Crucial Role of Anions on the Deprotonation of the Cationic Dihydrogen Complex trans-[FeH(η²-H₂)(dppe)₂]⁺

The kinetics of reaction of the dihydrogen complex trans-[FeH(η²-H₂)(dppe)₂]⁺ with an excess of NEt₃ to form cis-[FeH₂(dppe)₂] shows a first-order dependence with respect to both the metal complex and the base. The corresponding second-order rate constant only shows minor changes when the solvent is changed from THF to acetone. However, the presence of salts containing the BF₄^-, PF₆^-, and BPh₄^- anions causes larger kinetic changes, the reaction being accelerated by BF₄^- and PF₆^- and decelerated in the presence of BPh₄^-. These results can be interpreted considering that the ion pairs formed by the complex and the anion provide a reaction pathway more efficient than that going through the unpaired metal complex. From the kinetic results in acetone solution, the stability of the ion pairs and the rate constant for their conversion to the reaction products have been derived. Theoretical calculations provide additional information about the reaction mechanism both in the absence and in the presence of anions. In all cases, the reaction occurs with proton transfer from the trans-dihydride to the base through intermediate structures showing FeH₂···N and FeH···H···N dihydrogen bonds, isomerization to the cis product occurring once the proton transfer step has been completed. Optimized geometries for the ion pairs show that the anions are placed close to the H₂ ligand. In the case of BPh₄^-, the bulky phenyls hinder the approach of the base and make the ion pairs unproductive for proton transfer. However, ion pairs with BF₄^- and PF₆^- can interact with the base and evolve to the final products, the anion accompanying the proton through the whole proton transfer process, which occurs with an activation barrier lower than for the unpaired metal complex.