chemistry

COLOUR OF HYDRATED TRANSITION METAL IONS AND THEIR COMPOUNDS

•        A substance is coloured, if it absorbs a part of white light and transmit the remaining light.

•        The colour of a substance is the complementary colour of that part of visible light which is absorbed by the substance.

•        Colour is due to the presence of partly filled d-orbtials, with unpaired electrons.

•        The metal ion possessing completely filled d-orbitals or completely vacant d-orbtials is colourless. Ex TiO₂, CuCl.

•        d-orbitals are degenerate in isolated gaseous metal ions.

•        d-orbitals of the metal ion in compounds or hydrated ions or complexes posses slightly different energies.

•        Under the influence of the anion of the compound or the water molecule in hydrated state these

d-orbitals of the metal ions split into 2 sets. It is known as d-orbital splitting.

•        One set consists of two orbitals - d_x2−y2,d_z2of higher energy and the other set consists three orbitals - d_xy,d_yz,d_zxof lower energy.

•        Electron in the lower energy d-orbital is promoted to higher energy d-orbital with in the same energy level.

•        Thus, the colour of transition metal ions involve d–d transitions.

•        This excitation is possible in visible region (1 = 400 – 750 nm) as the energy difference between the two sets is less.

•        [Ti(H₂O)₆]³⁺absorbs green and yellow lights and transmits pink colour.

•        The same metal ion may exhibit different colours in different oxidation states.

•	Fe++ - green ;	Fe+++ - yellow
•	Cr2+ - blue	Cr3+ - green   Cr6+ - yellow
•	Mn2+ - pink	Mn3+ - blue   Mn6+ - green

•        Sc³⁺, Ti⁴⁺, Mn⁷⁺- are colourless as all the d-orbitals in these ions are vacant

•        Cr⁶⁺and Mn⁷⁺ posses vacant d orbitals but their oxyanions like Cr₂O₇^2–, CrO₄^2–and MnO₄^–are coloured due to charge transfer phenomenon.

Zn⁺⁺and Cu⁺ are colour less as all the d orbitals are completely filled