Seventeen-electron compounds in an eighteen-electron world – An historical perspective

Beginning in about the 1920s, it was realized that the structures and stoichiometries of all known, structurally characterized metal carbonyls obeyed what came to be known as the effective atomic number (EAN) rule, whereby the sum of the electrons donated by the carbonyl ligands plus the number of electrons contained in the metal valence shell equals the number of valence electrons in the next inert (noble) gas element. While extraordinarily useful for predicting the constitutions of new metal carbonyls and their derivatives, universal acceptance of the validity of the EAN (or inert gas, noble gas, and, eventually 18-electron) rule resulted in a number of rule-breaking compounds being misrepresented. This historical perspective will show how several such organometallic compounds, initially formulated as obeying the 18-electron rule, were eventually recognized as 17-electron species with radical-like properties, the first to be known in what became an important branch of organotransition metal chemistry.