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 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 complexes in a way that is not pro ...
(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 five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). In the modern IUPAC nomenclature, this group is ...
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 synthesis and behavior of inorganic and organometallic compounds. This field covers chemical compounds that are not carbon-based, which are the subjects of organic chemistry. The distinction between the two disci ...
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
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
In organic chemistry, pentadienyl refers to the organic radic ...
)
2Cr (ML
4X
2) and (η
6–
benzene)
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 compound with the formula . The molecule is a complex consisting of two cyclopentadienyl rings bound to a central iron atom. It is an orange solid with a camphor-like odor, that sublimes above room temperature, a ...
and
Uranocene also serve as examples where LBN and coordination number differ. Ferrocene has two η
5 cyclopentadienyl ligands while Uranocene has two η
8 cyclooctatetraene 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 compounds. 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
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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