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Why Fe(CO)5 is colourless while Fe(bipy)(CO)3 is intensely purple in colour ?

The intense colour of the latter complex is strongly suggestive of a charge transfer transition and since the metal is already fully reduced (zero oxidation state), it is highly likely that this involves a MLCT transition. The π* levels of the bipy or CO ligands are possible acceptors but the fact that Fe(CO)5 doesn’t show this colour suggests that it is the bipy π* levels that are involved in Fe(bipy)(CO)3. Since there should be MLCT transitions to the CO π* levels as well, we assume that the lack of colour for Fe(CO)5 means that these transitions fall in the UV rather than the visible.

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