[Winga]

Why 4s electron(s) of 3d transition metals is/are counted as 3d orbitals's when the metal forming a complex with the ligands?

[Mitch]

give an example

[Winga]

For example,

e- configuration:
Ni: [Ar] 3d8 4s2

For the complex Ni(CO)4, Ni is said to be having 10 d-electrons, why?

[maxyoung]

the metal center is sp3 hybridized. The 4s orbital now is occupied by the ligands.  All the 10 e-s of the metal center now will occupy the d orbitals.

Also can see from the MO diagram. All the bonding orbitals are filled with electrons from the ligands. Electrons from the metal center will occupy the t2 and e* orbitals before they occupy the antibonding orbitals. So the ten electrons all willl be counted as d electrons in the complex.

[Winga]

Can we use stability approach to explain it?

For example, e- in 3d-orbital is more stable than that in 4s-orbital when the complex is formed.

[Mitch]

The reason is not straight forward you have to look at a molecular orbital diagram. What they have taught you is how to count a theoritical construct which they call "d-electron count" using electrons from d and s. The term d-electron count is useful because it describes how many electrons sit on their butt neither bonding nor going antibonding(depending of crystal field splitting--but ignore for now) and just sit in 3 d-orbitals of t_2g symmetry.