Combined kinetic and DFT studies on the stabilization of the pyramidal form of H3PO2 at the heterometal site of [Mo3M′S4(H2O)10]4+ clusters (M′ = Pd, Ni)

Kinetic and DFT studies have been carried out on the reaction of the [Mo3M'S-4(H2O)(10)](4+) clusters (M' = Pd, Ni) with H3PO2 to form the [Mo3M'(pyr-H3PO2)S-4(H2O)(9)](4+) complexes, in which the rare pyramidal form of H3PO2 is stabilized by coordination to the M' site of the clusters. The reaction proceeds with biphasic kinetics, both steps showing a first order dependence with respect to H3PO2. These results are interpreted in terms of a mechanism that involves an initial substitution step in which one tetrahedral H3PO2 molecule coordinates to M' through the oxygen atom of the P=O bond, followed by a second step that consists in tautomerization of coordinated H3PO2 assisted by a second H3PO2 molecule. DFT studies have been carried out to obtain information on the details of both kinetic steps, the major finding being that the role of the additional H3PO2 molecule in the second step consists in catalysing a hydrogen shift from phosphorus to oxygen in O-coordinated H3PO2, which is made possible by its capability of accepting a proton from P-H to form H4PO2+ and then transfer it to the oxygen. DFT studies have been also carried out on the reaction at the Mo centres to understand the reasons that make these metal centres ineffective for promoting tautomerization.