For example, potassium octachlorodimolybdate salt (K4[Mo2Cl8]) contains the [Cl4Mo≣MoCl4]4− anion, in which the two Mo atoms are linked to each other by a bond with order of 4.
A bond of order 6 is detected in ditungsten molecules W2, which exist only in a gaseous phase.
[8] Hückel molecular orbital theory offers another approach for defining bond orders based on molecular orbital coefficients, for planar molecules with delocalized π bonding.
For more elaborate forms of molecular orbital theory involving larger basis sets, still other definitions have been proposed.
[11] A standard quantum mechanical definition for bond order has been debated for a long time.
[12] A comprehensive method to compute bond orders from quantum chemistry calculations was published in 2017.
Pauling suggested a value of 0.353 Å for b, for carbon-carbon bonds in the original equation:[13] The value of the constant b depends on the atoms.
This definition of bond order is somewhat ad hoc and only easy to apply for diatomic molecules.