why are transition elements coloured and can form very good complexes?What is relativistic effect?

Most properties of light atoms and molecules can be explained in a non-relativistic quantum mechanical description. When one goes down in the periodic system, where the nuclei get heavier, the relativistic effects will get larger and can eventually dominate the chemical processes.
Sometimes it is even necessay to explain the properties of light atoms by relativistic influences. An example is the pair of ²D lines in the spectrum of the sodium atom, which are spit due to spin-orbit interaction. This spin-orbit coupling is one of the most well known shortcomings of the non-relativistic theory in chemistry.
Other important manifestations of relativity, besides spin-orbit coupling, are the mass-velocity and Darwin correction. These effects are not directly observable but they still can have an important contribution to the chemistry of elements. These two effects lead to the "relativistic contraction" of the s- and p-shells and to the "relativistic expansion" of the d- and f-shells. The contraction can give a stabilisation of the 6s shell in a gold atom which leads to an increase of the electron affinity of th e gold atom. This is also happening in the platinum atom.