10. Playing with a Full Deck:
       The Periodic Table
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       d Orbitals in Bonding

One prominent characteristic of all transition-metal complexes is their wide variety of colors, which indicates the absorption of visible light by electrons within the complex. This happens because the six octahedral ligands alter the energies of the d orbitals of the metal unequally, and the resulting spacing between different d-level energies is small enough to fall in the visible region. In the absence of ligands, all five of these d orbitals in a transition-metal ion have the same energy. Now imagine that six negative charges (the electron pairs on the ligands) are brought in toward the ion from an infinite distance, along the octahedral directions ±x, ±y, and ±z (see opposite page). The negative charges will come directly toward the maximum-probability lobes of the , and orbitals. If electrons occupy these orbitals their energies will be raised because of repulsion from the incoming ligands. In contrast, the ligands move between the lobes of the , and orbitals, so the energy of electrons in these orbitals is less perturbed. The original d-orbital energy level is split into two levels, as shown at the right, with an energy separation of A. These are called the t and e levels for reasons irrelevant to this discussion, but you can remember which is which by thinking of the letters as standing for "three-orbital" and "excited."

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