The 18-electron rule and the corresponding methods for counting the total valence electrons of transition metal complexes are among the most useful basic tools in modern inorganic chemistry, particularly in its application to organometallic species.

To explain the limitations of this rule, the coordination compounds of transition metals are divided into three broad classes, the characteristics of which are described in Rasmussen, S. C. ChemTexts 2015, 1 (1), No. 10, DOI: 10.1007/s40828-015-0010-4.

For example, the coordination compound $\ce{[Zn(en)3]^2+}$ does not satisfy this rule, since it contains 22 electrons. On the other hand, the compounds $\ce{[PtF6]^2-}$ and $\ce{[Fe(CO)4]^2-}$ do obey this rule, since both species have 18 electrons. Based on the previous article, I could not explain why the first species does not obey the rule and the other two do. I suppose it is linked to the nature of the ligand and the central metal atom, but I am not sure.

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    $\begingroup$ Zinc often isn't considered a transition metal $\endgroup$
    – Mithoron
    Jul 6 '21 at 14:31
  • $\begingroup$ In the notes with which I study it does appear as such. @Mithoron $\endgroup$
    – Carlos
    Jul 6 '21 at 15:27
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    $\begingroup$ This earlier edit should not have been accepted, for a couple of reasons: (1) no need to use MathJax for plain numbers in the title. MathJax in the title should be reserved for very special situations, where there is absolutely no alternative. (2) Chemical formulae should be written using $\ce{...}$, not $...$. Please consider reading: FAQ: How can I format math/chemistry expressions on Chemistry Stack Exchange?. $\endgroup$
    – orthocresol
    Jul 6 '21 at 15:33
  • $\begingroup$ The $8$-electron and the $18$-electron rules are rules. They are often useful. But they have plenty of exceptions. Look at $\ce{ NO}$, $\ce{NO2, ClO2, B2H6, MnO2, CuSO4, K3[Fe(CN)_6]}$, etc. $\endgroup$
    – Maurice
    Jul 6 '21 at 16:47
  • $\begingroup$ I understand that there are exceptions to the rule, but I do not know how to explain its origin based on the article I have included in the question. That is the reason for my query. @Maurice $\endgroup$
    – Carlos
    Jul 7 '21 at 12:53

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