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Why do we add one electron to the ligands that have an odd electron number?

For example the cyclopentadienyl ligand has five electrons that can be donated to the metal but when we count the electrons by the ionic method we add one, so it become total of 6 electrons donated to metal.

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In the ionic method of counting, you are not counting neutral compounds. This is reasonable on a molecular basis: the ligand is, in fact, not cyclopentadiene or the cyclopentadienyl radical (with four or five electrons to be donated) but the cyclopentadienyl anion $\ce{C5H5-}$. This species contains six electrons in its π system, meaning that six are donated.

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