[maartenmennes]

Hi guys,
I am new to this forum so I don't really know how it works. I am kind of stuck on two questions.

- In [Cr(NH3)Cl]2+ so with Cr(III), If the z axis contains the Cr-Cl bond, and Cl- is not as good a sigma-donor as NH3, how would the d orbital energy level diagram be different from that of [Cr(NH3)6]3+ ?

And:

-If the z axis contains the Cr-Cl bond, how would the d orbital energy level diagram be different from that of [Cr(NH3)6]3+ through pi interactions with the Cl- ligand?

For the first one, I thought that if Cl- is a worse sigma-donor than NH3, the bond length along the z axis would be longer, so the orbitals containing z component would become stabilized..

And for the second one I am really unsure. Cl- is a pi donor so it would likely form pi interactions with the Chromium ion, but how would that change the d orbital level diagram?

I hope you guys can help me!

Maarten

[gritch]

I haven't been in an inorganic chemistry class in a while so I'm probably a bit rusty but I'll try to help.

- In [Cr(NH3)Cl]2+ so with Cr(III), If the z axis contains the Cr-Cl bond, and Cl- is not as good a sigma-donor as NH3, how would the d orbital energy level diagram be different from that of [Cr(NH3)6]3+ ?

Let's think of what d orbitals would interact with the Cl along the z-axis. We would get interaction with the dz², dyz, and dxz orbitals. Of these orbitals only the dz² has the correct symmetry for a sigma interaction with the axial ligand (dyz and dxz interact through pi bonding which is for the next question). Since Cl is a weaker sigma donor than NH₃ we would expect less interaction between the Cl ligand and the d orbitals leading to less splitting of the orbitals. I would predict a decrease in the difference of energies of the the MOs corresponding to the dz² orbitals.

-If the z axis contains the Cr-Cl bond, how would the d orbital energy level diagram be different from that of [Cr(NH3)6]3+ through pi interactions with the Cl- ligand?

In the case of pi interactions we would see a shifting of the dxz and dyz. If Cl is a better pi donor as you suggested we would expect the opposite trend: that is an increase the difference of energies for orbitals corresponding to the dxz and dyz orbitals.