tell some shorthand method to learn spectrochemical series....n please explain crystal field theory again.... I am not getting this explanation
Spectrochemical series
Ligands are arranged in a series in the increasing order of the field strength as follows:
Ligands for whichΔo(crystal-field splitting)
Ligands for whichΔo(crystal-field splitting) P (pairing energy), are called strong-field ligands, and form low-spin complexes.
Crystal-Field Splitting in Tetrahedral Coordination Entities
The splitting ofd-orbitals in a tetrahedral crystal field is shown in the given figure.
Δt= (4/9)Δo
The orbital-splitting energies are not sufficiently large for forcing pairing, and therefore, low-spin configurations are rarely observed.
Colour in Coordination Compounds
The colour of the coordination compounds is attributed toddtransition of electrons.
For example, the complex [Ti(H2O)6]3+is violet in colour, which is due toddtransition (shown in the figure)
The energy of yellow-green region is absorbed by the complex, and then, the electron gets excited fromt2glevel to theeglevel, i.e.,
The relationships between the wavelength of light absorbed and the colour observed in some coordination entities are listed in the given table.
Coordination entity
Wavelength of light absorbed (nm)
Colour of light absorbed
Colour of coordination entity
[CoCl(NH3)5]2+
535
Yellow
Violet
[Co(NH3)5(H2O)]3+
500
Blue-green
Red
[Co(NH3)6]3+
475
Blue
Yellow-orange
[Co(CN)6]3
310
Ultraviolet
Pale yellow
[Cu(H2O)4]2+
600
Red
Blue
[Ti(H2O)6]3+
498
Blue-green
Purple
Inthe absence of ligand, crystal-field splitting does not occur; hence, the substance is colourless. For example, the removal of water from [Ti(H2O)6]Cl3on heating renders it colourless. However, the complex [Ti(H2O)6]3+is violet in colour.
Limitations ofCrystal-Field Theory
Anionic ligands are assumed to exert the greatest splitting effect. But the anionic ligands are found at the lower end of the spectrochemical series.
Crystal-field theory does not take into account the covalent character of bonding between ligand and the central metal atom.
