Question

As I was reading about nephelauxetic effects, which is the effect that atomic d-orbitals are bigger in a complex than in gaseous metal ions. The Racah interelectronic repulsion parameter gets smaller, indicating that doubly occupied d-orbitals are bigger in complexes than in an atom (why only doubly occupied though, shouldn't there also be less repulsion between, lets say singly occupied d-orbitals?). I wondered what reasons there could be for that. The two most cited reasons are:

decreased effective nuclear charge, partially neutralized by the ligand sphere
formation of covalent bond character with the ligand
The first option seems fine enough for me, but 2) makes little sense for me. The ligand field splitting arises because the partially filled d-orbitals want to avoid the double occupied ligand orbitals, therefore assuming a position where the electronic repulsion is smallest.

Why would they form covalent bonds then, or is that just a consequence of MO theory that we have to acccept?

Answer 1

In CFT, the reasons behind the nephelauxetic effect include electrostatic interactions and the degree of covalency in bonds within complexes. Crystal field splitting and the nature of ligands influence color and magnetic properties.

Step-by-step explanation:

The question concerns the reasons behind the nephelauxetic effect and the role of ligands in influencing electronic properties of metal complexes as described by crystal field theory (CFT).

In regards to why covalent bonding characteristics are mentioned even though CFT primarily considers electrostatic interactions, it is essential to note that while CFT simplifies the interaction as purely electrostatic, in reality, there is some degree of covalency in the bonds within metal complexes.

This level of covalency implies some overlap between metal d-orbitals and ligand orbitals, hence the formation of molecular orbitals. The nature of the ligands, their ability to induce crystal field splitting, and the resulting high-spin or low-spin configurations directly influence observable properties like color and magnetism of the complexes.