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Electron counting

Formalism used for classifying compounds

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Atoms are called "electron-deficient" when they have too few electrons as compared to their respective rules, or "hypervalent" when they have too many electrons. Since these compounds tend to be more reactive than compounds that obey their rule, electron counting is an important tool for identifying the reactivity of molecules. While the counting formalism considers each atom separately, these individual atoms (with their hypothetical assigned charge) do not generally exist as free species.

Counting rules

Two methods of electron counting are "neutral counting" and "ionic counting". Both approaches give the same result (and can therefore be used to verify one's calculation).

  • The neutral counting approach assumes the molecule or fragment being studied consists of purely covalent bonds. It was popularized by Malcolm Green along with the L and X ligand notation. It is usually considered easier especially for low-valent transition metals.
  • The "ionic counting" approach assumes purely ionic bonds between atoms.

It is important, though, to be aware that most chemical species exist between the purely covalent and ionic extremes.

Neutral counting

  • Neutral counting assumes each bond is equally split between two atoms.
  • This method begins with locating the central atom on the periodic table and determining the number of its valence electrons. One counts valence electrons for main group elements differently from transition metals, which use d electron count.
E.g. in period 2: B, C, N, O, and F have 3, 4, 5, 6, and 7 valence electrons, respectively.
E.g. in period 4: K, Ca, Sc, Ti, V, Cr, Fe, Ni have 1, 2, 3, 4, 5, 6, 8, 10 valence electrons respectively.
  • One is added for every halide or other anionic ligand which binds to the central atom through a sigma bond.
  • Two is added for every lone pair bonding to the metal (e.g. each Lewis base binds with a lone pair). Unsaturated hydrocarbons such as alkenes and alkynes are considered Lewis bases. Similarly Lewis and Bronsted acids (protons) contribute nothing.
  • One is added for each homoelement bond.
  • One is added for each negative charge, and one is subtracted for each positive charge.

Ionic counting

  • Ionic counting assumes unequal sharing of electrons in the bond. The more electronegative atom in the bond gains electron lost from the less electronegative atom.
  • This method begins by calculating the number of electrons of the element, assuming an oxidation state.
E.g. for a Fe2+ has 6 electrons
S2− has 8 electrons
  • Two is added for every halide or other anionic ligand which binds to the metal through a sigma bond.
  • Two is added for every lone pair bonding to the metal (e.g. each phosphine ligand can bind with a lone pair). Similarly Lewis and Bronsted acids (protons) contribute nothing.
  • For unsaturated ligands such as alkenes, one electron is added for each carbon atom binding to the metal.

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