Coordination Compounds - Result Question 65
####72. Atomic number of $Cr$ and $Fe$ are respectively 25 and 26 , which of the following is paramagnetic?
[2002]
(a) $[Cr(CO)_6]$
(b) $[Fe(CO)_5]$
(c) $[Fe(CN)_6]^{-4}$
(d) $[Cr(NH_3)_6]^{+3}$
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Solution:
(d) $Cr^{3+}$ has $4 s^{0} 3 d^{3}$ electronic configuration with 3 unpaired electrons, hence paramagnetic. In other cases, pairing of $d$-electrons take place in presence of strong field ligands such as $CO$ or $CN^{-}$.
In $Cr(CO)_6$ molecule 12 electrons are contributed by $CO$ group and it contain no odd electron
$[Cr(CO)_6]$
$4 s$
- diamagnetic
$[Fe(CO)_5]$
$4 s$
$4 p$
$3 d$
- diamagnetic
$[Cr(NH_3)_6]^{+3}$
- Paramagnetic
