Colour Due to Charge Transfer)

Colour due to charge transfer By Abhishek Dubey and Ganesh Karthik in Chemistry World (Files) · Edit Doc Ligand-to-metal charge transfer LMCT complexes arise from transfer of electrons from MO with ligand like character to those with metal like character. This type of transfer is predominant if complexes have ligands with relatively high energy lone pairs (example S or Se) or if the metal has low lying empty orbitals. Many such complexes have metals in high oxidation states (even d0). These conditions imply that the acceptor level is available and low in energy. Consider a d6 octahedral complex (example IrBr63-). The t2g levels are filled as shown in Figure 1. Consequently an intense absorption is observed around 250 nm corresponding to a transition from ligand σ MO to the empty eg MO. However, in IrBr62- that is a d5 complex two absorptions, one near 600 nm and another near 270 nm, are observed. This is because two transitions are possible, one to t2g (that can n ow accommodate one more electron) and another to eg. The 600 nm band corresponds to transition to the t2g MO and the 270 nm band to the eg MO. Figure 1. MO diagram showing ligand to metal c harge transfer for a d6 octahedral complex Another thing to note is that CT bands might also arise from transfer of electrons from nonbonding orbitals of the ligand to the eg MO. Trend of LMCT energies Oxidation Number +7 MnO4- < TcO4- < ReO4- +6 CrO42- < MoO42- < WO42- +5 VO43- < NbO43- < TaO43- The energies of transitions correlate with the order of the electrochemical series. The metal ions that are most easily reduced correspond to the lowest energy transitions. The above trend is consistent with transfer of electrons from the ligand to the metal, thus resulting in a reduction of metal ions by the ligand. Examples include: 1. MnO4- : The permanganate ion having tetrahedral geometry is intensely purple due to strong absorption involving charge transfer from MO derived primarily from filled oxygen p orbitals to empty MO derived from manganese(VII). 2. CdS: The color of artist’s pigment cadmium yellow is due to transition from Cd2+ (5s) ← S2-(π). 3. HgS: it is red due to Hg2+ (6s) ← S2-(π) transition.

Colour Due to Charge Transfer)

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