The 18-electron rule is a useful closed-shell guideline for many low-valent transition-metal complexes, filling nine valence orbitals. It is not universal: many early-metal complexes have fewer electrons, while stable square-planar complexes are often 16-electron.

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The 18-electron rule is a useful guideline in coordination chemistry and organometallic chemistry that suggests that stable metal complexes often have a total of 18 valence electrons. This rule helps predict the stability and reactivity of transition metal complexes, particularly those involving d-block elements.