Kinetic, DFT and TD-DFT studies on the mechanism of stabilization of pyramidal H3PO3 at the [Mo3M′S4(H2O)10]4+ clusters (M′ = Pd, Ni)

The kinetics of reaction between the [Mo3M ' S-4(H2O)(10)](4+) clusters (M ' = Pd, Ni) and H3PO3 has been studied in 4.0 M Hpts/Lipts (pts(-) = p-toluenesulfonate). For both complexes there is an initial kinetic step with small absorbance changes that corresponds to substitution of the water coordinated to Pd by a molecule of tetrahedral H3PO3. For the Pd complex, tautomerization of H3PO3 occurs in a slower kinetic step with much larger absorbance changes; it leads to formation of [Mo3Pd(pyr-H3PO3)S-4(H2O)(9)](4+) in which H3PO3 adopts a pyramidal structure, but the process is not as favored as for H3PO2. The kinetics of this second step is independent of the concentration of H3PO3 but dependent on the concentration of Hpts on the supporting electrolyte. For the Ni complex, the second step is severely hindered and its kinetics could not be studied. DFT calculations indicate that tautomerization of H3PO3 is expected to be less favoured than that of H3PO2, both processes being less favored at the Ni cluster than at its Pd analogue. With regard to the tautomerization mechanism, the calculations indicate that the mechanism previously proposed for H3PO2 can be the same for H3PO3, although the initial H-shift can also occur through a protonation-deprotonation sequence with participation of external protons instead of a second molecule of the phosphorus acid. TD-DFT studies have been also carried out to understand the similarity between the spectra of the starting complex and the reaction intermediate formed in the first kinetic step as well as the large spectral changes associated to the tautomerization process. Although it contains contributions from several transitions, the intense band observed for clusters containing coordinated pyr-H3PO3 involves essentially ligand-to-metal charge-transfer (LMCT) transitions.