[akalovexx]

When a solution of [CrCl6]3- is dissolved in water the measured value of Δ increases by
10 – 20%.  When [FeCl4]2- is dissolved in water the measured value of  Δ virtually
doubles.  Explain these observations

Im not quite sure, But my reasoning so far:

 [CrCl6]3- is octehedral where as [FeCl4]2- is tetrahedral; however [FeCl4]2- has 4/9Δ splitting where as  Δ is much greater for the octehedral complex. Both are high spin due to weak field ligands.
When the lone pairs of electrons on the water are attracted to the metal cation it will not add much to the already great value of  Δ for the octehedral complex but will have a much greater effect for the tetrahedral complex.

Im not sure if im on the right lines here....

any help much appreciated.

[Schrödinger]

I think the explanation is a bit different. This is just my viewpoint, it may be far from the truth though :

Lone pairs indeed need to be considered. But remember, that Cl^- has 3 lone pairs even after coordination. And water has only one after coordination. The effect of decrease in Δ when π-donor ligands are considered, I hope, is known to you. Hence when you remove one LP (in going from Cl^- to water), the decrease in Δ if lifted and hence there is an increase.

The amount of increase is lesser with the Cr³⁺ complex because Cr is in a higher oxidation state, in which the dπ system has only 2 electrons. As against the Fe²⁺ complex which has 6 electrons in its d system. Hence the greater relief of repulsion, which means, greater increase.