Bond order is the number of chemical bonds between a pair of atoms. For example, in diatomic nitrogen N≡N the bond order is 3, in acetylene H−C≡C−H the bond order between the two carbon atoms is also 3, and the C−H bond order is 1. Bond order gives an indication of the stability of a bond. Isoelectronic species have same bond number.
In molecules that have resonance or nonclassical bonding, bond order may not be an integer. In benzene, the delocalized molecular orbitals contain 6 pi electrons over six carbons essentially yielding half a pi bond together with the sigma bond for each pair of carbon atoms, giving a calculated bond order of 1.5. Furthermore, bond orders of 1.1, for example, can arise under complex scenarios and essentially refer to bond strength relative to bonds with order 1.
Bond order in molecular orbital theory
In molecular orbital theory, bond order is also defined as half the difference between the number of bonding electrons and the number of antibonding electrons as per the equation below. This often but not always yields similar results for bonds near their equilibrium lengths, but it does not work for stretched bonds. Bond order is also an index of bond strength and is also used extensively in valence bond theory.
Generally, the higher the bond order, the stronger the bond. Bond orders of one-half may be stable, as shown by the stability of H+
2 (bond length 106 pm, bond energy 269 kJ/mol) and He+
2 (bond length 108 pm, bond energy 251 kJ/mol).
The bond order concept is used in molecular dynamics and bond order potentials. The magnitude of the bond order is associated with the bond length. According to Linus Pauling in 1947, the bond order is experimentally described as
where is the single bond length, is the bond length experimentally measured, and b is a constant, depending on the atoms. Pauling suggested a value of 0.353 Å for b, for carbon-carbon bonds in the original equation:
The value of the constant b depends on the atoms.
The above definition of bond order is somewhat ad hoc and only easy to apply for diatomic molecules. Hückel MO theory offers another approach for defining bond orders based on MO coefficients. Since the theory divides bonding into a sigma framework and a pi system, the Hückel definition is only applicable to planar molecules with delocalized π bonding. Assuming a bond order contribution of 1 from the sigma component it gives a total bond order (σ + π) of 1.67 for benzene rather than the commonly cited 1.5, showing some degree of ambiguity in how the concept of bond order is defined. A standard quantum mechanical definition for bond order has been debated for a long time. A comprehensive method to compute bond orders from quantum chemistry calculations was recently published.
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- Pauling, Linus (March 1, 1947). "Atomic Radii and Interatomic Distances in Metals". Journal of the American Chemical Society. 69 (3): 542–553. doi:10.1021/ja01195a024.
- IUPAC Gold Book bond order