[arvind1990]

In complexes , a strong ligand such as CN, can force the electron to be paired even though the sub shells are not singly occupied.
This is against Hund's Rule.

Can this be considered as a defect or drawback of Hund's rule??

[Dan]

No, because the d orbitals are not degenerate in a complex.

Hund's rule of max multiplicity: For a given configuration, the term with the greatest multiplicity lies lowest in energy.

I does not specify, for example in an Fe(II) octahedral complex, whether the lowest energy configuration is t_2g⁴ e_g² or t_2g⁶, it only specifies that the lowest energy term in the former case has S=2.

[arvind1990]

Hi,

I didn't get you.Can you explain me more in detail siting examples.

[Dan]

In a complex, the d orbitals are no longer equal in energy - this is due to ligand field splitting. It will be very difficult to explain this without diagrams, so read a little about crystal field theory/ligand field theory if you are not familiar with this.

Think about B. As I'm sure you agree, the ground electronic configuration of B is [He]2s²2p¹. Hund's rule does not imply that the lowest energy configuration is [He]2s¹ 2p², even though this configuration could have higher multiplicity. Hund's rule can not be used to predict the lowest energy configuration - it says that for a given configuration, the lowest energy term is that with the highest multiplicity - so it says that if the configuration in boron is [He]2s¹ 2p² (which it isn't, but bear with me), then the lowest term of that specific configuration is the one in which the two electrons in the p orbitals are unpaired.

Now, Fe(II) has 6 d electrons. But it is not enough to say that the lowest energy configuration is [Ar]3d⁶,  for a complex - because the configuration 3d⁶ is not specific enough. In a complex, the d orbitals must be considered seperately from each other.
In an octahedral comlex, it turns out that three of the d orbitals are lower in energy (which have t_2g symmetry), and the other two d orbitals are higher in energy (which have e_g symmetry).
So the label 'd' is useless (if the energy difference is large). We must consider the d orbitals (or "d subshell") as t_2g and e_g "sub-subshells".

So just as Hund's rule does not predict whether B is [He]2s²2p¹ or [He]2s¹ 2p²,

it does not predict whether an octahedral Fe(II) complex is [Ar]t_2g⁶  or [Ar]t_2g⁴  e_g²

Sorry, that was a bit long, but hopefully you can see what I'm getting at!

[arvind1990]

Hi,

Thank You For your reply.
You have given that Hund's rule does not predict whether B is [He]2s2 2p1 or [He]2s1 2p2,
but,it states that the electrons must be occupied the second time after each of the subshell are singly occupied.i.e., it states clearly that B is[He]2s2 2p1 and not [He]2s1 2p2.

By the way, can you explain me what do you mean by t2g and eg symmetry. I would be glad if you spend time on this.
                 Sorry..

[Dan]

Sorry! I was talking specifically about Hund's rule of maximum multiplicity, I should have made that clearer, my fault. The Aufbau principle (filling of lowest energy orbitals before starting to fill the next) does indeed start treading on thin ice when the energy seperation between the levels is small, because the Aufbau principle does not consider the energy cost of pairing two electrons.
I interpreted your first post as questioning the validity of Hund's rule of maximum multiplicity for transition metal complexes - which it is valid for.

t_2g and e_g are symmetry labels. They provide information about the behaviour of the orbitals on application of several different symmetry operations. If you want to know more about that, read about "group theory". The symmetry of the orbitals affects how they interact with ligands (this is the origin of ligand field splitting) but they are just labels.