An unusual square tetrapalladium sheet sandwich complex, [Pd-4(mu(4)-C9H9) (mu(4)-C8H8)][B(Ar-f)(4)] has been isolated and structurally characterized. X-ray structure analysis showed that the square palladium sheet is flanked by the nine-membered cyclononatetraenyl and eight-membered cyclooctatetraene ligands to form a square metal sheet molecular sandwich structure. The coordinative unsaturation of the complex was revealed by phosphine titration experiments.