Factors Influencing Stability of Metal Complexes

We can identify a number of factors that contribute to the size of stability constants, and it is appropriate to summarize and review effects. In doing so, we will also attempt to divide the effects into those more associated with the metal and those more associated with the ligand, since both contribute to the partnership and inevitably each brings something to the metal-ligand marriage. 

Variation of stability constant (K) with charge/radius ratio for some M(OH)(, 1)+ complexes. This is a reasonable representation of how K varies will ion charge and size, but obviously the effect of charge has some dominance, given the drop in K for Al3+ and Be2+ ions despite their large charge/radius ratio. 

The above simple effects are not the sole metal-centric influences on K, as is evident from examining trends in experimental values with a wide range of ligands. This is particularly evident when one examines a series of transition metal 2+ ions forming complexes with ligands data with ammonia appears in Table 5.1. Overall, between Mn2+ and Zn2+, there is a general trend in K values essentially irrespective of ligand of the order  

The relative sizes of the cations vary in a not so different fashion, with radii following the trend  

In this series, we are dealing, for high-spin complexes, with octahedral systems. Only for Cu2+ do we have an especially distorted octahedral shape. Electrons placed in the lower /2g level are stabilized by 0.4A Q, whereas those placed in the upper g level are destabilized 

---