R.S. Mulliken proposed an electronegativity scale in which the Mulliken electronegativity, ΧM is related to the electron affinity EAv (a measure of the tendency of an atom to form a negative species) and the ionization potential IEv (a measure of the tendency of an atom to form a positive species) by the equation:
ΧM = (IEv + EAv)/2
The subscript v denotes a specific valence state - so for trigonal boron compounds, a values of electronegativity can be defined for sp2 hybrid orbitals. If the values of IE and EA are in units of MJ mol-1, then the Mulliken electronegativity ΧM can be expressed on the Pauling scale by the relationship:
ΧM = 3.48[(IEv + EAv)/2 - 0.602]
Values presented here are largely from reference 1. Mulliken's proposals are to be found in references 2 and 3. Tables of values are also given in references 4 and 5 as well.
Mulliken-Jaffe electronegativities are tabulated for a number of orbitals as well as a consolidated set of values for various orbitals suitable to correlate with Pauling values:
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