Factors Affecting Magnitude of ∆o:
There are several factors which affect the magnitude of crystal field splitting (∆o) of d orbitals.
∆o for [Co(H2O)6] 2+ = 9200 cm–1
And ∆o for [Co(H2O)6]3+ = 20760 cm–1
This is because; the higher the o.s. of metal causes the ligands to approach more closely to it and, therefore, the more repulsion which causes more splitting.
CO > CN– > R3P > C6H5–, CH3– > NO2– > Phen, bpy > NH2OH > SO32–, en > Py, NH3 > gly > NCS– > H2O > O2– > C2O42–, > C2H5OH > OH– > urea > F > N3– > Cl– > SCN– >– S2– > Br– > I–
The order of field strength of the common ligand is independent of the nature of the metal cation and geometry of the complex. Usually,
Carbon donor > N-donor > O-donor > X-donor
I : the nature of the ligand
II : the charge on the metal ion
III : the size of metal cation
∆o > ∆t
High Spin and Low Spin Complexes
We saw how to calculate the CFSE of complexes having metal ion of d1, d2 and d3 configurations. Let’s discuss how to calculate CFSE of higher d electron metal complexes. For a d4 ion, two arrangements are available: the four electrons ma y occupy the t 2g set wit h the configuration t2g4eg0, or may singly occupy four d orbitals i.e. t2g3eg1.
High spin configuration Low spin configuration
First configuration is called high spin and the second is called the low spin.
High spin complexes or spin free complexes are formed in the presence of weak field ligands (oxygen and halogen donor ligands) while low spin or spin paired complexes are formed in the presence of strong field ligands (carbon and nitrogen donor ligands). In low spin complexes we forcibly pair up electrons. Hence pairing energy is introduced while calculating CFSE of low spin complexes according to the ground state configuration.
For example, the occupation of the 3d orbitals in weak and strong field Fe3+ (d5) complexes is shown below.
Pairing Energy: The energy required to pair up two unpaired electrons in a orbital is called pairing energy. It is denoted by P.
If ∆o > P, low spin complexes are formed.
If ∆o < P, high spin complexes are formed.
If ∆o = P, high spin and low spin complexes exist in equilibrium.
CFSE in High and Low Spin Complexes:
d4 Configuration:
High Spin CFSE = nt2g × (-0.4∆o) + neg × (0.6∆o) = 3 × (-0.4∆o) + 1 × (0.6∆o) CFSE = -0.6∆o | Low Spin CFSE = nt2g × (-0.4∆o) + neg × (0.6∆o) |
d5 Configuration:
High Spin CFSE = 0 | Low Spin CFSE = 5 × (–0.4∆o) + 0 × (0.6∆o) + 2P CFSE = –2.0 ∆o + 2P |
d6 Configuration:
High Spin CFSE = 4 × (–0.4∆o) + 2 × (0.6∆o) CFSE = –0.4∆o | Low Spin CFSE = 6 × (–0.4∆o) + 0 × (0.6∆o) + 2P CFSE = –2.4∆o + 2P |
Pairing energy is only taken for those electrons which are not paired in the ground state and we are forcibly pairing up them in the complex.
d7 Configuration:
High Spin CFSE = 5 × (–0.4∆o) + 2 × (0.6∆o) CFSE = –0.8∆o | Low Spin CFSE = 6 × (–0.4∆o) + 1 × (0.6∆o) + P CFSE = –1.8∆o+ 2P |
d8 Configuration:
CFSE = 6 × (–0.4∆o) + 2 × (0.6∆o)
CFSE = –1.2∆o
d9 Configuration:
CFSE = 6 × (–0.4∆o) + 3 × (0.6∆o)
CFSE = –0.6∆o
d10 Configuration:
CFSE = 6 × (–0.4∆o) + 4 × (0.6∆o) = 0
The following table summarises the CFSE values of all the electronic configurations in an octahedral complex.
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1. What are the factors affecting the magnitude of Δo in coordination chemistry? |
2. How does the nature of the metal ion affect the magnitude of Δo in coordination chemistry? |
3. How does the nature of the ligands affect the magnitude of Δo in coordination chemistry? |
4. How does the oxidation state of the metal ion affect the magnitude of Δo in coordination chemistry? |
5. How does the geometry of the complex affect the magnitude of Δo in coordination chemistry? |
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