What does auxiliary valence mean?

Answer 1

This is an old term used by Werner (1866-1919) in a description of the bonding of transition metal complexes. As an example in the complex CoCl3.6NH3 he proposed that Co bonded to the 6 ammonia molecules using its "auxiliary" or secondary valence and to the chloride by its main valence. Up until then there was debate as to how Co with a valency of could attach 6 NH3 in addition to bonding with 3 Cl.

This essentially the model that is now accepted and we term NH3 as ligands and the Co(NH3)63+ (now known to be octahedral) , as a complex of Co3+

Werner is sometimes called the father of coordination chemistry.

Answer 2

Auxiliary valence typically refers to the capacity for additional bonding or coordination by an atom in a molecule. These extra bonding opportunities can arise from the presence of empty or partially filled valence orbitals that allow the atom to form additional bonds beyond its usual valence requirements, leading to the concept of auxiliary (or extra) valence for that atom.