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Maximum absorption wavelength directly proportional to the donor number of the ligand for a Cu2+ 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 Cu2+ centre, so 9 d electrons. 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 dx2-y2 and dz2 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?

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