According to the valence bond theory, the metal atom or ion under the influence of ligands can use its (n-1)d, ns, np, nd orbitals for hybridization to yield a set of equivalent orbitals of definite geometry. These hybridized orbitals are allowed to overlap with ligand orbitals that can donate electron pairs for bonding. It is usually possible to predict the geometry of a complex from the knowledge of its magnetic behaviour on the basis of the valence bond theory. Consider the formation of \([Co(NH_3)_5Cl]Cl_2\) and answer the following question:
The coordination number of cobalt in the above coordination entity is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 6
The coordination number of the metal is the number of ligand donor atoms directly bonded to it, i.e. the ligands inside the square bracket. In \( \ce{[Co(NH3)5Cl]Cl2} \) the coordination sphere holds five NH3 and one Cl−:
5 + 1 = 6
The two \( \ce{Cl^{-}} \) written outside the bracket are counter ions; they are not bonded to cobalt.
NCERT (2026-27 edition):
Coordination Number – “The coordination number (CN) of a metal ion in a complex can be defined as the number of ligand donor atoms to which the metal is directly bonded. For example, in the complex ions, [PtCl6]2– and [Ni(NH3)4]2+, the coordination number of Pt and Ni are 6 and 4 respectively. Similarly, in the complex ions, [Fe(C2O4)3]3– and [Co(en)3]3+, the coordination number of both, Fe and Co, is 6 because C2O42– and en (ethane-1,2-diamine) are didentate ligands.”