All cluster-model calculations were performed by considering one unit of the anion, i.e. [M(O 2 ) 4 ] x -, embedded in an implicit water solvent using the polarizable continuum model (PCM). This approach has the advantage of producing mechanistic insights without the need to consider surface terminations present in periodic boundary conditions. Moreover, many of the known crystal structures of these systems contain lattice waters; 10 the implicit solvent is a way of partially accounting for this reality and has been used to model chemically similar systems. It should be cautioned, however, that implicit solvation models fail to properly account for specific solute-solvent interactions, notably hydrogen bonding. DFT geometry optimizations were performed in the Gaussian 16 Rev. A.03 software package with the range-separated hybrid functional CAM-B3LYP and default integration grid. By benchmarking against other commonly used functionals (see Tables and), qualitative trends were demonstrated to be insensitive to functional choice and inclusion of empirical dispersion effects. All thermochemical values presented include zero-point vibrational energy and thermal corrections at 298.15 K. The metal centers Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W were modelled