could either enter the t 2g level and pair with an existing electron, or (ii) it could avoid paying the price of the pairing energy by occupying the e g level. Which of these possibilities occurs, depends on the relative magnitude of the crystal field splitting, D o and the pairing energy, P (P represents the energy required for electron pairing in a single orbital). The two options are: (i) If D o < P, the fourth electron enters one of the e g orbitals giving the configuration 2g g t e . Ligands for which D o < P are known as weak field ligands and form high spin complexes.
(ii) If D o > P, it becomes more energetically favourable for the fourth electron to occupy a t 2g orbital with configuration t 2g e g . Ligands which produce this effect are known as strong field ligands and form low spin complexes. Calculations show that d to d coordination entities are more stable for strong field as compared to weak field cases. Fig.
. : d orbital splitting in an octahedral crystal field Fig. . : d orbital splitting in a tetrahedral crystal field.
In the previous Unit, we learnt that one of the most distinctive properties of transition metal complexes is their wide range of colours. This means that some of the visible spectrum is being removed from white light as it passes through the sample, so the light that emerges is no longer white. The colour of the complex is complementary to that which is absorbed. The complementary colour is the colour generated from the wavelength left over; if green light is absorbed by the complex, it appears red.
Table . gives the relationship of the different wavelength absorbed and the colour observed. . .
Colour in Coordinaton entity Wavelength of light absorbed (nm) Colour of light absorbed Colour of coordination entity Table . : Relationship between the Wavelength of Light absorbed and the Colour observed in some Coordination Entities [CoCl(NH ) ] Yellow Violet [Co(NH ) (H