(a) In
Ni(NH3)6]2+, Ni is present as
Ni2+ ion having outer electronic configuration.
Here,
NH3 is regarded as a weak field ligand which can not go for pairing when present in
d -subshell.
Due to presence of unpaired electrons it is paramagnetic in nature.
(b) In
[Ni(CN)4]2−,Ni is present as
Ni2+ ion and
CN is a strong field ligand,
Thus, has the following electronic configuration.
The above configuration has no unpaired electron, thus, is diamagnetic in nature.
(c) In
[Ni(CO)4], oxidation state of
Ni is zero and show the following electronic configuration.
Note Here, electrons of 4 s migrate towards
3d and give
sp3 hybridisation.
As
Ni(CO)4 has no unpaired electron so, it is diamagnetic in nature.
(d) In [Co(NH
3)6]3+, Co is in
Co3+ state and show the electronic configuration as follows
Due to presence of
NH3 as ligand,
[Co(NH3)6]3+ has no unpaired electron and therefore, is diamagnetic in nature.
Hence, option (a) is the correct.