# How to name Hg[Co(SCN)4]?

I have basically written the entire name except for the oxidation state of $\ce{Co}$.

Mercury tetrathiocyanatocobaltate( )

to know what is the oxidation state of cobalt, I must know the oxidation state of the mercury cation. But mercury can have $+1$ and $+2$ oxidation states. How do I know which one is it in this compound?

• You don't (that is, unless you know the crystal structure and stuff). In fact it is +2. – Ivan Neretin Dec 8 '17 at 13:53
• So, I cannot write the name? It's a question from my homework. – Raknos13 Dec 8 '17 at 13:56
• Then write (II) and be done with it. I don't see how you were supposed to figure that out. – Ivan Neretin Dec 8 '17 at 14:20
• It's Mercury(II) tetrathiocyanatocobaltate(II). – Nilay Ghosh Dec 8 '17 at 15:43
• It couldn't be Hg (I) Co (III) compound. There would need to be Hg2 group and cobalt is too strong oxidant for it to be present. – Mithoron Dec 8 '17 at 18:03

TL;DR: There is no inner- or outer coordination spheres in this complex. Both cobalt(II) and mercury(II) have nearly ideal tetrahedral coordination environment with $4$ $\ce{N}$ and $4$ $\ce{S}$ atoms, respectively, hence the proper name would be cobalt(II) mercury(II) tetrathiocyanate. Mercury(I) readily undergoes disproportionation, so assuming mercury(II) is the safest option.

Actually, denoting an inner coordination sphere is not correct for this compound. First crystal structure [1] has been assigned to cobalt(II) tetrathiocyanatomercurate(II) $\ce{Co[Hg(SCN)4]}$:

It appears reasonably certain that the $\ce{S}$ atom is attached to the $\ce{Hg}$ in tetrahedral co-ordination, with $\ce{S-Hg-S}$ angles of 120° and 104°.

But two decades later the structure has been refined again, by the group including the author of the original publication. This time the geometry has been established more precisely (ICSD#36062), and it turned out that there is an infinite network of $\ce{Co^2+}$ and $\ce{Hg^2+}$ cations cross-linked via thiocyanate ligands, and the new suggested name was cobalt mercury thiocyanate $\ce{Co(SCN)4Hg}$ [2]:

The combination of two tetrahedrally coordinated atoms, $\ce{Hg}$ and $\ce{Co}$, has produced a most unusual arrangement in which the $\ce{Hg}$ and $\ce{Co}$ atoms are held apart by four spirals, each containing $4$ $\ce{SCN}$ bridges, which are interlinked so that any one $\ce{SCN}$ bridge takes part in eight spirals.

[...] Each such spiral is a spring holding the $\ce{Hg}$ and $\ce{Co}$ atoms apart and straining the bonds in the process. It is almost certainly this strain which flattens the tetrahedral coordination round $\ce{Hg}$ and $\ce{Co}$ in the $c$ direction. If the arrangement could be reproduced mechanically it would probably provide the ideal spring mattress!

### References

1. Jeffery, J. W. Nature 1947, 159 (4044), 610. DOI: 10.1038/159610a0.
2. Jeffery, J. W.; Rose, K. M. Acta Cryst B 1968, 24 (5), 653–662. DOI: 10.1107/S0567740868002980.

Name of this complex is. Mercury tetrathiocyanato cobaltate (2) .

First take the stable oxidation state one .

Another factor is Hg +2 a soft acid and SCN- is a soft ligand their is soft -soft interactions... Cobalt is in +2 oxidation state which is a borderline acid but due to symbiosis it behaves more like soft acid than hard .