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

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 `"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 `"Fe"^(3+)`.
• Thus it would have `2` pairs of electrons in its `t_(2g)` orbitals and `1` electron in its third `t_(2g)` 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_(2g)` set, i.e. it would be more kinetically inert/stable.

A filled `t_(2g)` 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.