How Do Vacant d-Orbitals Facilitate the Formation of Transition Metal Complexes?

[chatelier]

one reason for transition metal to form complexes is that it has some vacant d-orbitals

but i wonder how this can really help the formation of complexes

as in a complex, electrons are donated by ligands to stabilize the positive charge of metal cation

there is just no need to promote electrons from lower energy level to higher one, isn't there?

so how does the vacant d-orbital have a role in forming the complexes?

 

[GCT]

I'm not sure what you're referring to here, but you should consider the orbitals involved in whichever two particular reactions you're comparing.

Also, you may be interested in the subject of charge transfer complexes, which does involve excitation.