Why are electrons added in the ionic counting method?
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 &#x3c0; system, meaning that six are donated.