爲什麼[Cu(H2O)4]2+的顏色比[Cu(NH3)4]2+的顏色淺

題目：

爲什麼[Cu(H2O)4]2+的顏色比[Cu(NH3)4]2+的顏色淺

解答：

First,in aqueous solution you have [Cu(H2O)6]2+ and [Cu(NH3)4(H2O)2]2+.
They both have a distorted octahedral structure with two H2O's located on the axial positions (D4h symmetry).For simplicity,just treat them as in Oh symmetry.central Cu2+'s 5 d orbitals will be splitting into two levels,Eg and T2g,with T2g lower in energy.
Cu2+ has 9 d electrons,6 in T2g orbitals,and 3 in Eg orbitals separately.So the Eg orbitals higher in energy are not completely occupied.An electron in T2g orbitals can absorb a photon (Visible range),and be excited into Eg orbital.
The energy difference Delta(E) between T2g and Eg orbitals will decide the color of the complex ions.NH3 ligand is a stronger field ligand than H2O molecule,so [Cu(NH3)4(H2O)2]2+ will have a larger Delta(E) than that of [Cu(H2O)6]2+.That is why [Cu(NH3)4(H2O)2]2+ in aqueous solution absorbs yellow-green color and gives out its complementary deep blue color,and [Cu(H2O)6]2+ absorbs orange yellow color and gives out pale blue color.