Why is Sc3+ colourless while Ti3+ coloured? (Atomic number Sc = 21, Ti =22)
Ti3+ is a 3d1 system. The colour is due to d–d transition. There is one d electron present in the 3d subshell. When light falls on the Ti3+ complex, the t2g electron is excited to the eg level. This excitation takes place in the greenish yellow region (≈5000 A°) and the rest is transmitted. The complementary colour is transmitted which is violet.
Sc3+ is a 3d0 system. There are no d electrons; hence, d–d transition is not possible. Hence, Sc3+ is colourless.