Why would there be differences in UV/Vis spectrum? $\ce{[NEt4]2[NiBr4]}$ dissolved in $\ce{MeCN}$ and $\ce{[NEt4]2[NiBr4]}$ dissolved in $\ce{H2O}$ have different UV/visible spectra in the visible region. How would they differ, for example would one of them have more peaks? Also how would the molar extinciton coefficients differ and what do these numbers tell you?
$\ce{H2O}$ does not absorb the radiation almost at all, so it will not interfere in the absorbance of the analyte we are analysing, thats why we use water almost always as a solvent in instrumental methods of analysis.Therefore $\ce{MeCN}$ wil absorb some of the radiation so then of course the absorbance of the analyte will also change ( decrease ) and the spectrum will be different ( the peaks of the analyte will be lower ).