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Cations of 3d- elements particularly, the last four ones such as Cu2+ Zn2+ Co2+/Co3+and Ni2+ form metal-ammine complexes readily in aqeous solution, while the cations Mn2+ Cr3+ and Fe3+ usually dont. In the case of Fe3+, I learnt that since it is a highly charged cation, behaving as an acid it normally doesn't form complexes with ammonia. But, Mn2+ is not so acidic. Still, it doesn't form the complexes. What could be the reason behind it?

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  • $\begingroup$ I guess it has more to do with Lewis acidity (see HSAB theory) of the ion rather than pH acidity since complexes are about sharing electrons rather than exchanging protons and electrons. For example, ammonia is Lewis base and a ligand because of the free electron pair, but ammonium (NH4+) does not behave like that at all. Iron is a borderline Lewis acid so it may just be the ammine complex exists but is unfavored. It can be unfavored in water as the complex formation would require large activation energy but supplying the energy as heat would break the compex... $\endgroup$
    – Libor
    Commented Feb 15 at 16:35

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