Insight into Compounds with Cu(H₂O)₆²⁺ Units: New Ideas for Understanding Cu²⁺ in Tutton Salts

In the last 65 years the properties of Tutton salts containing Cu2+ cations have been interpreted on the basis of elongated complexes induced by a static Jahn–Teller effect (JTE). Through the analysis of experimental data and the results of first-principles calculations, we show here that such an idea, though widely followed, is not correct.

By contrast, this work proves that the local geometry of Cu(H2O)62+ units in Tutton salts actually arises from a compressed octahedron although hidden by an additional orthorhombic instability fully unrelated to the JTE. For understanding this conclusion, it is crucial to consider the effects of the internal electric field, E R( r ), created by the rest of the lattice ions on the electrons localized in the Cu(H2O)62+ unit.

Indeed, the E R( r ) field in Tutton salts opens a gap between ∼x2–y2 and ∼3z2–r2 antibonding molecular orbitals that favors a hole in ∼3z2–r2 and triggers an orthorhombic distortion in the XY plane that reasonably explains available experimental data. The conditions responsible for the orthorhombic instability are discussed pointing out the singularity of Cu2+ complexes in the realm of 3d divalent cations.

For the sake of completeness the properties of Cu(H2O)62+ units in trigonal lattices, where a JTE is clearly observed, are analyzed in detail and compared to results of Cu2+ cations in cubic lattices. In the trigonal compounds, the force constant of the Jahn–Teller mode is shown to be smaller than that for hard ligands like O2– or F– but comparable to the softer ligand Cl–.

This fact helps to promote the orthorhombic instability in the Cu(H2O)62+ complex when the hole is no longer in the ∼x2–y2 orbital but in ∼3z2–r2.