Metal-ligand bonding orbitals definition

If the metal-ligand bonding is not symmetric due to the M-L axis, two AOM parameters are generally required for describing the energy effect on the metal d-orbitals (cf. Sect. 2.2). The definitions for the corresponding quantities and... 

While compounds with a formal MM quadruple bond have been isolated and studied since the mid-1960s, the first report of a compound with a formal MM quintuple bond was as recent as 2005. Bond order is a concept that is difficult to distinguish experimentally. For example, there are dichromium compounds with formal quadruple bonds that have shorter Cr—Cr bond lengths than those with quintuple bonds. Mixing of the metal and ligand molecular orbitals complicates trying to assign a definitive electronic structure in theoretical studies and also reduces the calculated bond order between the metals. Therefore, we use the term quintuple bond in this section to simply illustrate that five electron pairs are involved in the metal-metal interaction [82]. 

However, this definition cannot be applied to the many organometallic compounds in which two or more adjacent atoms of a ligand interact with the central metal atom through what is often a combination of cr, n and 8 bonding (the labels cr, ji or 8 referring to the symmetry of the orbital interactions between ligand and central atom). 

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