Do the tetramminecopper(II) and hexathiocyanatoferrate(III) complexes obey the 18-electron rule?

Can anyone tell me whether $\ce{[Cu(NH3)4]^2+}$ and $\ce{[Fe(SCN)6]^3-}$ obey the 18-electron rule, and if not, why? I found $\ce{[Cu(NH3)4]^2+}$ and $\ce{[Fe(SCN)6]^3-}$ have 17 electrons, was I wrong?





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> I found $\ce{[Cu(NH3)4]^2+}$ and $\ce{[Fe(SCN)6]^3-}$ have 17 electrons, was I wrong?
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You are correct for the iron(III) complex, which has an uneven electron count and is a $\mathrm{d^5}$ system. You are almost correct for copper(II), but that complex is actually $\ce{[Cu(NH3)4(H2O)2]^2+}$ where the two aqua-ligands are further away from the metal centre. In any case, it is a $\mathrm{d^9}$ system and has 21 electrons.


No system in which the central metal has an uneven electron number can fulfill the 18-electron rule unless there are radical ligands around which happens rather rarely.


So what is the benefit of the rule? Well, complexes involving 18 electrons — that would be ten d-electrons, two s-electrons and six p-electrons in extremely simplified terms — are often noted to be more stable than others with respect to decomposition. So iron forms a pentacarbonyliron(0) compound ($\mathrm{d^8}$) with ten electrons coming from the five carbon monoxide ligands. This compound is rather stable with respect to oxidation and decomposition while a hexacarbonyliron(0) or a tetracarbonyliron(0) are not. But you should take this 18-electron rule with a much larger grain of salt then the octet rule for main group elements.


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Only for some complexing agents do we typically expect the 18-electron rule to hold. The best candidates are very "soft-base" ligands with strong pi-electron interactions, like CO. Ammonia and thiocyanate do not do this.


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