Effects of Crystal Field Stabilization Energy (CFSE)? Coordination Chemistry

Crystal field Stabilization Energy (CFSE) is defined as the decrease in energy by the preferential filling of electrons in lower laying d orbitals.
1. Magnetic properties: Magnetic property of a complex depends on number of unpaired electrons, and the number of unpaired electrons in a complex depends on splitting of d orbitals. If the field produced by the ligands is strong, then 10Dq is greater than the pairing energy P and we get low spin or spin-paired complex. On the other hand if the field produced by the ligands is weak then 10Dq is less than Pairing energy and high spin complex is formed. E.g. [FeF6] where Floride is week field ligand, thus it splits d orbital to a lesser extent, In this case the energy gap 10Dq is less than the pairing energy so pairing will not take place and electron will be placed in both t2g and eg orbitals. Here all the electrons are unpaired so this becomes high spin complex and is strongly paramagnetic in nature. On the other hand Fe(CN)6-4 where cynaide is strong field ligand and this splits d orbitals in larger extent, that means now 10Dq is very high or we can say 10Dq is higher than the pairing energy. In this case pairing of electrons takes place as the energy gap between t2g and eg is very high. Now as most of the electrons are paired, this complex becomes low spin complex. And it is weakly paramagnetic in nature. This means the amount of energy gap between t2g and eg orbitals decides if the molecule will be low spin or high spin, in other words, it determines the magnetic property of the complex

2. Spectral Properties: In coordinate compounds new energy orbitals t2g and eg are produced due to splitting of d orbitals. And the energy gap between these orbital is almost equal to the energy of the radiation of visible light. And therefore when dd transition of electron from t2g to eg orbital takes place, the emitted radiation have energy in visible region that means from 400 to 800nm wavelength. And therefore these compounds appear highly coloured.

3. Variation in Ionic Radius: As we know that t2g orbitals are directed in-between the ligands and eg orbitals are directed towards the ligands, So when electrons are added to t2g orbitals, there will be less repulsion than the case when electrons are added to eg orbitals, this means as number of electrons are added to t2g orbitals, ionic radii goes on decreasing and when they are adde to eg orbitals ionic radiai goes on increasing, If we plot a graph of ionic radiai Vs the number of electrons added, It will look like this. This is the case of low spin complexes where first only t2g orbitals are filled with 6 electrons, but when 7th electron is added to eg orbital ionic radii starts increasing and it keeps on increasing as more and more electrons are added. Incase of high spin complexes where instead of pairing 4the electron goes to eg orbitals, ionic radii start increasing form the 4th electron itself, but it again starts decreasing as 6th electron is added back to t2g orbitals and finally when 8th electron goes to eg orbital ionic redii starts increasing.

4. Lattice Energy: The lattice energy is the amount of energy released when ions are combined to make a crystalline solid compound. That means the amount of energy released when crystalline solids are formed.  Lattice energy depends on the ratio of charge to ionic radii of both the ions. So if ionic radii is high lattice energy will be low and vice versa. In the graph we have shown the lattice energy of divalent complexes of first transition series with fluoride ions. As ionic radius decreases from Ca2+ to Zn2+ ion, a double humped curve with two peaks is observed. The ions Ca2+ (do), Mn2+ (d5) and Zn2+ (d10) lie on a nearly straight line because for these metal complexes CFSE is zero. Two maxima on each curve are observed at V2+ and Ni2+. This is because in weak ligand field (halides are weak ligands) V2+ (d) and Ni2+ (d) have maximum CFSE (12Dq).

5. Hydration Energy: Hydration energy is the amount of energy released when one mole of ions undergoes hydration or salvation. That means the amount of energy released when we dissolve one mole of crystalline solid in water. As in 1st series of transition elements in their +2 state, the ionic radii decrease from Sc to Zn, which bring the ligands or H2O closer to the metal ion, resulting in the increased electrostatic attraction between the metal ion and the ligands, consequently the values of hydration energy increase. Now if we plot the hydration energy of divalent ions of first row transition elements. It shows that there is no smooth increase as expected instead it shows maxima at V2+(d) and Ni2+(d). This is because the ionic radii for V and Ni are highest in case of strong field ligand like H2O.