Geometrical Isomerism in Coordination Complexes

In coordination chemistry, geometrical isomerism arises when different spatial arrangements of ligands are possible around a central metal ion. These isomers have the same chemical formula but differ in the relative positions of the ligands.

Coordination Entity: [Co(NH3)3(NO2)3]

The coordination entity given in the question is [Co(NH3)3(NO2)3]. This complex contains a central cobalt (Co) ion surrounded by three ammonia (NH3) ligands and three nitrite (NO2) ligands. The formula indicates that these ligands are coordinated to the metal ion.

Octahedral Geometry

The coordination number of the central cobalt ion is 6, which suggests an octahedral geometry. In an octahedral complex, the metal ion is surrounded by six ligands arranged at the corners of an octahedron. These ligands can be positioned in different ways, resulting in different geometrical isomers.

Facial (fac) Isomer

When all three nitrogen (N) atoms of the amine ligands occupy adjacent positions at the corners of an octahedral face, the geometrical isomer formed is known as the facial (fac) isomer. In this arrangement, the three amine ligands are adjacent to each other, forming a face of the octahedron.

The fac isomer can be represented as:

[Co(NH3)3(NO2)3] (fac)

The prefix "fac" indicates that the ligands are arranged in a facial manner. In this case, the nitrite ligands are positioned in the remaining three corners of the octahedron.

Other Possible Isomers

In addition to the fac isomer, there are other possible isomers in octahedral complexes:

1. Cis Isomer: In this isomer, two identical ligands are adjacent to each other, either on the same face or on opposite faces of the octahedron. However, in the given coordination entity, all three nitrogen atoms of the amine ligands are adjacent, so it cannot be the cis isomer.

2. Trans Isomer: In this isomer, two identical ligands are on opposite faces of the octahedron. Again, in the given coordination entity, all three nitrogen atoms of the amine ligands are adjacent, so it cannot be the trans isomer.

3. Mer Isomer: In this isomer, three identical ligands are arranged around the metal ion in a meridional manner. However, in the given coordination entity, the nitrogen atoms of the amine ligands are adjacent on the same face, so it cannot be the mer isomer.

Therefore, the only possible geometrical isomer formed in the coordination entity [Co(NH3)3(NO2)3] is the facial (fac) isomer.