Water ligands form dative covalent bonds with the central transition metal ion. That would mean a molecule is formed, just like sulfur dioxide or any other.

The transition metal ion does not form dipole ion attraction with water as does sodium chloride, but forms a covalent bond. If so, why is the resulting mixture a solution, and not a solid?

  • 2
    $\begingroup$ You have it all backwards. Solutions of transition metal salts are not all that different from the solution of NaCl. Besides, a hydrated transition metal ion may be a part of a solid all right. $\endgroup$ – Ivan Neretin Nov 3 '17 at 5:07
  • $\begingroup$ For some reason it keeps getting repeated that transition metals dissolved in water were conceptionally different in any way to sodium chloride dissolved in water. This is not the case! Sodium also undergoes complex formation with water molecules. The difference to many (but not all) transition metals is that the complexes are structurally highly fluctional and thus not well defined. $\endgroup$ – Jan Nov 20 '17 at 9:49

The transition metal ion does not form dipole ion attraction with water as does sodium chloride

Actually it does. What we refer to as the complex is only the innermost hydration shell. This comples is still positively charged so the water molecules around this complex will still tend to orient themselves oxygen-inwards to build up dipole-dipole interactions (and hydrogen bonds) with the proper complex. Likewise, you should not consider dissolved sodium ions as in sodium chloride any different. They, too, have a hydrate shell which can be described as a coordination complex. The key difference is that the hydrate shell is very labile and no precise geometry can be given. There is little energy cost associated with water molecules diffusing away or attaching back. But other than that, the two are the same.

Whether or not the complex ion (e.g. $\ce{[Fe(H2O)6]^2+}$) is in solution or in a solid depends on what you did with the compound. A lot of hydrates are actually cationic aquacomplexes; for example, solid $\ce{CuSO4.5H2O}$ can be described as $\ce{[Cu2(H2O)8(\mu{-}SO4)2] . 2H2O}$ instead, showing that we have complexes in the solid state, too.


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