The same considerations apply to iron complexes in biological systems. Blood is red because the iron-porphyrin complex in hemo globin absorbs green light. Chlorophyll is green because the magnesium-porphyrin complex absorbs light at the blue and red ends of the spectrum but not in the middle (see below). This rather paradoxical behavior of the main photosynthetic pigment in nature not absorbing light at wavelengths where solar energy is most abundant (5000Å) is remedied by having carotene and similar molecules nearby to trap these more plentiful wavelengths and pass the electronic energy on to chlorophyll for use in synthesis. In previous chapters we saw the two main sources of closely spaced electronic energy levels - delocalized aromatic rings and transition-metal complexes - and the consequent absorption of visible light. Herne and chlorophyll combine both sources in a single molecule.

You may have wondered why this section was entitled "d Orbitals in Bonding," when we have seen no covalent bonding so far between metal ion and ligands. The simple crystal field theory that we have been using to explain energy-level splitting is indeed a purely electrostatic theory, which assumes that the metal remains an ion and the lone pairs remain on the ligands.

In the more realistic molecular orbital treatment, six orbitals from the metal ion, one s, three p, and the two d orbitals that point toward the ligands, are combined with six ligand orbitals to produce twelve molecular orbitals, six of them bonding and six antibonding. The six electron pairs furnished by the ligands are used to fill the bonding orbitals and make covalent bonds from the metal to the ligands. The , and metal orbitals are not involved in the combining process because they have the wrong symmetry to combine with the s orbitals from the ligands. The end result is the same as obtained from crystal field theory. After six covalent bonds have been formed between metal and ligands, the three unused d orbitals and all of the outer electrons on the metal ion remain and can be given the kind of treatment we used with crystal field theory. The undisturbed energy level of these three orbitals corresponds to the t level, and the e level corresponds to the lowest two of the six antibonding molecular orbitals Crystal field theory assumes that the bonds between metal and ligands are ionic, and molecular orbital theory assumes them to be covalent. As usual, the truth lies somewhere in between.