Effects of relativistic motion of electrons on the chemistry of gold and platinum

Experimental evidence proving the unique stabilization of the 6s orbital in platinum and gold is presented. The conclusions are drawn from the chemical reactivities, of both elements, as well as from structural and spectroscopic features of selected compounds. In particular, the opening of a band gap in transparent CsAu and Cs₂Pt, backed by band structure calculations, are regarded conclusive indications of Au⁻ and Pt²⁻ to exist as closed shell species in these compounds.