(1) The transition metal ions has have incompelety filled (n-1) d-orbitlas.
(2) d-orbitals are degenerte orbitals, i.e., all five suborbitals of d-orbitals namely `d_(xy) , d_(yz), d_(x^(2) - y^(2)), d_(z^(2))` have nearly equal energy in a free metal ions.
(3) In compounds , in the pressnce of anions or complexing ligands, degenergy is lost and these five degenerate d-orbitals splits into two groups as follows.
(i) One group of d-orbitals have lower energy and consists of three orbitals namely `d_(xy)d_(xy)` and `d_(xy)` .They are called `t_(2g)` orbitals .
(ii) The second group of d-orbitals have higher energy and consist two orbitals `d_(x^(2) - y^(2))` and `d_(z^(2))` and they are called `e_(g)` orbitals.
(4) They energy differenec `DeltaE` between these two sets of d-orbitals is called crystal be field splitting energy (CFSE).
(5) `DeltaE` has very small value equal to the energy of electromagnetic radiation in the visible region `(DeltaE hu = (hc)/(lambda))`
(6) There ariese a transition of one or more unpaired electrons form `t_(2g)` orbitals to `e_(g)` orbitals due to the absorption of radiation in the visible region.
(7) When the electron d-orbitals `(t_(2g))` it transmits the light that is complementary in colour to colour of radiation absorbed. This imparts a characteristic colour to the compound or a metal ion.
For example in the figure the transition of one elecrons `(3d^(1))` in the complex `[Ti(H_(2)O)_(6)]^(3+)` is shown.
