Answer in brief.
With the help of the crystal field, energy-level diagram explain why the complex [Cr(en)3]3⊕ is coloured?
i. The central metal ion titanium has electronic configuration 3d3 and the electrons occupy one of the t2g orbitals.
d - d transition in d' system
ii. The absorption of the wavelength of light promotes an electron from the t2g level to eg level. Therefore, in the complex [Cr(en)3]3+, d-d transitions are possible. Hence, the complex is coloured.