Consequences of Crystal Field Splitting

From a more complete set of data similar to those shown in Table 17.3, it is possible to rank metal ions in terms of the splitting of the d orbitals produced by a given ligand. The series for many common metal ions can be given as follows. 

This series illustrates clearly the effects of charge and position in the periodic table that were described earlier. 

The heat of hydration of an ion is related to its size and charge (see Chapter 7). However, in this case the aqua complex that is formed causes the d orbitals to be split in energy, and if the metal ion has electrons in the d orbitals, they will populate the t2g orbitals, which have lower energy. This results in 

Number of electrons Weak field (Dq) Strong field (Dq) 

The graph shows what has become known as the double-humped appearance that reflects the fact that the ligand field stabilization energy for the aqua complexes begins at 0, increases to 12 Dq, then drops to 0 on going from d° to d5 and repeats the trend on going from d6 to d10 (see Table 17.4). 

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