# Maximum absorption wavelength directly proportional to the donor number of the ligand for a copper(II) coordination complex

I am looking at a set of results showing a directly proportional relationship for an increasing maximum absorption wavelength (UV-Vis spectra) with increasing donor number of solvent ligands. The complex has a $$\ce{Cu^2+}$$ centre with $$\mathrm{d^9}$$ electronic configuration. Assuming a tetragonal configuration, how does this tie in with ligand strengths?

I am not sure what I can deduce here as I thought the more available a lone pair (basicity) would be able to interact more strongly, a higher wavelength indicates the energy gap between the $$\mathrm d_{x^2-y^2},$$ and $$\mathrm d_{z^2}$$ orbital has decreased. So, has there been more significant Jahn–Teller distortion, or is the splitting parameter somehow smaller?

Where does my reasoning fall down?