Paramagnetism of [Co(NH3)6]Cl3

Coordination Complex
The complex [Co(NH3)6]Cl3 consists of a central cobalt ion coordinated to six ammonia ligands and three chloride ions.

Electronic Configuration of Cobalt
Cobalt has the electronic configuration [Ar] 3d7 4s2. In the complex, each ammonia ligand acts as a weak field ligand, causing no pairing of electrons in the d orbitals of cobalt.

Number of Unpaired Electrons
Since cobalt has 7 d-electrons and no pairing occurs due to the weak field ligands, there are 3 unpaired electrons in the d orbitals of the cobalt ion.

Paramagnetic Nature
A substance is considered paramagnetic if it contains unpaired electrons. In the case of [Co(NH3)6]Cl3, the presence of 3 unpaired electrons makes it paramagnetic.

Explanation
Even though the complex has a coordination number of 6 with ligands that are weak field ligands, the lack of electron pairing in the d orbitals leads to the presence of unpaired electrons, resulting in its paramagnetic nature.

Conclusion
In summary, [Co(NH3)6]Cl3 is paramagnetic due to the presence of 3 unpaired electrons in the d orbitals of the central cobalt ion, which arises from the weak field ligands not causing electron pairing.