# Why does Cobalt have different coordination numbers in its complexes with water and thiocyanate?

I'd like to know why does cobalt(II), $\ce{Co^2+}$, form $\ce{[Co(H2O)6]^2+}$ complex with water while it forms $\ce{[Co(SCN)4]^2-}$ with the thiocyanate? Why is there $\ce{6H2O}$ but only $\ce{4 SCN-}$? And is the cobalt's coordination number 6 in these cases? Thanks.

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• the coordination number is the number of coordinate bonds to the metal ion: six for the hexaaqua complex and four for the tetrathiocyanate complex. as for why I too am dying to know as I haven't been able to find any good explanations on the internet – bon Jan 10 '15 at 21:50
• Water, being neutral, has a stabilizing effect on positive ions, such as $H^+$. Thiocyanate has a negative charge. I think that after 4 ions have clustered around the cobalt, that the charge repel more thiocyanates from bonding to the cobalt. – LDC3 Jan 10 '15 at 22:06
• That's a good thought @LDC3, but if that were all there was to it, wouldn't you expect the thiocyanate complex to have two water ligands as well? However, when you add $SCN^-$ to a solution of hexaaquo cobalt (II), you get $[Co(SCN)_4]^2-$, not $[Co(SCN)_4(H_2O)_2]^2-$. I am still scratching my head on this one .. water and thiocyanate are both weak field ligands, so I would expect the same coordination geometry for both of them. I must be missing something. – dtmoore1971 Jan 11 '15 at 0:23
• Maybe you need to take into account the geometry in addition of charge stabilisation. $\ce {H2O}$ is bended while $\ce {SCN-}$ is linear. I'm not sayin' that's the answer but maybe it should be taken into account :x – Babounet Jan 11 '15 at 10:24

Firstly, in water Co(II) is not exclusively $\ce{[Co(H2O)6]^2+}$, there is also a small amount of tetrahedral $\ce{[Co(H2O)4]^2+}$, 0.08% at room temperature increasing substantially at high temperature. Canadian Journal of Chemistry, 1980, 58(14): 1418-1426.