, the reactivity of OH with Ni(X2C2H2)2 and Ni(X2C2H2)2- (where X = S or Se) was investigated
From the thermodynamics, it found that the OH radical attacks a backbone C-atom of the Ni(S2C2H2)2 complex. For the Ni(Se2C2H2)2 complex, the OH is predicted to target the ligating chalcogen atom. The significance of this is that with the attack of OH to a backbone C-atom, the thermodynamic cost to lose a proton or hydrogen atom ranges from exergonic to marginally endergonic depending on the oxidation state of the complex. Notably, such a process results in a rearrangement of the complex, likely leading to deactivation of the catalyst. Where OH has attacked a ligating chalcogenide atom, the thermodynamic cost to lose a proton or hydrogen is endergonic regardless of oxidation state of the complex.
Where OH attacks a coordinating chalcogenide atom, the thermodynamics for the addition of a proton was considered. At the present level of theory, it was found that for the dithiolene and diselenolene monoanionic complexes, the addition of a proton is marginally endergonic. However, following protonation, the loss of water is significantly exergonic and results in the regeneration of the neutral non-oxidized Ni-complex. Given the greater tendency for OH to attack Se versus S it may be speculated that the use of diselenolene ligands may offer a means to protect the Ni-complex from damaging OH radicals due to the thermodynamic tendency for OH to attack Se atom of the diselenolene complexes not seen in the dithiolene complexes.
