Ncert Solutions Chemistry Class 12th

Compounds containing carbonyl

Ligands only are known as homoleptic carbonyl. Such types of compounds are formed by most of the transition metals. These metal carbonyls always have simple, well-defined structures. In metal carbonyls the metal - carbonyl bond possess both s and p.character. M-C-bond is sigma bond. It is formed by the donation of lone pair of electrons of the carbonyl carbon into the vacant orbital of the metal. The M-C pi bond is formed by the donation of a pair of electron from a filled d orbital of a metal into the vacant antibonding? orbital of carbon monoxide. Such type of metal to ligand bonding creates a synergic effect which strengthens the bond between CO and metal.

Overall charge balance in [Cr (NH₃)₆]³⁺ complex:

X + 6 (0) = + 3 X = + 3

Cr is in + 3 oxidation state.

Electronic configuration of Cr in + 2 state: 3d³ . Now ammonia is a weak field ligand so it not causes pairing of the unpaired electron and undergoes hybridisation to form 6 sp³d² hybrid orbitals filled by the six ammonia ligands. It's geometry is octahedral with unpaired electrons and hence is paramagnetic complex.

In case of [Ni (CN)₄]^2– ion :

Overall charge balance in [Ni (CN)₄]^2–complex:

X + 4 (-1) = -2 X = + 2

Ni is in + 2 oxidation state.

Electronic configuration of Ni in + 2 state: 3d⁸. Now cyanide ion is a strong field ligand so it causes pairing of the unpaired electron and undergoes hybridisation to form 4 dsp² hybrid orbitals filled by the four cyanide ligands. It's geometry is square planar with no unpaired electrons and hence is diamagnetic complex.

The difference between the energies of the two set of the d orbitals is called as crystal field splitting energy (CFSE). The degenerate d orbitals split into two levels i.e t_2g and e_g level due to the presence of the ligands. This splitting of the degenerate orbitals due to the ligand is called as crystal field splitting and the energy difference between the two levels is called as crystal field splitting energy.

After the splitting of the degenerate orbitals has taken place the filling of the electrons takes place. Now first 3 electrons goes into the lower energy three t_2g orbitals. The fourth electron can be filled in two ways:

It can enter the t_2g orbitals causing pairing up of the electron (giving rise to t_2g⁴e ⁰ electronic configuration) or it can enter into higher energy e_g orbital (giving rise to t_2g³e ¹ electronic configuration). If the CFSE value of ligand is more than energy required for the pairing up of the electron then it will result in the pairing of electron (2 case) but if the CFSE value of ligand is less than the pairing energy of the electron then it will cause the electron to enter in higher energy e_g orbital. (1 case)

The strong ligands have higher splitting power of d orbitals of the central metal ion, whereas weak ligand has relatively lower splitting power of d orbitals of the central metal ion. The energy difference between t_2g and e_g sets of d orbitals is CFSE. The strength of the ligands depend on the magnitude of Δ . Strong ligands have larger value of CFSE and in case of weak ligands the CFSE values are smaller. The common ligands can be arranged in a series in the order of their decreasing field strength, as follows.

This series depends on the power of splitting the d orbitals and is called spectrochemical series, The order of field strength of the common ligands neither depends on the geometry of the complex the nature of central metal ion.