Ligand field stabilization energies trends

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). 

Oxidation Elements from Sc to Mn can form oxidation states up to the group number (e.g. MnOj) These states become increasingly strongly oxidizing, and lower states are more stable for later elements. The M3+/M2+ couple shows a trend related to ionization energies and ligand field stabilization energy. 

Two contributions to this trend are (a) the general decrease in electropositive character resulting from increased effective nuclear charge (see Topic HI) and (b) ligand field stabilization energies (see Topic H2). which increase the stability of complexes with ligands higher in the spectrochemical series than water in all ions except Mn (dr) and Zn (d ). 

Figure 3 Hydration enthalpies of divalent hexa-aqua metal complexes from the first row of the d-block as a function of the number of d-electrons from Ca " to. Both dashed lines illustrate linear trends when weak-field ligand field stabilization energies are subtracted from the experimental hydration enthalpies.

Figure 5.11. The hydration enthalpy of M + ions of the first row of the d elements. The straight lines show the trend when the ligand field stabilization energy has been subtracted from the observed values ].

Another factor that affects trends in the stability constants of complexes formed by a series of metal ions is the crystal field stabilization energy. As was shown in Chapter 17, the aqua complexes for +2 ions of first-row transition metals reflect this effect by giving higher heats of hydration than would be expected on the basis of sizes and charges of the ions. Crystal field stabilization, as discussed in Section 17.4, would also lead to increased stability for complexes containing ligands other than water. It is a pervasive factor in the stability of many types of complexes. Because ligands that form tt bonds... 

There are several other types of thermodynamic data that reflect the ligand field stabilization caused by splitting the d orbitals. For example, the lattice energies of the MC12 (where M is a +2 transition metal ion) compounds also show a double humped shape when plotted as shown in Figure 19.8. However, these types of data will not be discussed because the trends follow naturally from the principles that have already been presented. 

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