Atomic number of Mn Fe and Co are 25, 26 and 27 respectively. Which of the following inner orbital octahedral complex ions are diamagnetic?

a. ${\left[\mathrm{Co}{\left({\mathrm{NH}}_{3}\right)}_{6}\right]}^{3+}$

b. ${\left[\mathrm{Mn}{\left(\mathrm{CN}\right)}_{6}\right]}^{3-}$

c.  ${\left[Fe{\left(CN\right)}_{6}\right]}^{4-}$

d.  ${\left[\mathrm{Fe}{\left(CN\right)}_{6}\right]}^{3-}$

Choose the correct option
1. (a, b)
2. (b, c)
3. (c, d)
4. (a, c)

Molecular orbital electronic configuration of CO3+ in ${\left[\mathrm{Co}{\left({\mathrm{NH}}_{3}\right)}_{6}\right]}^{3+}$

Number of unpaired electron=0

Magnetic property = Diamagnetic

Molecular orbital electronic configuration of

Number of unpaired electron=2

Magnetic property = Paramagnetic

Molecular orbital electronic configuration of  is

Number of unpaired electron=2

Magnetic property = Diamagnetic

Molecular orbital electronic configuration of

Number of unpaired electron = 2

Magnetic property = Paramagnetic

Thus, ${\left[\mathrm{Co}{\left({\mathrm{NH}}_{3}\right)}_{6}\right]}^{3+}$ and ${\left[Fe{\left(CN\right)}_{6}\right]}^{4-}$ are diamagnetic.

Hence, correct choices are options (a) and (c).