The ion [Co (NH3) 6]2+

isoctahedral and high spin. This complex is:

A. paramagnetic, with 1 unpaired electron.

B. paramagnetic, with 3 unpaired electrons.

C. paramagnetic, with 4 unpaired electrons.

D. paramagnetic, with 5 unpaired electrons.

E. diamagnetic.

The answer would be B.

As paramagnetic with 3 unpaired electrons. Since there are 6 ligands around the Co+2 ion it isoctahedral and these ligands are neutral. This makes the overall charge on the complex + 2 and therefore comes from the configuration for Co+2 which is [Ar] 3d7. Since it is in high spin you must fill all the orbitals with at least one electron and then pair up any that remain. If you do this, 3 unpaired electrons remain. Para magnetism occurs in substances with unpaired electrons.