waLLcandy - 11-11-2011 at 22:34
I have two square planar complexes:
cis-(NH3)2PtCl2
trans-(NH3)2PtCl2
I'm trying to determine which has the greater (spin allowed) d-d electronic transition. This problem should relate to the fact that the trans complex
is symmetric to inversion, while the cis complex isn't. (Maybe)
I found some very vague example that related this to circular dichroism--but everything I found online became too technical from there.
Just by looking at the orientation of the metal d orbitals and comparing them to the positions of the strong field ligands (NH3) & the weak field
ligands (Cl) got me no where.
francis - 12-11-2011 at 06:17
Yes, you are right about the symmetry of the complexes.
There is another selection rule, called the Laporte selection rule, which says that d -> d transitions in centrosymmetric complexes are forbidden.
The trans isomer has an inversion center (ie is centrosymmetric) so d->d transitions are Laporte forbidden, and will be weaker than the cis isomer.
waLLcandy - 17-11-2011 at 13:30
Thank you very much! I didn't think it was so simple. Another quick question: Which do you think would have the greater influence on d-d transitions,
the ligands or the oxidation state of the metal? Maybe this isn't something that can be easily generalized.
francis - 20-11-2011 at 22:09
The identity of the metal, the charge, the ligand's identity and the complex geometry all play a part in determing the crystal field splitting energy
(ie the energy of d->d transitions).
For the charge: higher charge allows greater splitting (the ligand can get closer to the metal)
For the identity of the metal: across a row of metals (if ligands, geometry and charge doesn't change), the magnitude of splitting doesn't change too
much.
But as you go down a group in the periodic table, the splitting energy changes by 25%-50% for each row down. (Since the larger 4d and 5d orbitals are
able to overlap with ligand orbitals more effectively, raising the energy of the complex).
But according to my lecture notes from last year,
Ligands have an intrinsic capacity to increase delta (the crystal field splitting energy), which does not vary greatly from metal to metal.
And the spectrochemical series arrange the ligands in order of their ability to influence the splitting.
So a bunch of factors come into play, but if you had to generalise and say charge vs ligand identity (for a given metal) I would say the identity of
the ligand would play the greater role in determining the magnitude of the d->d transitions.