# In the compound, CaNa[Fe(CN)_6], what ligands are in the coordination sphere?

May 30, 2016

You didn't specify which coordination sphere.

In stackrel(+2)"Ca"stackrel(+1)"Na"[stackrel(+3)"Fe " stackrel(-"1 each")(("CN")_6)] as-written, a mixed calcium-sodium hexacyanoferrate(III) complex:

• The first coordination sphere involves what is directly coordinating with ${\text{Fe}}^{3 +}$, which are the cyanide ions.
• The second coordination sphere involves anything coordinating from the outside with the first coordination sphere. These species in the second coordination sphere are the sodium and calcium cations.
• Iron would be a ${d}^{5}$ metal, since it is supposedly ${\text{Fe}}^{3 +}$.
• Thus it would have $2$ pairs of electrons in its ${t}_{2 g}$ orbitals and $1$ electron in its third ${t}_{2 g}$ orbital.

Now, to me something seems off.

Could you mean "CaNa"_2["Fe"("CN")_6], where the first coordination sphere has a $4 -$ charge instead of a $3 -$ charge?

In this case, iron(II) would be a ${d}^{6}$ metal, with a full ${t}_{2 g}$ set, i.e. it would be more kinetically inert/stable.

A filled ${t}_{2 g}$ set usually implies poor kinetic favorability with electron donors trying to react with it, as they would try to donate into the high-lying (usually antibonding) ${e}_{g}$ set and increase the antibonding character of the metal-ligand bonds, thus weakening the metal-ligand bonds.