Settling for 16 electrons

The 16-electron rule is an extension on the same simple idea expressed by the 18-electron rule — that is, that for an organometallic complex, stability is reached when the valence electrons sum to a certain value. In this case, that value is 16 electrons. 

For one of several possible reasons a metal center may accept only 16 electrons total, so another ligand with two more electrons can t fit. This can occur when the central atom is too small to fit enough electron-bearing atoms or molecules around it it may find relative stability with only 16 electrons, or it may be a transition state in a cataljdic reaction. 

Some more stable examples include IrCl(CO), commonly known as Vaska s complex. Sometimes the atoms that are the source of the electrons are too large to fit around the smaller metal atoms, and so a 16-electron rule is more applicable. Or in rare cases such as V(CO)gthe metal center is so small it can t fit a seventh coordination site. 

Exceptions to the 18-electron occur, for example where 16 electrons is a meta stable transition state in catalytic reactions, but these are often very short lived intermediate species. In a catalytic cycle the catalyst toggles back and forth between an 18 electron and 16 electron species. 

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