This salt contains the nonahydridorhenate(VII) anion, [ReH9]2−, which is a rare example of a coordination complex bearing only hydride ligands.

[3] This compound exhibits strongly reducing properties, and slowly yields hydrogen gas when dissolved in water.

Three more hydrogen ligands define a triangle lying parallel to the base and crossing the prism in its center (see figure).

Recent density functional theory calculations on [ReH9]2− indicate that this dianion adopts the D3h⇌C4v⇌D3h pathway in gas phase and solution, such interconversion originally proposed by Muetterties[12] featuring a capped square antiprism structure as transition state has very low energy barrier.

In K2[ReH9] solid, intramolecular motions of [ReH9]2− include (1) circle-dance mechanism (resembling Matisse's painting Dance (II)) and (2) three-arm turnstile rotation.