The metal complex that is diamagnetic is (Atomic number: Fe, \( 26 \); Cu, \( 29 \))
\( \mathrm{K_3[Cu(CN)_4]} \)
\( \mathrm{K_2[Cu(CN)_4]} \)
\( \mathrm{K_3[Fe(CN)_4]} \)
\( \mathrm{K_4[FeCl_6]} \)