HOME

TheInfoList



OR:

The covalent bond classification (CBC) method is also referred to as the LXZ notation. It was published by M. L. H. Green in 1995 as a solution for the need to describe covalent compounds such as
organometallic Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and s ...
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 charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
. Instead of simply assigning a charge to an atom in the molecule (i.e. the oxidation state), the covalent bond classification method analyzes the nature of the
ligand In coordination chemistry, a ligand is an ion or molecule (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 elect ...
s surrounding the atom of interest, which is often a
transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
. According to this method, there are three basic types of interactions that allow for coordination of the ligand. The three types of interaction are classified according to whether the ligating group donates two, one, or zero electrons. These three classes of ligands are respectively given the symbols L, X, and Z.


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 Electron 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 rule is used with Lewis structures for main group eleme ...
, or donate two electrons to the metal when using the donor pair method of electron counting. Regardless of whether it is 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 atoms ...
. /sup> 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. /sup> The bonds formed between these ligands and the metal are
dative covalent bonds In coordination chemistry, a coordinate covalent bond, also known as a dative bond, dipolar bond, or coordinate bond is a kind of two-center, two-electron covalent bond in which the two electrons derive from the same atom. The bonding of metal ...
, 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. /sup> 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 spe ...
, such as BR3. /sup>


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 io ...
, number of d-electrons,Spessard, Gary; Miessler, G. Organometallic Chemistry: 2nd edition. Oxford University Press, 2010; pg. 59-60. valence number and the ligand bond number /sup> 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 of 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. /sup> In other words, the equivalent neutral class is the representation of the complex as though there was no charge.


References

{{Reflist Chemical bonding