- Electron counting
Electron counting is a formalism used for classifying compounds and for explaining or predicting electronic structure and bonding. Many rules in chemistry rely on electron-counting:
Octet rulefor main group elements, especially the lighter ones such as carbon, nitrogen, and oxygen,
*Eighteen electron rule in
inorganic chemistryand organometallic chemistryof transition metals,
Polyhedral skeletal electron pair theoryfor cluster compounds, including transition metals and main group elements such as boronincluding Wade's rules for polyhedralcluster compounds, including transition metals and main group elements and mixtures thereof.
Atoms that do not obey their rule are called "electron-deficient" when they have too few electrons to achieve a "
noble gasconfiguration", 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.
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.
* 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
halideor other anionic ligand which binds to the central through a sigma bond.
* 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.
* Calculate the number of electrons of the element, assuming an oxidation state:e.g. for a Fe2+ has 6 electrons:S2- has 8 electrons
* Add two for every
halideor other anionic ligand which binds to the metal through a sigma bond.
* 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
The numbers of electrons "donated" by some ligands depends on the geometry of the metal-ligand ensemble. Perhaps the most famous 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 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: C4- contributes 8 electrons, each proton contributes 0 each: 8 + 4(0) = 8 electrons.: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+), C4- 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: S2- 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.
*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. 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. 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. 18 possible), we can anticipate that TiCl4 will be a good Lewis acid. 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. 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.: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!), C4- 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!
Wikimedia Foundation. 2010.
Look at other dictionaries:
Electron shell — Periodic table with electron shells An electron shell may be thought of as an orbit followed by electrons around an atom s nucleus. The closest shell to the nucleus is called the 1 shell (also called K shell ), followed by the 2 shell (or L shell … Wikipedia
Electron-multiplying CCD — An electron multiplying CCD (EMCCD, also known as an L3Vision CCD, L3CCD or Impactron CCD) is a charge coupled device in which a gain register is placed between the shift register and the output amplifier. The gain register is split up into a… … Wikipedia
Polyhedral skeletal electron pair theory — In chemistry the polyhedral skeletal electron pair theory provides electron counting rules useful for predicting the structures of clusters such as borane and carborane clusters. The electron counting rules were originally formulated by Kenneth… … Wikipedia
18-Electron rule — The 18 electron rule is a rule of thumb used primarily in transition metal chemistry for characterizing and predicting the stability of metal complexes. Valence shells of a transition metal can accommodate 18 electrons: 2 in each of the five d… … Wikipedia
d electron count — The d electron count is a chemistry formalism used to describe the electron configuration of the valence electrons of a transition metal center in a coordination complex. The d electron count is an effective way to understand the geometry… … Wikipedia
Photoemission electron microscopy — (PEEM, also called photoelectron microscopy, PEM) is a widely used type of emission microscopy. PEEM utilizes local variations in electron emission to generate image contrast. The excitation is usually produced by UV light, synchrotron radiation… … Wikipedia
Surface-conduction electron-emitter display — A surface conduction electron emitter display (SED) is a flat panel display technology that uses surface conduction electron emitters for every individual display pixel. The surface conduction emitter emits electrons that excite a phosphor… … Wikipedia
Lewis structure — The Lewis structure of water. Lewis structures (also known as Lewis dot diagrams, electron dot diagrams, and electron dot structures) are diagrams that show the bonding between atoms of a molecule and the lone pairs of electrons that may exist in … Wikipedia
Nitric oxide — Not to be confused with nitrous oxide or nitrogen oxides. For other uses, see NO (disambiguation). Nitric oxide … Wikipedia
Organometallic chemistry — n Butyllithium, an organometallic compound. Four lithium atoms are shown in purple in a tetrahedron, and each lithium atom is bound to a butyl group (carbon is black, hydrogen is white). Organometallic chemistry is the study of chemical compounds … Wikipedia