Stability by using charge/radius rule

I found a question



> 
> Which is more stable among the following?
> 
> 
> (a) $\ce{CaCO3}$
> 
> 
> (b) $\ce{MgCO3}$
> 
> 
> 


Answer : (a)


Explanation was given as:



> 
> $\ce{Mg^2+}$ has smaller radius than $\ce{Ca^2+}$ and thus charge/radius of $\ce{Mg^2+}$ is greater , hence it is less stable.
> 
> 
> 


While when reading about the coordination compound, I found the sentence



> 
> Higher the charge of central metal atom ion i.e., greater ionic potential
> charge/radius then greater is the stability.
> 
> 
> 


In first case **higher the ratio less is the stability**.


In second case **higher the ratio more is the stability**.


Is the difference in the answers in both cases due to a difference in the salts (i.e. simple and complex salt) or is any one of them wrong?




Please allow me to answer the question in two parts.


Stability of carbonates
=======================


We observe this order when we compare the **thermal stability of group I-A carbonates**: $$\ce{Li2CO3 \lt Na2CO3 \lt K2CO3 \lt Rb2CO3 \lt Cs2CO3}$$


On similar grounds, the thermal stability of group II-A carbonates can be predicted.


This order is in accordance with [Fajans' Rules](https://en.wikipedia.org/wiki/Fajans%27_rules). Here's the table we need to glance at from the Wikipedia page.



> 
> $$\begin{array}{|c|c|} \hline
> \mathrm{Ionic} & \text{Covalent}\ \\ \hline
> \text{Low positive charge} & \text{High positive charge} \\
> \text{Large cation} & \text{Small cation} \\
> \text{Small anion} & \text{Large anion} \\ \hline \end{array}$$
>  Polarization will be increased by high charge and small size of the cation.
> 
> 
> 


Thus, larger the ratio of charge and radius, higher is the polarization power of the cation, more is the *covalent character* in the compound and lesser is the *ionic character*; Ionic compounds are more stable when exposed to heat. A **higher ratio** will therefore mean **lesser stability**.


Stability of complex compounds
==============================


**Thermodynamic stability of complexes**, on the other hand, is determined by the [Irving–Williams series](https://en.wikipedia.org/wiki/Irving%E2%80%93Williams_series), according to which general stability sequence of high spin octahedral metal complexes for the replacement of water by other ligands is: 
$$\ce{Mn^2+ < Fe^2+ < Co^2+ < Ni^2+ < Cu^2+ > Zn^2+}$$
Owing to this observation, a lot of theories were formulated to explain this phenomenon. One such theory was that since Crystal Field Theory (CFT) was based on the idea that a purely electrostatic interaction existed between the central metal ion and the ligands, the stability of complexes *should be somehow* related to the size by charge ratio (otherwise known as the ionic potential) of the central metal atom. [[Source](http://wwwchem.uwimona.edu.jm/courses/IrvWill.html)]


An interesting thing to note here is that Wikipedia has stated in its [article](https://en.wikipedia.org/wiki/Irving%E2%80%93Williams_series) that:



> 
> None of the explanations can satisfactorily explain the success of the Irving–Williams series in predicting the relative stabilities of transition metal complexes.
> 
> 
> 


Thus from manganese till copper, it can be said that **higher the ionic potential**, **more is the stability**.


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