Why does D- block elements form coloured compounds?

Answer 1

Transition metals form ions with partly filled d-orbitals. There are 5 d-orbitals which each can contain two electrons. These five d-orbitals are degenerate - they have the same energy - when there are no ligands around the metal. When a ligand approaches the metal ion, the electrons from the ligand will be closer to some of the d-orbitals and farther away from others. The electrons in the d-orbitals and the electrons in the ligand repel each other (because they're both negatively charged), and so d-electrons closer to the ligands will have a higher energy than ones further away because they feel more repulsion. Thus, the d-orbitals will split in energy. What determines the way that the orbitals split is the orientation of the ligands with respect to the metal d orbitals. If there are six ligands there will most likely be one along each axis, so the complex will have octahedral symmetry. The dxy, dxz and dyz orbitals will be lower energy than the dz2 and dx2-y2, which will have higher energy, because the former group are further from the ligands than the latter. In a tetrahedral crystal field splitting, the lower energy orbitals will be dz2 and dx2-y2, and the higher energy orbitals will be dxy, dxz and dyz - the opposite way round to the octahedral case.
The color of a complex depends on:

the nature of the metal ion, specifically the number of electrons in the d orbitals
the arrangement of the ligands around the metal ion (for example geometric isomers can display different colors)
the nature of the ligands surrounding the metal ion. The stronger the ligands then the greater the energy difference between the split high and low 3d groups.
The complex formed by the d block element zinc (though not strictly a transition element) is colorless, because the 3d orbitals are full - no electrons are able to move up to the higher group

Answer 2

If there are any gaps interior the d-orbitals of the steel ion, then photons of seen easy could be absorbed, and an electron could be excited from a decrease orbital to an more suitable orbital - the place there become a area of interest. for the reason that a particular frequency of sunshine has been bumped off, the relax (meditated) easy seems colored.