Q.20 (a) Write the formula to calculate EAN.
(b) Explain formation of [Co(NH3)6]3+ complex ion with respect to :
(i) Type of hybridisation
(ii) Magnetic property
Best answer
Answer: (a) EAN = Z − X + Y; (b) d2sp3, octahedral, diamagnetic.
(a) EAN = Z − X + Y, where Z = atomic number of the metal, X = number of electrons lost in forming the metal ion (its oxidation state), Y = number of electrons donated by the ligands (2 × number of donor atoms).
(b) Co (Z = 27): [Ar]3d74s2; Co3+: [Ar]3d6. NH3 is a strong field ligand, so the six 3d electrons pair up in three 3d orbitals, leaving two 3d orbitals empty.
(i) Hybridisation: two 3d, one 4s and three 4p orbitals mix to give six d2sp3 hybrid orbitals, which accept six lone pairs from six NH3 molecules. The complex is octahedral and inner orbital (low spin).
(ii) Magnetic property: all electrons are paired (no unpaired electron), so [Co(NH3)6]3+ is diamagnetic.
Note: the printed paper writes the ion as [CO(NH3)6]3+; this is a misprint for cobalt (Co), since CO (carbon monoxide) cannot carry a 3+ charge here. The answer is for [Co(NH3)6]3+.
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