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.