In the inorganic chemistry and organometallic chemistry of transition metals, electron counting is a formalism used for characterizing a compound and for understanding its electronic structure and bonding. Inorganic chemistry is the branch of chemistry concerned with the properties and reactions of inorganic 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 the d-block of the periodic table, including zinc and scandium. ... Dutch East India Company bond, issued in 1623. ...

The valence shells of a transition metal are filled when they contain 18 electrons: 2 each in the 5 d orbitals, or 10 total; 2 each in the 3 p orbitals, or 6 total; and finally 2 in the single valence shell s orbital. The valence shell is the outermost shell of an atom, which contains the electrons most likely to account for the nature of any reactions involving the atom and of the bonding interactions it has with other atoms. ... Properties The electron is a subatomic particle. ...

The electrons contributed by the metal atom or ion are summed with the electrons contributed by each ligand (ie, those valence electrons of each ligand participating in some way in a bonding interaction with the metal, and not otherwise occupied only in intraligand bonding or in lone-pairs not interacting with the metal center). An ion is an atom or group of atoms with a net electric charge. ... In chemistry, a ligand is an atom, ion or functional group that is bonded to one or more central atoms or ions, usually metals generally through co-ordinate covalent bond. ...

A compound or ion which satisfies this 18 electron rule is considered to be qualitatively more stable than other configurations or electronic states of the molecule.

The "18 electron rule" applies mostly to organometallic complexes, compounds in which there are metal-carbon bonds. Coordination compounds without metal-carbon bonds don't usually obey the "18 electron rule."

There are two different approaches one can use when counting electrons, each arriving at the same total. The constituents (ie, metal and ligands) can be regarded as ions, or as neutral species.

Using ferrocene as an example, and using the neutral approach first, the iron atom has 8 valence electrons. Each of the two cyclopentadiene radicals contributes 5 electrons, totalling 10 electrons from the ligands. Ferrocene Fe(C5H5)2 is the prototypical metallocene, a type of organometallic chemical compound, consisting of two cyclopentadienyl rings bound on opposite sides of a central iron atom and forming an organometallic sandwich compound. ... General Name, Symbol, Number iron, Fe, 26 Chemical series transition metals Group, Period, Block 8, 4, d Appearance lustrous metallic with a grayish tinge Atomic mass 55. ... Cyclopentadiene, with molecular formula C5H6, is a clear, colorless, liquid organic chemical, with an odor reminiscent of turpentine or camphor. ... Radical is derived from the Latin word radix, which means root. In various fields of endeavor, it can mean: Sciences in chemistry, either an atom or molecule with at least one unpaired electron, or a group of atoms, charged or uncharged, that act as a single entity in reaction. ...

 10+8=18 

Using the ionic approach, iron is taken in its common oxidation state Fe²⁺, contributing only 6 valence electrons. However, the cyclopentadiene moieties are counted as aromatic cyclopentadiene anions, contributing 6 electrons each as well. The oxidation state or oxidation number is defined as the sum of negative and positive charges in an atom, which indirectly indicates the number of electrons it has accepted or donated. ... An anion is an ion with negative charge. ...

 6+6+6=18 

The utility of electron 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 create by formally removing one of the cyclopentadienyl ligands from ferrocene and replacing it with some number of carbon monoxide ligands? Carbon monoxide, chemical formula CO, is a colourless, odourless, flammable and highly toxic gas. ...

Using the ionic approach, removing one cyclopentadienyl anion yields a cationic fragment containing one cyclopentadienyl (Cp) fragment and 12 valence shell electrons. Since each carbon monoxide ligand contributes 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: In chemistry, a cationic species is one that contains a full positive charge. ...

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. 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

 Cp 5 Fe 8 CO 4 Fe 1 ---- 18