Ligand bond number (LBN) represents the effective total number of ligands or ligand attachment points surrounding a metal center, labeled M. More simply, it represents the number of
coordination sites occupied on the metal. Based on the
covalent bond classification method (from where LBN is derived), the equation for determining ligand bond number is as follows:
:LBN = L + X + Z
Where L represents the number of neutral ligands adding two electrons to the metal center (typically
lone electron pairs,
pi-bonds and
sigma bonds. Most encountered ligands will fall under this category. X represents covalent-bonding ligands such as
halogen
The halogens () are a group in the periodic table consisting of six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and the radioactive elements astatine (At) and tennessine (Ts), though some authors would ...
anions. Z represents, though rarely encountered electron accepting ligands or
dative bond forming ligands. The ligand bond number convention is most commonly encountered within
inorganic chemistry
Inorganic chemistry deals with chemical synthesis, synthesis and behavior of inorganic compound, inorganic and organometallic chemistry, organometallic compounds. This field covers chemical compounds that are not carbon-based, which are the subj ...
and it's related fields
organometallic chemistry
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 so ...
and
bioinorganic chemistry.
Comparisons with
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 ...
On comparison to the classical coordination numbers, some major differences can be seen. For example, (
η5–
cyclopentadienyl Cyclopentadienyl can refer to
* Cyclopentadienyl anion, or cyclopentadienide,
** Cyclopentadienyl ligand
* Cyclopentadienyl radical, •
* Cyclopentadienyl cation,
See also
* Pentadienyl
{{Chemistry index ...
)
2Cr (ML
4X
2) and (η
6–
benzene
Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...
)
2Cr (ML
6) both have a LBN of 6 as compared to classical coordination numbers of 10 and 12.
Well known complexes such as
Ferrocene
Ferrocene is an organometallic chemistry, organometallic compound with the formula . The molecule is a Cyclopentadienyl complex, complex consisting of two Cyclopentadienyl anion, cyclopentadienyl rings sandwiching a central iron atom. It is an o ...
and
Uranocene
Uranocene, U(C8H8)2, is an organouranium compound composed of a uranium atom sandwiched between two cyclooctatetraene, cyclooctatetraenide rings. It was one of the first Organoactinide chemistry, organoactinide compounds to be synthesized. It is a ...
also serve as examples where LBN and coordination number differ. Ferrocene has two η
5 cyclopentadienyl ligands while Uranocene has two η
8 cyclooctatetraene
1,3,5,7-Cyclooctatetraene (COT) is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as nnulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature. Because of ...
ligands; however, by covalent bond classification the complexes are found to be ML
4X
2 and ML
6X
4.
This corresponds to LBN values of 6 and 10 respectively, even though the total coordination numbers would be 10 and 16. The usefulness of LBN to describe bonding extends beyond just
sandwich compound
In organometallic chemistry, a sandwich compound is a chemical compound featuring a metal bound by hapticity, haptic, covalent bonds to two arene compound, arene (ring) ligands. The arenes have the formula , substituted derivatives (for example ...
s. Co(CO)
3(NO) is a stable
18-electron complex in part due to the bonding of the
NO ligand in its linear form. The donation of the lone pair on the nitrogen makes this complex ML
4X, containing 18 electrons. The traditional coordination number here would be 4, while the CBC more accurately describes the bonding with a LBN of 5. In simple cases, the LBN is often equal to the classical coordination number (ex. Fe(CO)
5, etc.)
Ligand bond plots
{{multiple issues, section=yes,
{{confusing, section, date=June 2021
{{cleanup, section, reason=contains unencyclopedic and vague images that should be replaced or removed., date=June 2021
The LBN for transition metals trends downward from left to right across the periodic table. This trend is highlighted in the LBN plots of Groups 3 through 10. Groups exhibit trends, but the LBN for individual complexes can vary.
File:Ligand Bond Number Group 3.jpg
File:Group 4.jpg
File:Group 5.jpg
File:Group 6.jpg
File:Group 7.jpg
File:Group 8.jpg
File:Group 9.jpg
File:Group10.jpg
References
Chemical bonding