  Protactinium ($Z$ = 91) is a very rare actinide with peculiar
physico-chemical properties. Indeed, although one may naively think that it
behaves similarly to either thorium or uranium by its position in the periodic
table, it may in fact follow its own rules. Because of the quite small energy
gap between its valence shells (in particular the 5$f$ and 6$d$ ones) and also
the strong influence of relativistic effects on its properties, it is actually
a challenging element for theoretical chemists. In this article, we combine
experimental information, chemical arguments and standard first-principle
calculations, complemented by implicit and explicit solvation, to revisit the
stepwise complexation of aqueous protactinium(V) with sulfate and oxalate
dianionic ligands (SO$_4$$^{2-}$ and C$_2$O$_4$$^{2-}$, respectively). From a
methodological viewpoint, we notably conclude that it is necessary to at least
saturate the coordination sphere of protactinium(V) to reach converged
equilibrium constant values. Furthermore, in the case of single complexations
(i.e. with one sulfate or oxalate ligand bound in the bidentate fashion), we
show that it is necessary to maintain the coordination of one hydroxyl group,
present in the supposed [PaO(OH)]$^{2+}$ precursor, to obtain coherent
complexation constants. Therefore, we predict that this hydroxyl group is
maintained in the formation of 1:1 complexes while we confirm that it is
withdrawn when coordinating three sulfate or oxalate ligands. Finally, we
stress that this work is a first step toward the future use of theoretical
predictions to elucidate the enigmatic chemistry of protactinium in solution.
