Electron counting - Citizendia

Electron counting is a formalism used for classifying compounds and for explaining or predicting electronic structure and bonding. A chemical bond is the physical process responsible for the attractive interactions between Atoms and Molecules and which confers stability to diatomic and polyatomic Many rules in chemistry rely on electron-counting:

Octet rule for main group elements, especially the lighter ones such as carbon, nitrogen, and oxygen,
Eighteen electron rule in inorganic chemistry and organometallic chemistry of transition metals,
Polyhedral skeletal electron pair theory for cluster compounds, including transition metals and main group elements such as boron including Wade's rules for polyhedral cluster compounds, including transition metals and main group elements and mixtures thereof. The octet rule is a simple chemical Rule of thumb that states that Atoms tend to combine in such a way that they each have eight Electrons in Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the The 18-electron rule is a Rule of thumb used primarily in transition metal chemistry for characterizing and predicting the stability of metal complexes Inorganic chemistry is the branch of Chemistry concerned with the properties and behavior of Inorganic compounds This field covers all Chemical compounds Organometallic chemistry is the study of Chemical compounds containing bonds between Carbon and a Metal. In Chemistry, the term transition metal (sometimes also called a transition element) has two possible meanings It commonly refers to any element in In Chemistry the polyhedral skeletal electron pair theory provides Electron counting rules used to predict the structure of electron deficient clusters. In Chemistry, a cluster is an ensemble of bound Atoms intermediate in size between a Molecule and a bulk Solid. Boron (ˈbɔərɒn is a Chemical element with Atomic number 5 and the chemical symbol B. Dihedral is the upward angle from horizontal of the wings or tailpane of a Fixed-wing aircraft or the wing of a Bird.

Atoms that do not obey their rule are called "electron-deficient" when they have too few electrons to achieve a noble gas configuration, or "hypervalent" when they have too many electrons. History Noble gas is translated from the German noun de ''Edelgas'' first used in 1898 by Hugo Erdmann to indicate their extremely low level of reactivity 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.

1 Counting rules
- 1.1 Neutral counting
- 1.2 Ionic counting
2 Electrons donated by common fragments
- 2.1 "Special cases"
3 Examples of electron counting
4 See also

Counting rules

Two styles of electron counting are popular and both give the same result. The neutral counting approach assumes the molecule or fragment being studied consists of purely covalent bonds. It is usually considered easier especially for low-valent transition metals. The "ionic counting" approach assumes purely ionic bonds between atoms. It rewards the user with a knowledge of oxidation states, which can be valuable. One can check one's calculation by counting employing both approaches, though it is important to be aware that most chemical species exist between the purely covalent and ionic extremes.

Neutral counting

Locate the central atom on the periodic table and determine the number of its valence electrons. One counts valence electrons for main group elements differently from transition metals.

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.

Add one for every halide or other anionic ligand which binds to the central through a sigma bond. A halide is a Binary compound, of which one part is a Halogen Atom and the other part is an element or radical that is less
Add two 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.
Add one for each homoelement bond.
Add one for each negative charge, and subtract one for each positive charge.

Ionic counting

Calculate the number of electrons of the element, assuming an oxidation state

e. g. for a Fe²⁺ has 6 electrons

S^2- has 8 electrons

Add two for every halide or other anionic ligand which binds to the metal through a sigma bond. A halide is a Binary compound, of which one part is a Halogen Atom and the other part is an element or radical that is less
Add two 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, count the number of carbon atoms binding to the metal. Each carbon atom provides one electron.

Electrons donated by common fragments

Ligand	Electrons contributed (neutral counting)	Electrons contributed (ionic counting)
X	1	2 (X^-; X = F, Cl, Br, I)
H	1	2 (H^-)
H	1	0 (H⁺)
O	2	4(O^2-)
N	3	6 (N^3-)
NR₃	2	2 (NR₃; R = H, alkyl, aryl)
CR₂	2	4 (CR₂^2-)
Ethylene	2	2 (C₂H₄)
cyclopentadienyl	5	6(C₅H₅^-)
benzene	6	6 (C₆H₆)

"Special cases"

The numbers of electrons "donated" by some ligands depends on the geometry of the metal-ligand ensemble. A halide is a Binary compound, of which one part is a Halogen Atom and the other part is an element or radical that is less Hydride is the name given to the negative Ion of Hydrogen, H− Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 An oxide is a Chemical compound containing at least one Oxygen atom as well as at least one other element In chemistry a nitride is a compound of Nitrogen with a less Electronegative element where nitrogen has an Oxidation state of -3 Amines are Organic compounds and Functional groups that contain a basic Nitrogen Atom with a Lone pair. A transition metal carbene complex in Organometallic chemistry is a compound bearing a formal carbon-metal bond. Structure This Hydrocarbon has four Hydrogen Atoms bound to a pair of Carbon atoms that are connected by a Double bond. A cyclopentadienyl complex is a Metal complex with one or more cyclopentadienyl groups (C5H5&minus abbreviated as Cp Benzene, or benzol, is an organic Chemical compound and a known Carcinogen with the molecular formula C 6 H 6 Perhaps the most famous example of this complication is the M-NO entity. Nitrosyl refers to the Diatomic species with the formula N[[oxygen O]] 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 η-3 vs. η-1 allyl. Another unusual ligand from the electron counting perspective is sulfur dioxide.

Examples of electron counting

CH₄, for the central C

neutral counting: C contributes 4 electrons, each H radical contributes one each: 4+4(1) = 8 valence electrons

ionic counting: C^4- contributes 8 electrons, each proton contributes 0 each: 8 + 4(0) = 8 electrons. Methane is a Chemical compound with the molecular formula. It is the simplest Alkane, and the principal component of Natural gas.

Similar for H:

neutral counting: H contributes 1 electron, the C contributes 1 electron (the other 3 electrons of C are for the other 3 hydrogens in the molecule): 1 + 1(1) = 2 valence electrons.

ionic counting: H contributes 0 electrons (H⁺), C^4- contributes 2 electrons (per H), 0 + 1(2) = 2 valence electrons

conclusion: Methane follows the octet-rule for carbon, and the duet rule for hydrogen, and hence is expected to be a stable molecule (as we see from daily life)

H₂S, for the central S

neutral counting: S contributes 6 electrons, each hydrogen radical contributes one each: 6+2(1) = 8 valence electrons

ionic counting: S^2- contributes 8 electrons, each proton contributes 0: 8+2(0) = 8 valence electrons

conclusion: with an octet electron count (on sulfur), we can anticipate that H₂S would be pseudotetrahedral if one considers the two lone pairs. Hydrogen sulfide (or hydrogen sulphide) is the Chemical compound with the formula H 2 S.

SCl₂, for the central S

neutral counting: S contributes 6 electrons, each chlorine radical contributes one each: 6+2(1) = 8 valence electrons

ionic counting: S²⁺ contributes 4 electrons, each chloride anion contributes 2: 4+2(2) = 8 valence electrons

conclusion: see discussion for H₂S above. Sulfur dichloride is the Chemical compound with the formula SCl2 Notice that both SCl₂ and H₂S follow the octet rule - the behavior of these molecules is however quite different.

SF₆, for the central S

neutral counting: S contributes 6 electrons, each fluorine radical contributes one each: 6+6(1) = 12 valence electrons

ionic counting: S⁶⁺ contributes 0 electrons, each fluoride anion contributes 2: 0+6(2) = 12 valence electrons

conclusion: ionic counting indicates a molecule lacking lone pairs of electrons, therefore its structure will be octahedral, as predicted by VSEPR. Sulfur hexafluoride is an Inorganic compound with the formula. Valence shell electron pair repulsion (VSEPR theory (1957 is a model in Chemistry, which is used for predicting the shapes of individual Molecules based One might conclude that this molecule would be highly reactive - but the opposite is true: SF₆ is inert, and it is widely used in industry because of this property.

TiCl₄, for the central Ti

neutral counting: Ti contributes 4 electrons, each chlorine radical contributes one each: 4+4(1) = 8 valence electrons

ionic counting: Ti⁴⁺ contributes 0 electrons, each chloride anion contributes two each: 0+4(2) = 8 valence electrons

conclusion: Having only 8e (vs. Titanium tetrachloride or titanium(IV chloride is the Chemical compound with the formula TiCl4 18 possible), we can anticipate that TiCl₄ will be a good Lewis acid. The 18-electron rule is a Rule of thumb used primarily in transition metal chemistry for characterizing and predicting the stability of metal complexes Indeed, it reacts (in some cases violently) with water, alcohols, ethers, amines.

Fe(CO)₅

neutral counting: Fe contributes 8 electrons, each CO contributes 2 each: 8 + 2(5) = 18 valence electrons

ionic counting: Fe(0) contributes 8 electrons, each CO contributes 2 ech: 8 + 2(5) = 18 valence electrons

conclusions: this is a special case, where ionic counting is the same as neutral counting, all fragments being neutral. Iron pentacarbonyl, also known as iron carbonyl, is the compound with formula (5 Since this is an 18-electron complex, it is expected to be isolable compound.

Ferrocene, (C₅H₅)₂Fe, for the central Fe:

neutral counting: Fe contributes 8 electrons, the 2 cyclopentadienyl-rings contribute 5 each: 8 + 2(5) = 18 electrons

ionic counting: Fe²⁺ contributes 6 electrons, the two aromatic cyclopentadienyl rings contribute 6 each: 6 + 2(6) = 18 valence electrons on iron. Ferrocene is the Organometallic compound with the formula Fe(C5H52 A cyclopentadienyl complex is a Metal complex with one or more cyclopentadienyl groups (C5H5&minus abbreviated as Cp

conclusion: Ferrocene is expected to be an isolable compound.

Please Note: These examples show the methods of electron counting, they are a formalism, and don't have anything to do with real life chemical transformations. Most of the 'fragments' mentioned above do not exist as such; they cannot be kept in a bottle: e. g. the neutral C, the tetraanionic C, the neutral Ti, and the tetracationic Ti are not free species, they are always bound to something, for neutral C, it is commonly found in graphite, charcoal, diamond (sharing electrons with the neighboring carbons), as for Ti which can be found as its metal (where it shares its electrons with neighboring Ti atoms!), C^4- and Ti⁴⁺ 'exist' only with appropriate counterions (with which they probably share electrons). So these formalisms are only used to predict stabilities or properties of compounds!

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