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Octahedral Complexes 

Inner Orbital Complexes: 

[Co(CN)6]3-: In this complex the oxidation state (O.S.) of cobalt is +3. The valence shell electronic configuration is 3d6. The CN- ligands are strong, therefore pairing of 3d electrons takes place. This lead to the formation of d 2sp3 configuration with the combination of two 3d, 4s and three 4p orbitals. These six hybrid orbitals overlap with six filled orbitals of ligands forming six coordinate bonds. The geometry of the complex is octahedral and since there are no unpaired electrons left, this complex is diamagnetic.

 Octahedral & Tetrahedral Complexes | Inorganic Chemistry
Octahedral & Tetrahedral Complexes | Inorganic Chemistry

Mn(CN)6]3–: In this complex the O.S. of Mn is +3 and its valence shell configuration is 3d4. Since CN is a strong field complex, pairing takes place according to the Hund’s rule of maximum multiplicity.
The pairing lead to the formation of six d2sp3 hybrid orbitals thereb y forming an octahedral complex. Since there are two unpaired electrons in the complex, it is paramagnetic. 

 Octahedral & Tetrahedral Complexes | Inorganic Chemistry
Octahedral & Tetrahedral Complexes | Inorganic Chemistry Octahedral & Tetrahedral Complexes | Inorganic Chemistry

The magnetic moment for the above complexes can be calculated by the following formula: Magnetic moment  Octahedral & Tetrahedral Complexes | Inorganic ChemistryBohr’s magneton,         where  n = no. of unpaired e-s

For example, for   Octahedral & Tetrahedral Complexes | Inorganic Chemistry= 2.82 B.M., (paramagnetic)

 

[V(NH3)6]3+: In this ion the O.S. of vanadium is +3 and its valence shell electronic configuration is 3d2. Alt hough, the NH3 is a strong ligand, this will not lead to the pairing of electrons because there are enough inner orbitals available for hybridisation. The orbitals combine to form six d2sp3 hybrid orbitals having octahedral geometry. The complex is paramagnetic since there are two unpaired electrons.

Octahedral & Tetrahedral Complexes | Inorganic Chemistry
Octahedral & Tetrahedral Complexes | Inorganic ChemistryOctahedral & Tetrahedral Complexes | Inorganic Chemistry
 

n = 2,   Octahedral & Tetrahedral Complexes | Inorganic Chemistry = B.M. = 2.88 B.M., (paramagnetic)

 

Outer Orbital Complexes: 

[CoF6]3–: In this the outer shell electronic configuration of cobolt is 3d6. F is a weak ligand, hence there will be no pairing of 3d electrons of the metal cation. This led to the formation of six sp3d2 orbitals with the help of outer 4d orbitals. The geometry is octahedral and the complex is paramagnetic with correspondence to four unpaired electrons.

Octahedral & Tetrahedral Complexes | Inorganic Chemistry
Octahedral & Tetrahedral Complexes | Inorganic Chemistry
Octahedral & Tetrahedral Complexes | Inorganic Chemistry
n = 4,  Octahedral & Tetrahedral Complexes | Inorganic Chemistry= 4.90 B.M. (paramagnetic)

 

[Fe[H2O]5(NO)]2+ ion: This complex is formed in the blue ring test of nitrate ion. In this the o.s. of Fe is +1 because NO exist in +1 oxidation state in complexes of Fe. Hence the valence shell electronic configuration is 3d64s1. The H2O is a weak field ligand where as NO+ is a strong field ligand. This single NO+ ligand lead to the pairing of only two unpaired electrons, thereby making valence shell electronic configuration 3d6. Therefore sp3dhybridisation takes place and an octahedral complex is formed. The magnetic moment corresponds to the four unpaired electrons.

Octahedral & Tetrahedral Complexes | Inorganic Chemistry

In presence of Noligand 

Octahedral & Tetrahedral Complexes | Inorganic Chemistry

Octahedral & Tetrahedral Complexes | Inorganic Chemistry
 n = 3, Octahedral & Tetrahedral Complexes | Inorganic Chemistry  B.M. = 3.87 B.M. (paramagnetic)

 

Tetrahedral Complexes

Tetrahedral complexes are either sp3 or sd3 hybridised. Examples are shown below.

[NiCl4]2–In this the outer shell electronic configuration of Ni (+2 o.s.) is 3d8. Since Cl is a weak ligand, pairing of 3d electrons does not take place. None of the 3d orbitals are vacant. The vacant 4s and 4p orbitals hybridised to give four equivalent sp3 hybrid orbitals thereb y forming tetrahedral geometry. There are two unpaired electrons. Hence, the magnetic moment of the complex is 2.44.

Octahedral & Tetrahedral Complexes | Inorganic ChemistryOctahedral & Tetrahedral Complexes | Inorganic Chemistry

n = 2,     Octahedral & Tetrahedral Complexes | Inorganic Chemistry   = 2.82  B.M. (paramagnetic)


[Ni(CO)4]: In this complex the o.s. of Ni is zero. Hence the electronic configuration of outermost shell is 3d84s2. Since CO is a strong ligand, pairing of electrons do occur and after pairing there is no vacant 3d orbitals left. Thus the vacant orbital available for hybridisation are 4s and 4p which combine to give four sp3 hybrid orbitals. This will form tetrahedral geometry and the magnetic moment is zero as there are no unpaired electrons. Therefore the complex is diamagnetic.

Octahedral & Tetrahedral Complexes | Inorganic Chemistry

In presence of CO ligand

Octahedral & Tetrahedral Complexes | Inorganic Chemistry
Octahedral & Tetrahedral Complexes | Inorganic Chemistry

Octahedral & Tetrahedral Complexes | Inorganic Chemistry Diamagnetic

 

KMnO4: In the MnO4– complex ion, the o.s. of Mn is +7. The ground state configuration of M n is 3d54s2. In Mn7+ ion all the five 3d, 4s-orbitals are vacant. The vacant 4s and 3d orbitals hybridised to give sd3 hybrid orbitals. Thus Mn7+ ion is sd3 hybridised and form tetrahed ral geometry in t his complex ion as shown below: 

Octahedral & Tetrahedral Complexes | Inorganic Chemistry
Octahedral & Tetrahedral Complexes | Inorganic Chemistry  Octahedral & Tetrahedral Complexes | Inorganic Chemistry
                                                                                         Octahedral & Tetrahedral Complexes | Inorganic Chemistry

Similar arrangement is also observed for K2Cr2O7 (Cr2O72- ion).

The document Octahedral & Tetrahedral Complexes | Inorganic Chemistry is a part of the Chemistry Course Inorganic Chemistry.
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FAQs on Octahedral & Tetrahedral Complexes - Inorganic Chemistry

1. What is the difference between octahedral and tetrahedral complexes?
Ans. Octahedral complexes have six ligands surrounding the central metal ion, arranged in an octahedral shape, while tetrahedral complexes have four ligands arranged in a tetrahedral shape around the metal ion. The main difference between the two is the coordination number and geometry.
2. How do octahedral and tetrahedral complexes form?
Ans. Octahedral complexes usually form when a metal ion with a coordination number of six reacts with six ligands. The ligands bond with the metal ion in a way that produces an octahedral geometry. On the other hand, tetrahedral complexes form when a metal ion with a coordination number of four reacts with four ligands, resulting in a tetrahedral geometry.
3. What factors determine the geometry of octahedral and tetrahedral complexes?
Ans. The geometry of octahedral and tetrahedral complexes is determined by several factors, including the nature of the ligands, the size of the metal ion, and the electron configuration of the metal ion. Ligands with a large size or high charge density tend to favor octahedral geometry, while ligands with a smaller size or lower charge density favor tetrahedral geometry.
4. Are octahedral complexes more stable than tetrahedral complexes?
Ans. The stability of octahedral and tetrahedral complexes depends on various factors such as ligand type, metal ion size, and electronic effects. In general, octahedral complexes are more stable than tetrahedral complexes due to the greater symmetry and more efficient packing of ligands around the metal ion in octahedral geometry. However, there are exceptions based on specific ligand-metal ion interactions.
5. What are some examples of octahedral and tetrahedral complexes in coordination chemistry?
Ans. Some examples of octahedral complexes include [Co(NH3)6]3+, [Ni(CN)6]4-, and [Fe(H2O)6]2+. These complexes have a coordination number of six and ligands arranged in an octahedral shape around the central metal ion. Examples of tetrahedral complexes include [PtCl4]2-, [CuCl4]2-, and [Fe(CO)4]. These complexes have a coordination number of four and ligands arranged in a tetrahedral shape around the metal ion.
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