Within each periodic group, there is an average increase of ∼ 5 % in M -O going from the first to second row transition metals (e.g. V → Nb) but a decrease of ∼ 1 % from row two to three (e.g. Nb → Ta). Lanthaninde contraction-a well-known effect of the poor shielding provided by 4f electrons-causes an unexpected similarity in atomic radii between the second and third row transition metals and can used to explain this pattern in M -O bonds. Finally, Houser's four coordinate index, τ 4 , 66 is a quantitative metric of tetrahedral distortion and has been calculated