Would you expect the wavelength of maximum absorbance for [Cu (NH3) 4]2 + to be greater than or less than the wavelength of maximum absorbance for Cu (H2O) 62+? Explain

Less

Explanation:

Since [Cu (NH3) 4]2 + and [Cu (H2O) 6]2 + are Octahedral Complexes the transitions between d-levels explain the majority of the absorbances seen in those chemical compounds. The difference in energy between d-levels is known as ΔOh (ligand-field splitting parameter) and it depends on several factors:

The nature of the ligand: A spectrochemical series is a list of ligands ordered on ligand strength. With a higher strength the ΔOh will be higher and thus it requires a higher energy light to make the transition. The oxidation state of the metal: Higher oxidation states will strength the ΔOh because of the higher electrostatic attraction between the metal and the ligand

A partial spectrochemical series listing of ligands from small Δ to large Δ:

I - < Br - < S2 - < Cl - < N3 - < F-< NCO - < OH - < C2O42 - < H2O < CH3CN < NH3 < NO2 - < PPh3 < CN - < CO

Then NH3 makes the ΔOh higher and it requires a higher energy light to make the transition, which means a shorter wavelength.