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How does the 18-electron rule help explain the stability of organometallic compounds?

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periodic table with group numbers and the s, p, d, and f blocks — organometallic compound The periodic table of the elements, displaying the group numbers and the s, p, d, and f blocks. Elements in the shaded area are the metals of organometallic chemistry.

The 18-electron rule is an empirical guideline used to explain the stability of many organometallic compounds, serving as the transition-metal analogue to the octet rule found in main-group chemistry.1

According to this rule, the most stable organometallic complexes are those in which the metal atom achieves a total of 18 valence electrons in its valence shell, a count that encompasses the metal's outer electrons plus those donated by the surrounding ligands. This total corresponds to the number of electrons in the valence shells of the noble gases, providing a configuration that is energetically favorable for many covalent complexes, such as metal carbonyls and metallocenes.2134

For example, in nickel tetracarbonyl, the neutral nickel atom provides 10 valence electrons, and four carbon monoxide ligands contribute two electrons each, resulting in a stable 18-electron configuration. While the rule is highly effective for predicting the structure and stability of many complexes, it is most applicable to covalent compounds; exceptions exist, particularly among metals on the far left or right of the d-block, such as the platinum metals, which often favor a 16-electron count.123