Cation–cation bond

Cation-cation interactions often occur between molecular cations of transition metals and f-elements (so-called yl cations). They are distinguished as a separate type of intermolecular interactions, since without taking them into account, cations usually repel each other in accordance with Coulomb's law.

Cation-cation interactions in f-elements

Of particular interest are cation-cation interactions between actinide cations - uranium, in the form of uranyl UO₂²⁺, neptunium in the form of neptunoyl NpO₂²⁺, plutonium and americium, which are essentially a manifestation of specific complex formation.^[1] They are a special case of intermolecular interactions that occur in molecular ions. It was first discovered when studying the behavior of neptunium(V) compounds in solutions of uranyl perchlorate.^[2] Great impact to crystallochemistry of cation-cation bonds was made by Mikhail Grigoriev.

The cation-cation interaction [NpO₂]⁺ determines the crystal structure of Np(V) compounds, and the strength of cation-cation bonds between neptunoyl ions in a solid is comparable to the strength of ordinary bonds with acid ligands,^[3] although in an aqueous solution, apparently, the role There are somewhat fewer of these connections. In an organic solution, the intensity of C-C interactions can remain high [1]. In the absence of mutual coordination of neptunium ions, Np(V) compounds can exhibit both structural similarity with An(VI) compounds and fundamental differences (showing a tendency to unite Np coordination polyhedra through common equatorial edges. Cation-cation interactions can be detected not only by data from structural studies, but also according to vibrational spectroscopy data.^[4] For uranyl, cation-cation interactions are also recorded in the gas phase.^[5]

Cation-cation interactions in transition metal ions

For transition metals, cation-cation interactions appear in nitrene complexes.^[6] Nitrenium ligands provide an excellent platform for the simple and efficient synthesis of extremely rare complexes that have positively charged ligands coordinated to positively charged metals to form stable cation-cation and cation-dication coordination bonds.^{[citation needed]}