JEE Main · 2005 · 08 May 2005 · Offline · Coordination Compounds

Which one of the following cyano complexes would exhibit the lowest value of paramagnetic behaviour?
(At. No.

Cr = 24, Mn = 25, Fe = 26, Co = 27

)

Answer: (D)

Solution

A strong-field ligand like

CN X^{-}

forces the

d

electrons to pair up as far as possible, so the least paramagnetic complex is the one left with no unpaired electrons. In

[Co (CN) X_{6}] X^{3 -}

the cobalt is

Co X^{3 +}

, a

d^{6}

ion, and the strong field drives all six electrons into the lower

t_{2 g}

set as

t_{2 g}^{6} e_{g}^{0}

, leaving none unpaired and giving a diamagnetic complex. The

d^{3}

chromium,

d^{4}

manganese and

d^{5}

iron analogues still keep three, two and one unpaired electrons respectively, so the cobalt complex shows the lowest magnetic moment.

JEE Main · 2005 · 08 May 2005 · Offline · Single correct option