The reaction of spirostannabifluorene with methyllithium gave lithium pentaorganostannate, which is an intermediate of the transmetalation reaction on the tin atom and the first example of a stable pentaorganostannate having five carbon substituents. The X-ray structural analysis reveals its slightly distorted trigonal bipyramidal structure, the apical bonds of which have a considerable s character. The unique structure of the lithium pentaorganostannate is also discussed by the aid of theoretical calculations.