At school we've just finished learning about Coordination Compounds.

Our textbook has defined Isomers as

Compounds having same molecular formula but different arrangement of atoms, due to which they differ in one or more physical properties are called isomers.

It further goes on to discuss the various kinds of structural isomerism seen in (transition-metal) complexes:

  1. Ionization Isomerism
  2. linkage isomerism
  3. ligand isomerism
  4. solvate/hydration isomerism
  5. coordination isomerism
  6. coordination-position isomerism
  7. polymerization isomerism

Polymerization isomers, according to our textbook, are

those complexes that have the same empirical formula, but differ in molecular mass by integral multiples of empirical mass.

e.g., $\ce{[Pt(NH3)2 Cl2]}$ and $\ce{[Pt(NH3)4][PtCl4]}$ are polymerization isomers.

Yet another book I've read, simply states that polymerization isomerism is not isomerism in the true sense of the word, without elaborating on this statement.

But looking at the definition for isomerism, the way I see it, polymerization isomerism fits the bill, so what exactly do the authors of the book imply by that statement? Are they wrong?

  • 8
    $\begingroup$ Your second book is of course right. NO is not an isomer of N2O2. I've never heard the term "polymerisation isomerism", sounds like a rather stupid category to me. ;-) $\endgroup$
    – Karl
    Aug 27, 2016 at 12:27
  • $\begingroup$ @Karl, I forgot to mention the examples given in the book, just did it now. Also I haven't come across 'Polymerization Isomerism' before, so perhaps it only applies to complexes (and not NO and N2O2) ? $\endgroup$ Aug 27, 2016 at 12:43
  • 4
    $\begingroup$ "Polymer" implies that the strucure of the monomer is retained. This is not the case for your example, so in this case i would say it's just wrong. $\endgroup$
    – Karl
    Aug 27, 2016 at 12:58
  • 1
    $\begingroup$ At the moment, that's rather my opinion than an answer. Let's wait what others have to say. $\endgroup$
    – Karl
    Aug 27, 2016 at 13:53

1 Answer 1


I’m pretty sure that polymerisation isomerism should mainly be considered a historical term nowadays. Going back in the history of science, all people knew at some point was which elements a specific compound is made out of. That worked well for main group metal salts but not too well for transition metal salts. Later, quantitative analysis established the law of multiple proportions, which basically says that if two compounds are made up of the same set of elements, the ratio of element mass ratios will always be an integer ratio. Sounds hard to understand, here’s an example:

$\ce{CuCl}$ has one chlorine per copper, $\ce{CuCl2}$ has two. So for every $x~\mathrm{g}$ of copper, one of the two will have $1~\mathrm{g}$ of chlorine, the other will have $2~\mathrm{g}$, and $1$ and $2$ form a simple integer ratio.

And all of a sudden there were compounds that were clearly distinct but gave the same ratio of elements, a simple example being $\ce{NO2}$ and $\ce{N2O4}$. To understand this, the term polymerisation isomer must have been introduced.

Nowadays, isomers are defined as having the same elemental composition on a per-fragment basis. So examples such as $\ce{N2O4}$ (which is a different molecule than $\ce{NO2}$) or the platinum complexes you give would no longer be considered isomers — neither constitutional ones nor stereoisomers. Hence why so many people said ‘your second book is right.’


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