We know a coordinate bond refers to a covalent bond in which one atom provides a lone pair and the other provides an empty orbital. When OH- gets a proton and becomes H2O, O provides a lone electron pair and the proton (H+) provides an empty orbital; when Na2O is dissolved by water, the oxygen ion has a similar change; also when H2O gets a proton and becomes H3O+. So, are the bonds in H3O+ all covalent bonds? I think if so, the definition is useless, for many bonds can also be coordinate bonds, e. g. when Mg3N2 is dissolved by acid, the nitrogen ion (N3-) gets 4 protons and form 4 coordinate bonds, just like H3O+.
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4$\begingroup$ You are correct. Once formed, a coordinate bond is no different from any other covalent bond. $\endgroup$– Ivan NeretinCommented Feb 24 at 7:59
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1$\begingroup$ And in H3O+ and NH4+ all bonds are equivalent. Coordinate/Dative bonding is solely a tool to help electron counting, it has no separate "reality" from covalent bonding. $\endgroup$– Ian BushCommented Feb 24 at 12:24
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In the examples You provided, it is true that the bonds can be seen as both coordinate and just covalent depending only on context. However, there are cases in which taking apart a covalent bond cannot result in one of the species retaining a lone pair intact, instead the molecule rearranges. Even though this kind of situation is rare, the definition is not completely "useless", since it helps also describe a process from which a molecule results along with its structure.
