NOTE: Important charges and non-bonding electrons are shown throughout the animation except during the transition state phase.
The anion produced in the first stage of the Mitsunobu reaction is basic enough to remove a proton from the formic acid producing the carboxylate anion.
Click the structures and reaction black arrows in sequence to view the 3D models and animations respectively
T. Y. S. But and P. Toy, Chem. - An Asian J., 2007, 2, 1340–1355.