Combined the results of the detailed absorption studies on 1-4 prove that BiI3 containing coordination complexes 3 and 4 absorb strongly in the visible region due to the presence of multiple transitions associated with the organic ligand, iodine and metal center due to the presence of strongly electron donating and highly diffused  orbitals on the iodine. In the case of 1 and 2, such lower energy bands in the visible region are absent due to the poor electron donating  orbitals. Therefore, the change in ancillary ligand from chloride to bromide in bi-octahedral bismuth complexes causes the enhancement of the oscillator strength of lower energy absorption transitions, while for iodine, in addition to the increase in absorption coefficient, a new lower energy absorption band in the visible region is observed.