The covalent bond classification (CBC) method, also referred to as LXZ notation, is a way of describing

covalent compound A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...

s such as organometallic complexes in a way that is not prone to limitations resulting from the definition of

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...

. Instead of simply assigning a

charge Charge or charged may refer to: Arts, entertainment, and media Films * ''Charge, Zero Emissions/Maximum Speed'', a 2011 documentary Music * ''Charge'' (David Ford album) * ''Charge'' (Machel Montano album) * '' Charge!!'', an album by The Aqu ...

(oxidation state) to an atom in the

molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

, the covalent bond classification method analyzes the nature of the

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

s surrounding the atom of interest.The CBC Method
/ref> According to this method, the interactions that allow for

coordination Coordination may refer to: * Coordination (linguistics), a compound grammatical construction * Coordination complex, consisting of a central atom or ion and a surrounding array of bound molecules or ions ** A chemical reaction to form a coordinati ...

of the ligand can be classified according to whether it donates two, one, or zero

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

s. These three classes of ligands are respectively given the symbols L, X, and Z. The method was published by Malcolm L. H. Green in 1995.

Types of ligands

X-type ligands are those that donate one electron to the metal and accept one electron from the metal when using the neutral ligand method of

electron counting 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 chemical bond, bondi ...

, or donate two electrons to the metal when using the donor pair method of electron counting. Regardless of whether they are considered neutral or anionic, these ligands yield normal

covalent bonds A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...

. A few examples of this type of ligand are H, halogens (Cl, Br, F, etc.), OH, CN, CH₃, and NO (bent). L-type ligands are neutral ligands that donate two electrons to the metal center, regardless of the electron counting method being used. These electrons can come from lone pairs, pi, or sigma donors. The bonds formed between these ligands and the metal are dative covalent bonds, which are also known as coordinate bonds. Examples of this type of ligand include CO, PR₃, NH₃, H₂O, carbenes (=CRR'), and alkenes. Z-type ligands are those that accept two electrons from the metal center, as opposed to the donation occurring with the other two types of ligands. However, these ligands also form dative covalent bonds like the L-type. This type of ligand is not usually used because in certain situations it can be written in terms of L and X. For example, if a Z ligand is accompanied by an L type, it can be written as X₂. Examples of these ligands are

Lewis acids A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...

, such as BR₃.

Uses of the notation

When given a metal complex and the trends for the ligand types, the complex can be written in a more simplified manner with the form . The subscripts represent the numbers of each ligand type present in that complex, M is the metal center, and Q is the overall charge on the complex. Some examples of this overall notation are as follows: Also from this general form, the values for electron count, oxidation state,

coordination number In chemistry, crystallography, and materials science, the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules or ions bonded to it. The ion/molecule/atom surrounding the central ion ...

, number of d-electrons, valence number and the ligand bond number can be calculated. Electron Count = Where is the group number of the metal. Oxidation State (OS) = Coordination Number (CN) = Number of d-electrons (dn) = = Valence Number (VN) = Ligand Bond Number (LBN) =

Other uses

This template for writing a metal complex also allows for a better comparison of molecules with different charges. This can happen when the assignment is reduced to its “equivalent neutral class". The equivalent neutral class is the classification of the complex if the charge was localized on the ligand as opposed to the metal center. In other words, the equivalent neutral class is the representation of the complex as though there were no charge.

References

{{Reflist Chemical bonding