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).
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.[3] It is usually considered easier especially for low-valent transition metals.[4]
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 countingedit
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 countingedit
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.
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.
For a water molecule (H2O), using both neutral counting and ionic counting result in a total of 8 electrons.
This figure of the water molecule shows how the electrons are distributed with the covalent counting method. The red ones are the oxygen electrons, and the blue ones are electrons from the hydrogen atoms.
Neutral counting
Atom
Electrons contributed
Electron count
H.
1 electron x 2
2 electrons
O
6 electrons
6 electrons
Total = 8 electrons
The neutral counting method assumes each OH bond is split equally (each atom gets one electron from the bond). Thus both hydrogen atoms have an electron count of one. The oxygen atom has 6 valence electrons. The total electron count is 8, which agrees with the octet rule.
This figure of the water molecule shows how the electrons are distributed with the ionic counting method. The red ones are the oxygen electrons, and the blue ones are electrons from hydrogen. All electrons in the OH bonds belong to the more electronegative oxygen.
Ionic counting
Atom
Electrons contributed
Electron count
H+
none
0 electron
O2-
8 electrons
8 electrons
Total = 8 electrons
Wtih the ionic counting method, the more electronegative oxygen will gain electrons donated by the two hydrogen atoms in the two OH bonds to become O2-. It now has 8 total valence electrons, which obeys the octet rule.
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.
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)
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.
The geometry of cis-Dichlorobis(bipyridine)ruthenium(II).
RuCl2(bpy)2 is an octahedral metal complex with two bidentate2,2′-Bipyridine (bpy) ligands and two chloride ligands.
Neutral counting
Metal/ligand
Electrons contributed
Electron count
Ru(0)
d8 (8 d electrons)
8 electrons
bpy
4 electrons x 2
8 electrons
Cl .
1 electron x 2
2 electrons
Total = 18 electrons
In the neutral counting method, the Ruthenium of the complex is treated as Ru(0). It has 8 d electrons to contribute to the electron count. The two bpy ligands are L-type ligand neutral ligands, thus contributing two electrons each. The two chloride ligands hallides and thus 1 electron donors, donating 1 electron each to the electron count. The total electron count of RuCl2(bpy)2 is 18.
Ionic counting
metal/ligand
electrons contributed
number of electrons
Ru(II)
d6 (6 d electrons)
6 electrons
bpy
4 electrons x 2
8 electrons
Cl-
2 electrons x 2
4 electrons
Total = 18 electrons
In the ionic counting method, the Ruthenium of the complex is treated as Ru(II). It has 6 d electrons to contribute to the electron count. The two bpy ligands are L-type ligand neutral ligands, thus contributing two electrons each. The two chloride ligands are anionic ligands, thus donating 2 electrons each to the electron count. The total electron count of RuCl2(bpy)2 is 18, agreeing with the result of neural counting.
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.
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 each: 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.
^Parkin, Gerard (2006). "Valence, Oxidation Number, and Formal Charge: Three Related but Fundamentally Different Concepts". Journal of Chemical Education. 83 (5): 791. Bibcode:2006JChEd..83..791P. doi:10.1021/ed083p791. ISSN 0021-9584. Retrieved 2009-11-10.
^Rasmussen, Seth C. (March 2015). "The 18-electron rule and electron counting in transition metal compounds: theory and application". ChemTexts. 1 (1). doi:10.1007/s40828-015-0010-4. ISSN 2199-3793.
^Green, M. L. H. (1995-09-20). "A new approach to the formal classification of covalent compounds of the elements". Journal of Organometallic Chemistry. 500 (1–2): 127–148. doi:10.1016/0022-328X(95)00508-N. ISSN 0022-328X.
^Green, M. L. H. (1995-09-20). "A new approach to the formal classification of covalent compounds of the elements". Journal of Organometallic Chemistry. 500 (1): 127–148. doi:10.1016/0022-328X(95)00508-N. ISSN 0022-328X.
January 01, 1970
electron, counting, chemistry, electron, counting, formalism, assigning, number, valence, electrons, individual, atoms, molecule, used, classifying, compounds, explaining, predicting, their, electronic, structure, bonding, many, rules, chemistry, rely, electro. 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 Octet rule is used with Lewis structures for main group elements especially the lighter ones such as carbon nitrogen and oxygen 18 electron rule 2 in inorganic chemistry and organometallic chemistry of transition metals Huckel s rule for the p electrons of aromatic compounds Polyhedral skeletal electron pair theory for polyhedral cluster compounds including transition metals and main group elements and mixtures thereof such as boranes 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 Contents 1 Counting rules 1 1 Neutral counting 1 2 Ionic counting 2 Electrons donated by common fragments 2 1 Special cases 3 Examples 4 See also 5 ReferencesCounting rules editTwo 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 3 It is usually considered easier especially for low valent transition metals 4 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 edit 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 edit 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 Electrons donated by common fragments editLigand 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 CO 2 2 CO 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 Special cases edit 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 h3 versus the h1 allyl Another unusual ligand from the electron counting perspective is sulfur dioxide Examples editH2O For a water molecule H2O using both neutral counting and ionic counting result in a total of 8 electrons nbsp This figure of the water molecule shows how the electrons are distributed with the covalent counting method The red ones are the oxygen electrons and the blue ones are electrons from the hydrogen atoms Neutral counting Atom Electrons contributed Electron count H 1 electron x 2 2 electrons O 6 electrons 6 electrons Total 8 electrons The neutral counting method assumes each OH bond is split equally each atom gets one electron from the bond Thus both hydrogen atoms have an electron count of one The oxygen atom has 6 valence electrons The total electron count is 8 which agrees with the octet rule nbsp This figure of the water molecule shows how the electrons are distributed with the ionic counting method The red ones are the oxygen electrons and the blue ones are electrons from hydrogen All electrons in the OH bonds belong to the more electronegative oxygen Ionic counting Atom Electrons contributed Electron count H none 0 electron O2 8 electrons 8 electrons Total 8 electrons Wtih the ionic counting method the more electronegative oxygen will gain electrons donated by the two hydrogen atoms in the two OH bonds to become O2 It now has 8 total valence electrons which obeys the octet rule 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 pseudo tetrahedral 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 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 RuCl2 bpy 2 nbsp The geometry of cis Dichlorobis bipyridine ruthenium II RuCl2 bpy 2 is an octahedral metal complex with two bidentate 2 2 Bipyridine bpy ligands and two chloride ligands Neutral counting Metal ligand Electrons contributed Electron count Ru 0 d8 8 d electrons 8 electrons bpy 4 electrons x 2 8 electrons Cl 1 electron x 2 2 electrons Total 18 electrons In the neutral counting method the Ruthenium of the complex is treated as Ru 0 It has 8 d electrons to contribute to the electron count The two bpy ligands are L type ligand neutral ligands thus contributing two electrons each The two chloride ligands hallides and thus 1 electron donors donating 1 electron each to the electron count The total electron count of RuCl2 bpy 2 is 18 Ionic counting metal ligand electrons contributed number of electrons Ru II d6 6 d electrons 6 electrons bpy 4 electrons x 2 8 electrons Cl 2 electrons x 2 4 electrons Total 18 electrons In the ionic counting method the Ruthenium of the complex is treated as Ru II It has 6 d electrons to contribute to the electron count The two bpy ligands are L type ligand neutral ligands thus contributing two electrons each The two chloride ligands are anionic ligands thus donating 2 electrons each to the electron count The total electron count of RuCl2 bpy 2 is 18 agreeing with the result of neural counting 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 each 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 See also editd electron count Tolman s ruleReferences edit Parkin Gerard 2006 Valence Oxidation Number and Formal Charge Three Related but Fundamentally Different Concepts Journal of Chemical Education 83 5 791 Bibcode 2006JChEd 83 791P doi 10 1021 ed083p791 ISSN 0021 9584 Retrieved 2009 11 10 Rasmussen Seth C March 2015 The 18 electron rule and electron counting in transition metal compounds theory and application ChemTexts 1 1 doi 10 1007 s40828 015 0010 4 ISSN 2199 3793 Green M L H 1995 09 20 A new approach to the formal classification of covalent compounds of the elements Journal of Organometallic Chemistry 500 1 2 127 148 doi 10 1016 0022 328X 95 00508 N ISSN 0022 328X Green M L H 1995 09 20 A new approach to the formal classification of covalent compounds of the elements Journal of Organometallic Chemistry 500 1 127 148 doi 10 1016 0022 328X 95 00508 N ISSN 0022 328X Retrieved from https en wikipedia org w index php title Electron counting amp oldid 1220233916, wikipedia, wiki, book, books, library,
