Which metal in the first transition series (3d series) exhibits a +1 oxidation state most frequently and why?

a. Cu is the only metal in the first transition series (3d series) which exhibits a +1 oxidation state more frequently. This is because the electronic configuration of Cu is 3d104s1 and after losing one electron it acquires a stable 3d10 configuration.
b. Complexes of transition metal ions (e.g. Ti3+ and V3+) are usually coloured due to d-d transitions within the partly filled d orbitals. Although there is a partially filled d level in scandium ([Ar]3d14s2), when it forms Sc3+, it loses all three outer electrons so the 3d level is empty. So, although scandium is a member of the d block like titanium and vanadium, its ion (Sc3+) hasn't got any d electrons left to move around. Sc3+ complexes are therefore colourless because no visible light is absorbed.