In chemistry, electron counting is a formalism for assigning a number of valence electrons to individual atoms in a molecule. It is used for classifying compounds and for explaining or predicting their electronic structure and bonding.[1] Many rules in chemistry rely on electron-counting:
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.
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).
It is important, though, to be aware that most chemical species exist between the purely covalent and ionic extremes.
Ligand | Electrons contributed (neutral counting) |
Electrons contributed (ionic counting) |
Ionic equivalent |
---|---|---|---|
X | 1 | 2 | X−; X = F, Cl, Br, I |
H | 1 | 2 | H− |
H | 1 | 0 | H+ |
O | 2 | 4 | O2− |
N | 3 | 6 | N3− |
NR3 | 2 | 2 | NR3; R = H, alkyl, aryl |
CR2 | 2 | 4 | CR2−2 |
Ethylene | 2 | 2 | C2H4 |
cyclopentadienyl | 5 | 6 | C5H−5 |
benzene | 6 | 6 | C6H6 |
The numbers of electrons "donated" by some ligands depends on the geometry of the metal-ligand ensemble. An example of this complication is the M–NO entity. When this grouping is linear, the NO ligand is considered to be a three-electron ligand. When the M–NO subunit is strongly bent at N, the NO is treated as a pseudohalide and is thus a one electron (in the neutral counting approach). The situation is not very different from the η3 versus the η1 allyl. Another unusual ligand from the electron counting perspective is sulfur dioxide.