pair with one of the t 2g electrons. In this scenario, the preferred configuration will be the one with lowest energy. If the octahedral crystal field splitting energy (Δ o ) is greater than the pairing energy (P), it is necessary to cause pairing of electrons in an orbital, then the fourth electron will pair up with an the electron in the t 2g orbital. Conversely, if the Δ o is lesser than P, then the fourth electron will occupy one of the degenerate higher energy e g orbitals.
For example, let us consider two different iron(III) complexes [Fe(H O) ] + (weak field complex; wave number corresponds to Δ o is 14000 cm - ) and [Fe(CN) ] - (Strong field complex; wave number corresponds to Δ o is 35000 cm - ). The wave number corresponds to the pairing energy of Fe + is 30000 cm - . In both these complexes the Fe + has d configuration. In aqua complex, the Δ o < P hence, the fourth & fifth electrons enter e g orbitals and the configuration is t 2g , e g .
In the cyanido complex Δ o > P and hence the fourth & fifth electrons pair up with the electrons in the t 2g orbitals and the electronic configuration is t 2g , e g . The actual distribution of electrons can be ascertained by calculating the crystal field stabilisation energy (CFSE). The crystal field stabilisation energy is defined as the energy difference of electronic configurations in the ligand field (E LF ) and the isotropic field/barycentre (E iso ). CFSE (ΔE o ) = {E LF } - {E iso } = {[n t2g (- .
)+n eg ( . )] Δ o + n p P} - {n' p P} Here, n t2g is the number of electrons in t 2g orbitals; n eg is number of electrons in e g orbitals; n p is number of electron pairs in the ligand field; & n' p is the number of electron pairs in the isotropic field