Consider the following low-spin complexes
K₃[Co(NO₂)₆], K₄[Fe(CN)₆], K₃[Fe(CN)₆], Cu₂[Fe(CN)₆] and Zn₂[Fe(CN)₆]
The sum of the spin-only magnetic moment values of complexes having yellow colour is ________ B.M. (answer in nearest integer)

Solution

Chemistry ma’am · 3 steps

First sort these complexes by colour, then count unpaired electrons only in the yellow ones.

1. Pick the yellow ones. K₃[Co(NO₂)₆] is yellow (Fischer's salt) and K₄[Fe(CN)₆] is lemon yellow. The other three are not yellow: ferricyanide is red, the copper salt is chocolate brown and the zinc salt is white.

   Complex	Central ion, low spin	Unpaired electrons	Colour
   K₃[Co(NO₂)₆]	Co³⁺, d⁶: t_(2g)⁶ e_g⁰	0	yellow
   K₄[Fe(CN)₆]	Fe²⁺, d⁶: t_(2g)⁶ e_g⁰	0	yellow
   K₃[Fe(CN)₆]	Fe³⁺, d⁵: t_(2g)⁵ e_g⁰	1	red
   Cu₂[Fe(CN)₆]	Fe²⁺, d⁶	0 on Fe	chocolate brown
   Zn₂[Fe(CN)₆]	Fe²⁺, d⁶	0 on Fe	white

2. Spin-only moment. Both yellow complexes have a d⁶ ion with strong-field ligands (NO₂⁻ and CN⁻). All six electrons pair up in t_(2g), so n = 0 and μ = √n(n+2) = 0 B.M. for each.
3. So. The sum is 0 + 0 = 0 B.M.

The common slip: 2. Counting K₃[Fe(CN)₆] as yellow adds its √3 = 1.73 B.M. for one unpaired electron and gives 2. That salt is red, so it is left out.

The idea to keep. A low-spin d⁶ ion with strong-field ligands fills t_(2g)⁶ completely, so it is diamagnetic. Remember the classic colours: ferrocyanide yellow, ferricyanide red, Fischer's salt yellow.

Answer from the NTA final answer key. Solution by PrepWiser.