Fluxionality, substitution, and hydrogen exchange at eight-coordinate rhenium(V) polyhydride centers

•Hydride fluxionality arises from A site to B site to A site inversions.•B site ligands are fluxional due to concomitant B site to A site to B site inversions.•Hydride ligands exchange with hydrogen from water even at low temperatures.•The hydride ligand for water hydrogen exchange involves only the B site hydride ligand.•Amine ligand for free amine exchange is first order in rhenium complex concentration.