light

Question - If transition elements have their colours due to lightenergy being absorbed causing electron transitions between d-orbitalswhen ligands are attached, why does [for instance] CoCl4 appear pinkwhilst Co(H2O)6 appear blue (e.g. by what means does a different ligandcause a difference in the way d-orbials split). Also (whilst it is not atransition element) why is graphite black as 2p-3s ground - excited stateexcitations are too high an energy leap for visible light to cause thetransitions so should not all the light be reflected thus appearing white.----------------------------------------------------------Second question first. Graphite is highly extended fused benzene ringsterminated with quinone groups and phenol groups. This allows the "pi"electrons to delocalize over large areas. This delocalization "collapses"the molecular orbitals to a point that they absorb all light in the visibleregion. Hence graphite appears black. If you look up the color of fusedbenzenes:: benzene, naphthalene, anthracene, pyrene,..., etc. you see thistrend beginning from colorless, to yellow, to orange, red,... and finallyblack. The difference in energy of the d-orbitals in transition metals thatproduces their color depends upon the chemical identity of the ionssurrounding a particular ion as well as the symmetry of the arrangement. Thesemi-empirical theoretical treatment of just how and why this occurs fallsunder the topic of "ligand field theory" or "crystal field theory". Most anyintermediate to advanced text on inorganic chemistry treats the topics indetail. Doing so is beyond the scope of a platform such as NEWTON BBS.Vince Calder=====================================================Let me at least address the graphite question...For graphite absorption you aren't dealing with atomic transitions (2p-> 3s) but molecular transitions (Pi -> Pi*). these transitions aremuch lower in energy and do absorb visible light.Greg Bradburn=====================================================