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What's the difference between the co-ordination compounds $\ce{Co.(H2O)6}$ and$\ce{[Co(H2O)6]^2+}$?

Farhan S
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$\ce{Co·6H2O}$ does not exist. But, if it would exist, il would be a special sort of metallic cobalt, surrounded by 6 water molecules fixed around each $\ce{Co}$ atom. This substance would be electrically neutral. It would exist in a sample containing no other substance.

$\ce{[Co(H2O)6[^{2+}}$ does exist. It is a charged species (cation) that exists, but does not exist alone. It must be accompanied by one or several negative ion, like $\ce{SO4^{2-}}$ or $\ce{2 Cl-}$. There must be the same number of positive charges and of negative charges in any sample containing this ion. So there must be as many $\ce{[Co(H2O)6]^{2+}}$ cations as say $\ce{SO4^{2-}}$ anions. In solution, these ions are independent and can react differently. But the number of positive charges must always be equal to the number of negative charges.

Maurice
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  • In general, how does $\ce{X.nH2O}$ differ from $\ce{X(H2O)n}$? – Farhan S May 10 '21 at 12:32
  • @Farhan S. If $\ce{H2O}$ is attached to the preceding metal $\ce{X}$, it means that there is a covalence between them. If you separate them by a point, it means that the attraction between them is weaker than a covalence, and only due to van der Wals forces. – Maurice May 10 '21 at 16:11