Molybdate Complexes of An(VI) with Li ⁺ , Na ⁺ , and Ag ⁺ Outer-Sphere Cations: Electronic Absorption Spectra of Np(VI) and Pu(VI) Molybdate Complexes in Solid State and in Solutions

New dimolybdate complexes of hexavalent actinides with composition Li[CH₃NH₃][UO₂(MoO₄)₂]·3H₂O (<b>1</b>), Li[H₃O][UO₂(MoO₄)₂]·4H₂O (<b>2</b>), Ag₂[UO₂(MoO₄)₂]·0.5H₂O (<b>3</b>), Na₂[UO₂(MoO₄)₂]·4H₂O (<b>4a</b>), Na₂[NpO₂(MoO₄)₂]·4H₂O (<b>4b</b>), Na₂[NpO₂(MoO₄)₂]·5H₂O (5<b>a</b>), and Na₂[PuO₂(MoO₄)₂]·5H₂O (<b>5b</b>) were isolated and structurally characterized. All compounds belong to the type of layered ones. Compounds <b>1</b>-<b>4ab</b> are characterized by the same structure of anionic layers, and the structure of complexes <b>5a</b>,<b>b</b> is different. The inclusion of an additional water molecule in complexes <b>5a</b>,<b>b</b> leads to a change in the structure of the anionic layer. Coordination polyhedra of An(VI) atoms are pentagonal bipyramids with oxygen atoms of AnO₂²⁺ dioxocations in the apical positions. The equatorial plane of the bipyramids is formed by oxygen atoms of five MoO₄^2- anions. Two crystallographically independent molybdate anions in the structures have different functions; one is tridentate-bridging and the other is bidentate-bridging. In hydrates with outer-sphere Na⁺ cations along the U(VI)-Np(VI)-Pu(VI) series, actinide contraction is manifested by a decrease in the interatomic distances in the AnO₂²⁺ cations. The average lengths of An-O_anion bonds in the equatorial planes of the coordination polyether change to a lesser extent. The spectra of Np(VI) and Pu(VI) compounds in the Vis-near-IR range were measured and compared with the spectra of proper solutions.