Why gold has two valencies ?
Answers
Gold is an element of group 11 ans their electron configuration is 4f14 5d10 6s1.
The most common valency is +1 which leaves 4f14 5d10 ‘f’ and ‘d’ orbitals full and the ‘s’ orbital empty, which gives stability.
The most common elemental step (and its reverse) in coordination chemistry and organo-metalic chemistry are oxidative addition/reductive elimination, which respetively increase/decrease the valence of the metal in 2.
Thus Au+3 is formed with a configuration 4f14 5d7 6s2. ‘f’ and ‘s’ orbitals are full and orbitals ‘d’ have 7 electrons. In a complex, where you have Au+3, these orbitals are further divided depending on the geometry. Au+3 is usually square planar, which mean a structure like that:
_ dx2-y2
_dxy (1 e-, half full)
_ _ _ dxz, dyz, dz2 (6 e-, full)
This configuration allow to have 3 full and stable orbitals and 1 half full, which is not so unstable.