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The 18-electron rule is a rule of thumb used primarily in transition metal chemistry for characterizing and predicting the stability of metal complexes. A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation Valence shells of a transition metal can accommodate 18 electrons: 2 in each of the five d orbitals (10 in total); 2 in each of the three p orbitals (6 in total); and 2 in the s orbital (see Electron counting). An electron shell may be crudely thought of as an Orbit followed by Electrons around an Atom nucleus. In Chemistry, the term transition metal (sometimes also called a transition element) has two possible meanings It commonly refers to any element in The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J Electron counting is a formalism used for classifying compounds and for explaining or predicting electronic structure and bonding. In practice, of course, these orbitals cannot directly accept electrons, otherwise one would encounter ions such as Fe10− and Pt8−. However, combination of these atomic orbitals with ligand orbitals gives rise to nine molecular orbitals which are either metal-ligand bonding or non-bonding. An atomic orbital is a Mathematical function that describes the wave-like behavior of an electron in an atom In Chemistry, a ligand is either an Atom, Ion, or Molecule (see also Functional group) that bonds to a central metal generally In Chemistry, a molecular orbital (or MO) is a region in which an Electron may be found in a Molecule. (There are also some higher energy anti-bonding orbitals). The complete filling of these nine lowest energy orbitals with electrons, whether those electrons originate from the metal or from any ligands, is the basis of the 18-electron rule. In Chemistry, a ligand is either an Atom, Ion, or Molecule (see also Functional group) that bonds to a central metal generally When the metal has 18 electrons, it has then achieved the same electron configuration as the noble gas at the end of the period. 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

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Application of the 18-electron rule

Many metal complexes do not satisfy the 18-electron rule. It is, however, especially useful for organometallic complexes of the Cr, Mn, Fe, and Co triads, and applies to compounds such as ferrocene, iron pentacarbonyl, chromium carbonyl and nickel carbonyl. Ferrocene is the Organometallic compound with the formula Fe(C5H52 Iron pentacarbonyl, also known as iron carbonyl, is the compound with formula (5 Chromium carbonyl, also known as chromium hexacarbonyl, is the Chemical compound with the formula Cr(CO6 Nickel carbonyl ( IUPAC name tetracarbonylnickel) is a colorless Organometallic complex that is a versatile reagent first described in In compounds such as these, the nine bonding molecular orbitals are all low in energy. Because putting electrons into them is a favourable process, and as each orbital can take two electrons, the greatest stability is achieved when there are a total of 18 electrons in these orbitals - this includes both the electrons that come from the metal, and those donated to it from the ligands. This is the basis of the 18-electron rule. This stability is such that much chemistry is guided by a metal's need to retain or get 18 electrons.

The ligands in a complex play an important role in determining whether or not it obeys the 18-electron rule. Generally, complexes that do obey the rule have ligands that are π-acids. This kind of ligand typically exerts a very strong ligand field, which causes the resultant molecular orbitals to be very low in energy and thus makes it good to fill them. Ligand field theory (LFT describes the bonding in Coordination complexes. Typical ligands include olefins, phosphines and carbonyls. In Organic chemistry, an alkene, olefin, or olefine is an unsaturated Chemical compound containing at least one Carbon Phosphine is the common name for phosphorus hydride (PH3 also known by the IUPAC name phosphane and occasionally phosphamine. In Organic chemistry, a carbonyl group is a Functional group composed of a Carbon Atom double-bonded to an Oxygen Metals form the best complexes with π-acids when the metal is in a low-oxidation state (because then you get good overlap of metal and ligand orbitals, and the metal can donate electrons back to the ligand in a synergic fashion), so complexes that obey the 18-electron rule generally have the metal in a low-oxidation state too.

This is not to say that all complexes with a low oxidation-state metal and π-acidic ligands have 18 electrons - see below for counterexamples. It is also not to say that if a metal is in a high oxidation state or does not have π-acidic ligands it cannot have 18 electrons.

Compounds that obey the 18 VE rule are typically "exchange inert," such as [Co(NH3)5Cl]2+ and [Fe(CN)6]4-.

As the rule is essentially the result of filling the valence orbitals of the metal by covalent bonding between metal and ligands, metals that display largely ionic chemistry don't obey it. This includes the s-block metals, the lanthanides and the actinides. Terminology The Trivial name " Rare earths " is sometimes used to describe all the lanthanoids together with Scandium and Yttrium History of the actinoid series From the earlier known chemical properties of actinium (89 up to uranium (92 indicating a relation to the Transition metals it was generally It can, however, be compared to the octet rule for carbon, which equally makes enough covalent bonds to fill its valence orbitals. 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

Using the 18-electron rule

The usefulness of the 18-electron rule becomes more apparent when one considers what chemical transformations or derivatives might be readily accessible. For example, what piano stool compound might one be able to prepare by formally removing one of the cyclopentadienyl ligands from ferrocene and replacing it with some number of carbon monoxide ligands?

Using the ionic approach, removing one cyclopentadienyl anion yields a cationic fragment containing one cyclopentadienyl (Cp) fragment and 12 valence shell electrons. sandwich compound in Organometallic chemistry is any Chemical compound containing a Metal Atom sandwiched between two arene units Carbon monoxide, with the chemical formula CO is a colorless odorless tasteless yet highly toxic Gas. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge Since each carbon monoxide ligand contributes 2 electrons (3 CO ligands give the requisite 6 electrons), it should be possible to create an iron-containing complex cation containing one cyclopentadienyl group, one iron atom, and 3 carbon monoxide ligands:

CpFe(CO)3+

What one finds is that the iron complex satisfies the 18 electron count another way, by forming a dimer with an Fe-Fe bond (see Cyclopentadienyliron dicarbonyl dimer). Cyclopentadienyliron dicarbonyl dimer is an Organometallic compound with the formula (C5H52Fe2(CO4 also abbreviated Counting electrons for just one iron center can be done by considering the other iron as contributing 1 electron to the count:

[CpFe(CO)2]2
neutral counting: Cp 5 + Fe 8 + 2 CO 4 + Fe 1 = 18

Another stable compound is obtained, when one small monoanionic ligand is used:

CpFe(CH3)(CO)2
Neutral counting: Cp 5 + Fe 8 + CH3 1 + 2 CO 4 = 18

Deviations from the 18-electron rule

The 18-electron rule is just that - a rule, not a law. Electron counting is a formalism used for classifying compounds and for explaining or predicting electronic structure and bonding. Many transition metal complexes do not follow this rule, and, furthermore, compounds which have fewer than 18 valence electrons tend to show enhanced reactivity. In fact, 18 electrons is often a recipe for non-reactivity in either a stoichiometric or catalytic sense. Stoichiometry (sometimes called reaction stoichiometry to distinguish it from composition stoichiometry is the Calculation of Quantitative (measurable Catalysis is the process in which the rate of a Chemical reaction is increased by means of a Chemical substance known as a catalyst The fact that the rule is broken often and is mainly a guide to unreactive species does not detract from its usefulness - it remains an invaluable guide for the classification of compounds and as a predictor of structures and mechanism. For example, 18-electron compounds almost invariably react with donor ligands via pathways that are dissociative, whereas most catalytic processes rely on agents that react via associative steps. 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 like processes.

Violations to the 18-electron rule can be classified according to four main classes of complexes:

Bulky ligands

Bulky ligands can preclude the approach of additional ligands that would allow the metal to achieve the 18 electron configuration. Similarly, it is often not possible to fit sufficient numbers of ligands around early metals to get to 18 electrons. Examples:

Sometimes such complexes engage in agostic interactions with the hydrocarbon framework of the bulky ligand. 12345-Pentamethylcyclopentadiene is a cyclic diolefin with the formula C5Me5H (Me = CH3 For example:

High spin complexes

High spin metal complexes have singly-occupied orbitals and may not have any empty orbitals which ligands could donate electron density into. Generally, 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. oxygen), or addition of a strong field ligand can cause electron-pairing, thus creating a vacant orbital that it can donate into. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Ligand field theory (LFT describes the bonding in Coordination complexes. 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.

'Inaccessible' high energy orbitals

"Late" transition metals, located toward the right half of the periodic table, often violate the 18-electron rule. Here, one or more of the d-orbitals is of high energy, and does not accept electrons. Examples:

Combinations

The above factors can sometimes combine. Vaska's complex is the Trivial name for the Chemical compound trans -chlorocarbonylbis(triphenylphosphineiridium(I which has the formula IrCl(CO2 Zeise's salt is the Chemical compound with the formula K]Cl3(C2H4] Examples include

Higher electron counts

Some complexes have more than 18 electrons. Titanium tetrachloride or titanium(IV chloride is the Chemical compound with the formula TiCl4 Examples:

Often cases where complexes have more than 18 valence electrons are attributed to electrostatic forces - the metal attracts ligands to itself to try and counterbalance its positive charge, and the number of electrons it ends up with is unimportant. Cobaltocene, Co(C5H52 is known as bis(cyclopentadienylcobalt(II or even "bis Cp cobalt Nickelocene is the Organometallic compound with the formula Ni( η5 -C5H52 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.

References

See also

Electron counting is a formalism used for classifying compounds and for explaining or predicting electronic structure and bonding. 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 Ligand field theory (LFT describes the bonding in Coordination complexes.
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