Why transition metals can have unpaired electrons in their compounds?

[sludger13]

Why can have transition metals unpaired electrons in their compounds? In correlates to their multiple oxidation states, but I still don't know the explanation of it, that would make me satisfied - I suppose it's mathematical, as molecular orbitals are creating. Or is there any explanation?

 

[cgk]

In most simple compounds electrons occur paired in bonds, lone pairs, and core orbitals. However, the degree to which forming bonds lowers the molecules energy (in comparison to every atom just keeping its own electrons in a high-spin configuration and not forming bonds) is related to the degree to which the atomic valence orbitals overlap, from which the bonds would be formed.

The main reason why many transition metal compounds can keep unpaired electrons is that overlap of their d electrons to their ligands' s- and p-orbitals is not large enough to make forming actual covalent bonds energetically favorable over both the ligands and the metal just keeping their own electrons, rather than sharing them.

 

[sludger13]

Thank you for your answer!
It's clear molecular orbital would have higher potential energy than atomic orbitals.

The main reason why many transition metal compounds can keep unpaired electrons is that overlap of their d electrons to their ligands' s- and p-orbitals is not large enough to make forming actual covalent bonds

I can't see why some of (d) electrons can have sufficient overlap and others can not, though all of them are (circa) equally far from a nucleus and have the same energy level. Is it due to electron repulsion? Can anybody explain me?
E.G. Iron (III) oxide has one unpaired (d) electron, right? Which orbital is that? What impede it to create -lower energy- bonding orbital?

 

[sludger13]

Can anyone just refer to some sources, please.

 

[DrDu]

d-orbitals don't point all in the same direction, so you can't expect all bonding overlaps to be equally strong. Fe2O3 is a high spin complex, meaning that all 5 d-electrons are unpaired.
See, e.g. http://en.wikipedia.org/wiki/Ligand_field_theory

 

[Barry Bonobo]

Everything is explained here sufficiently but if you want a source, I recommend Bowser's Inorganic Chem.