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

The way that the outer electrons in a transition-metal ion fill these two levels depends on the energy difference between the t and e levels, which is called the crystal-field splitting energy, A. The ion has three outer electrons, and in the ground state of the ion each electron occupies one of the three , and orbitals in the t level (right). has five outer electrons, and if the ligands around the iron produce only a small crystal-field splitting, A, then the electrostatic repulsion between electrons paired in the same orbital will ensure that one electron goes into each d orbital: three in the t level and two in the e. This is called a high-spin complex because the electron spins are not paired in the orbitals. These spins can be observed by magnetic measurements. If the ligands produce a large energy splitting, it may take less energy to pair electrons in the t orbitals than to place two electrons in the higher-energy e orbitals ( and ) where they must come close to the electrons of the ligands. In such a case four electrons are paired in the lower orbitals, thus leaving only one electron unpaired and creating a low-spin complex, as shown at the right.

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