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 IUPAC name of the above coordination entity is

Pentaamminechloridocobalt (III) chloride

The correct answer is OPTION 3 - Pentaamminechloridocobalt (III) chloride

• Oxidation State--To find the name, we first calculate the oxidation state of the central metal atom (Cobalt):

  • Let the oxidation state of Cobalt be x.

  • Ammine (NH3) is a neutral ligand, so its charge is 0.

  • Chlorido (Cl) is an anionic ligand, so its charge is -1.

  The calculation for the whole neutral compound is:

  x + 5(0) + 1(-1) + 2(-1) = 0

  x - 1 - 2 = 0

  x = +3

• Ligands: "Pentaammine" and "chlorido" (alphabetical order).

• Metal: It is a cationic complex, so we use Cobalt. "Cobaltate" is only for anionic complexes.

• Counter Ion: Named simply as chloride. Never use "di-" for ions outside the bracket.

Hence, the final IUPAC name is Pentaamminechloridocobalt (III) chloride