Cesium's off-the-map valence orbital
Journal article, 2017
The Td-symmetric [CsO4]+ ion, featuring Cs in an oxidation state of 9, is computed to be a minimum. Cs uses outer core 5s and 5p orbitals to bind the oxygens. The valence Cs 6s orbital lies too high to be involved in bonding, and contributes to Rydberg levels only. From a Molecular Orbital perspective, the bonding scheme reminds of XeO4: an octet of electrons to bind electronegative ligands, and no low-lying acceptor orbitals on the central atom. In this sense, Cs+ resembles hypervalent Xe.
Core electron reactivity