Question

which is most likely to be colourless..? [Co(H2o)6]2+,[Mn(CN)6]3-,[CrCl3(H2o)], Ag(NH3)2]+ and why?

Answer 1

you're looking for complex with the metal atom that has either a full set of d electrons, \(d^{10}\), or an empty set of d electrons, \(d^0\). This is because the transitions of electrons between d orbitals are of the same energies as light in the visible spectrum.