18-Electron rule
Encyclopedia
The 18-electron rule is a rule of thumb
used primarily for predicting formulas for stable metal complexes. The rule rests on the fact that valence shells of a transition metal
consists of nine valence orbitals, which collectively can accommodate 18 electron
s either as nonbinding electron pairs or as bonding electron pairs. Stated differently, combination of these nine atomic orbitals with ligand
orbitals gives rise to nine molecular orbital
s that are either metal-ligand bonding or non-bonding. When a metal complex has 18 valence electrons, it is said to have achieved the same electron configuration as the noble gas
at the end of the period. The rule and its exceptions are similar to the application of the octet rule
to main group elements. The rule is not helpful for complexes of the s-block metals, the lanthanides, and the actinides.
The rule was first proposed by British chemist Nevil Sidgwick in 1927.
, iron pentacarbonyl
, chromium carbonyl
, and nickel carbonyl
.
Ligands in a complex determine the applicability of the 18-electron rule. In general, complexes that obey the rule are composed at least partly of π-acid ligands. This kind of ligand exerts a very strong ligand field, which lowers the energies of the resultant molecular orbitals and thus favorably occupied. Typical ligands include olefins, phosphines, and CO
. Complexes of π-acids typically feature metal in a low-oxidation state. The relationship between oxidation state and the nature of the ligands is rationalized within the framework of π backbonding.
. In such cases, in general ligand exchange occurs via dissociative substitution
mechanisms, wherein the rate of reaction is determined by the rate of dissociation of a ligand. On the other hand, 18-electron compounds can be highly reactive toward electrophiles such as protons, and such reactions are associative in mechanism, being acid-base reactions.
Complexes with fewer than 18 valence electrons tend to show enhanced reactivity. Thus, the 18-electron rule is often a recipe for non-reactivity in either a stoichiometric or a catalytic
sense.
(IrCl(CO)(PPh3)2), [PtCl4]2− , and Zeise's salt
[PtCl3(η2-C2H4)]− . In such complexes, the dz2 orbital is doubly occupied and nonbonding.
Many catalytic cycle
s operate via complexes that alternate between 18e and square-planar 16 configurations. Examples include Monsanto acetic acid synthesis, hydrogenation
s, hydroformylation
s, olefin isomerizations, and some alkene polymerizations.
Other violations can be classified according to the kinds of ligands on the metal center.
Examples:
Sometimes such complexes engage in agostic interactions with the hydrocarbon framework of the bulky ligand. For example:
), or addition of a strong field
ligand can cause electron-pairing, thus creating a vacant orbital that it can donate into.
Examples:
Complexes containing strongly pi-donating ligands often violate the 18-electron rule. These ligands include fluoride (F− ), oxide (O2− ), nitride (N3− ), alkoxide (RO− ), and imide (oxide (RN2− ). Examples:
In the latter case, there is substantial donation of the nitrogen lone pairs to the Mo (so the compound could also be described as a 16 VE compound). This can be seen from the short Mo-N bond length, and from the angle Mo - N - C(R), which is nearly 180°.
Counter-examples:
In these cases, the M=O bonds are "pure" double bonds (i.e., no donation of the lone pairs of the oxygen to the metal), as reflected in the relatively long bond distances.
Often, cases where complexes have more than 18 valence electrons are attributed to electrostatic forces - the metal attracts ligands to itself to try to counterbalance its positive charge, and the number of electrons it ends up with is unimportant. In the case of the metallocenes, the chelating nature of the cyclopentadienyl ligand stabilizes its bonding to the metal. Somewhat satisfying are the two following observations: (i) cobaltocene is a strong electron donor, readily forming the 18-electron cobaltocenium cation and (ii) nickelocene tends to react with substrates to give 18-electron complexes, e.g. CpNiCl(PR3) and free CpH.
Rule of thumb
A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation. It is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination...
used primarily for predicting formulas for stable metal complexes. The rule rests on the fact that valence shells of a transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
consists of nine valence orbitals, which collectively can accommodate 18 electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s either as nonbinding electron pairs or as bonding electron pairs. Stated differently, combination of these nine atomic orbitals with ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
orbitals gives rise to nine molecular orbital
Molecular orbital
In chemistry, a molecular orbital is a mathematical function describing the wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region. The term "orbital" was first...
s that are either metal-ligand bonding or non-bonding. When a metal complex has 18 valence electrons, it is said to have achieved the same electron configuration as the noble gas
Noble gas
The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...
at the end of the period. The rule and its exceptions are similar to the application of the octet rule
Octet rule
The octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low (The octet rule is a chemical rule of thumb that states that atoms of low (...
to main group elements. The rule is not helpful for complexes of the s-block metals, the lanthanides, and the actinides.
The rule was first proposed by British chemist Nevil Sidgwick in 1927.
Applicability of the 18-electron rule
Although the majority of metal complexes do not satisfy the 18-electron rule, the rule usefully predicts the formulas for low-spin complexes of the Cr, Mn, Fe, and Co triads. Well-known examples include ferroceneFerrocene
Ferrocene is an organometallic compound with the formula Fe2. It is the prototypical metallocene, a type of organometallic chemical compound consisting of two cyclopentadienyl rings bound on opposite sides of a central metal atom. Such organometallic compounds are also known as sandwich compounds...
, iron pentacarbonyl
Iron pentacarbonyl
Iron pentacarbonyl, also known as iron carbonyl, is the compound with formula 5. Under standard conditions Fe5 is a free-flowing, straw-colored liquid with a pungent odour. This compound is a common precursor to diverse iron compounds, including many that are useful in organic synthesis. Fe5 is...
, chromium carbonyl
Chromium carbonyl
Chromium carbonyl, also known as chromium hexacarbonyl, is the chemical compound with the formula Cr6. At room temperature the solid is stable to air, although it does have a high vapor pressure and sublimes readily. Cr6 is zerovalent, meaning that Cr has a formal charge of zero, and it is called...
, and nickel carbonyl
Nickel carbonyl
Nickel carbonyl is the organonickel compound with the formula Ni4. This pale-yellow liquid is the principal carbonyl of nickel. It is an intermediate in the Mond process for the purification of nickel and a reagent in organometallic chemistry...
.
Ligands in a complex determine the applicability of the 18-electron rule. In general, complexes that obey the rule are composed at least partly of π-acid ligands. This kind of ligand exerts a very strong ligand field, which lowers the energies of the resultant molecular orbitals and thus favorably occupied. Typical ligands include olefins, phosphines, and CO
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
. Complexes of π-acids typically feature metal in a low-oxidation state. The relationship between oxidation state and the nature of the ligands is rationalized within the framework of π backbonding.
Consequences for reactivity
Compounds that obey the 18 VE rule are typically "exchange inert." Examples include [Co(NH3)5Cl]2+, Mo(CO)6, and [Fe(CN)6]4-Ferrocyanide
Ferrocyanide is the name of the anion Fe64−. In aqueous solutions, this coordination complex is relatively unreactive. It is usually available as the salt potassium ferrocyanide, which has the formula K4Fe6....
. In such cases, in general ligand exchange occurs via dissociative substitution
Dissociative substitution
Dissociative substitution describes a pathway by which compounds interchange ligands. The term is typically applied to coordination and organometallic complexes, but resembles the Sn1 mechanism in organic chemistry. The opposite pathway is associative substitution, being analogous to Sn2 pathway...
mechanisms, wherein the rate of reaction is determined by the rate of dissociation of a ligand. On the other hand, 18-electron compounds can be highly reactive toward electrophiles such as protons, and such reactions are associative in mechanism, being acid-base reactions.
Complexes with fewer than 18 valence electrons tend to show enhanced reactivity. Thus, the 18-electron rule is often a recipe for non-reactivity in either a stoichiometric or a catalytic
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
sense.
16e complexes
A popular class of complexes that violate the 18e rule are the 16e complexes with d8 configurations. Examples are especially prevalent for derivatives of the cobalt and nickel triads. Such compounds are typically square-planar. The most famous example is Vaska's complexVaska's complex
Vaska's complex is the trivial name for the chemical compound trans-chlorocarbonylbisiridium, which has the formula IrCl[P3]2. This square planar diamagnetic organometallic complex consists of a central iridium atom bound to two mutually trans triphenylphosphine ligands, carbon monoxide, and a...
(IrCl(CO)(PPh3)2), [PtCl4]2
Zeise's salt
Zeise's salt, potassium trichloroplatinate, is the chemical compound with the formula KPtCl3]·H2O. The anion of this air-stable, yellow, coordination complex contains an η2-ethylene ligand. The anion features a platinum atom with a square planar geometry.-Preparation:This compound is commercially...
[PtCl3(η2-C2H4)]
Many catalytic cycle
Catalytic cycle
A catalytic cycle in chemistry is a term for a multistep reaction mechanism that involves a catalyst . The catalytic cycle is the main method for describing the role of catalysts in biochemistry, organometallic chemistry, materials science, etc. Often such cycles show the conversion of a...
s operate via complexes that alternate between 18e and square-planar 16 configurations. Examples include Monsanto acetic acid synthesis, hydrogenation
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
s, hydroformylation
Hydroformylation
Hydroformylation, also known as oxo synthesis or oxo process, is an important industrial process for the production of aldehydes from alkenes. This chemical reaction entails the addition of a formyl group and a hydrogen atom to a carbon-carbon double bond...
s, olefin isomerizations, and some alkene polymerizations.
Other violations can be classified according to the kinds of ligands on the metal center.
Bulky ligands
Bulky ligands can preclude the approach of the full complement of ligands that would allow the metal to achieve the 18 electron configuration.Examples:
- Ti(neopentyl)4 (8 VE)
- Cp*Pentamethylcyclopentadiene1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic diolefin with the formula C5Me5H . 1,2,3,4,5-Pentamethylcyclopentadiene is the precursor to the ligand 1,2,3,4,5-pentamethylcyclopentadienyl, which is often denoted as Cp*...
2Ti(C2H4) (16 VE) - V(CO)6 (17 VE)
- Cp*Cr(CO)3 (17 VE)
- Pt(PtBu3)2 (14 VE)
- Co(norbornyl)4 (11 VE)
- [FeCp2]+ (17 VE)
Sometimes such complexes engage in agostic interactions with the hydrocarbon framework of the bulky ligand. For example:
- W(CO)3[P(C6H11)3]2 has 16 VE but has a short bonding contact between one C-H bond and the W center.
- Cp(PMe3)V(CHCMe3) (14 VE, diamagnetic) has a short V-H bond with the 'alkylidene-H', so the description of the compound is somewhere between Cp(PMe3)V(CHCMe3) and Cp(PMe3)V(H)(CCMe3).
High-spin complexes
High-spin metal complexes have singly occupied orbitals and may not have any empty orbitals into which ligands could donate electron density. In general, there are few or no π-acidic ligands in the complex. These singly occupied orbitals can combine with the singly occupied orbitals of radical ligands (e.g., oxygenOxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
), or addition of a strong field
Ligand field theory
Ligand field theory describes the bonding, orbital arrangement, and other characteristics of coordination complexes. It represents an application of molecular orbital theory to transition metal complexes. A transition metal ion has nine valence atomic orbitals, five d, one s, and three p orbitals...
ligand can cause electron-pairing, thus creating a vacant orbital that it can donate into.
Examples:
- CrCl3(THF)3 (15 VE)
- [Mn(H2O)6]2+ (17 VE)
- [Cu(H2O)6]2+ (21 VE, see comments below)
Complexes containing strongly pi-donating ligands often violate the 18-electron rule. These ligands include fluoride (F
- [CrO4]2
− (16 VE) - Mo(=NR)2Cl2 (12 VE)
In the latter case, there is substantial donation of the nitrogen lone pairs to the Mo (so the compound could also be described as a 16 VE compound). This can be seen from the short Mo-N bond length, and from the angle Mo - N - C(R), which is nearly 180°.
Counter-examples:
- trans-WO2(Me2PCH2CH2PMe2)2 (18 VE)
- Cp*ReO3 (18 VE)
In these cases, the M=O bonds are "pure" double bonds (i.e., no donation of the lone pairs of the oxygen to the metal), as reflected in the relatively long bond distances.
Pi-donating ligands
Ligands where the coordinating atom bear nonbonding lone pairs often stabilize unsaturated complexes. Metal amides and alkoxides often violate the 18e rule.Combinations of effects
The above factors can sometimes combine. Examples include- Cp*VOCl2 (14 VE)
- TiCl4 (8 VE)
Higher electron counts
Some complexes have more than 18 electrons. Examples:- CobaltoceneCobaltoceneCobaltocene, known also as biscobalt or even "bis Cp cobalt", is an organocobalt compound with the formula Co2. It is a dark purple solid that sublimes readily slightly above room temperature. Cobaltocene was discovered shortly after ferrocene, the first metallocene...
(19 VE) - NickeloceneNickeloceneNickelocene is the organonickel compound with the formula Ni2. Also known as bisnickel or NiCp2, this bright green paramagnetic solid is of enduring academic interest, although it yet has no practical applications....
(20 VE) - The hexaaqua copper(II) ion [Cu(H2O)6]2+ (21 VE)
Often, cases where complexes have more than 18 valence electrons are attributed to electrostatic forces - the metal attracts ligands to itself to try to counterbalance its positive charge, and the number of electrons it ends up with is unimportant. In the case of the metallocenes, the chelating nature of the cyclopentadienyl ligand stabilizes its bonding to the metal. Somewhat satisfying are the two following observations: (i) cobaltocene is a strong electron donor, readily forming the 18-electron cobaltocenium cation and (ii) nickelocene tends to react with substrates to give 18-electron complexes, e.g. CpNiCl(PR3) and free CpH.
See also
- Electron countingElectron countingElectron 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 ruleOctet ruleThe octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low (The octet rule is a chemical rule of thumb that states that atoms of low (...
- Ligand field theoryLigand field theoryLigand field theory describes the bonding, orbital arrangement, and other characteristics of coordination complexes. It represents an application of molecular orbital theory to transition metal complexes. A transition metal ion has nine valence atomic orbitals, five d, one s, and three p orbitals...
- d electron countD electron countThe 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 and reactivity of transition metal complexes...