Question

What is meant by auxiliary valency?

Answer 1

Explanation:

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.