When I was doing my Bachelor thesis I worked a lot in Niobium ampoules as we were doing solid state experiments at high temperatures with a lot of fluoride. As this attacks glass we chose Niobium instead and it seems to be quite unreactive even at those high temperatures (working in a helium atmosphere to prevent oxidation).
So when I was finished with my work I descided to try some Niobium salts only to discover that there is no real aqueous chemistry of Niobium, Tantalum and a few other elements in the PSE. They don't form any Nb(III)-ions in solution besides complex-anions for example and certainly no real hexaqua-complex. Vandium above however and also Dubnium below (at lower pH values) show a lot of aqueous chemistry and also quite often pure aqua-complexes.
This surprised me a bit first as nearly all reactions involving Nb, Ta and a few others (I think W, too) usually end with the Oxide (pentoxide) which is super stable. So I though a bit about that and came up with another example. Not only does this resemble the chemistry of Polyvanadates like the reddish decavanadates and all the stuff that forms if you acidify ammonium metavanadate but also the reactions of silicic acid which, as you try to protonate silicates often form polysilicates and SiO2.
Then I remembered the deep red colores of polychromates. If you acidify potassium chromate with HCl which is 70% water you will get the dichromate solution which is orange if you use nitric acid which has 30% of water you will get the trichromates, tetrachromates and higher, red polychromates. And if you use conc. sulfuric acid you will recieve the trioxide. I assumed for a long time that it was a simple dehydration where a H2O will be taken from a H2CrO4 to form CrO3 but then I looked at the structure of CrO3 only to discover that it is a long chain of joined tetrahedra so a high polychromate.
If you look at the structure of Niobium(V)oxide it's pretty much the same only with octahedra. So I imagine a "niobate" as it can be dissolved in basic environments will do the same elimination of water to form long chains or networks which are then called oxides. And in this case those niobates and tantalates are so basic that anything pH 8 (or whatever that point is) is considered a strong acid for it (much into comparison of chromate for example) and this is why a pH7 aqueous chemistry of Niobium or Tantalum can't work.
If this was the case I am not sure why it's exactly in the middle with those few elements. I imagine if a metal is quite electronegative and has a short bond towards the oxygen it will pull the oxygen so close that the positive charge and the hydrogen influence each other (much like with the chemistry of Beryllium) and it will be easer to deprotonate it. So in my case the lower electronegativity of the Nb and Ta in comparison to the V(III) should increase the electron density on the oxygen more making it more basic. And if I compare the electronegativities those elements are some of the least electronegative ones (there isn't much Sc chemistry known and I think Ti, Zr and Hf don't have that much aqueous chemistry either).
But is there another maybe better explanation to why only those few elements show pretty much now real aqueous chemistry at all ?