Abstract: | The recently discovered and characterized Pd4(dppm)4(H)]2+ cluster catalyst ( 1 ; dppm = Ph2PCH2PPh2), slowly evolves in the presence of the reducing tetraphenylborate anion, to generate a new diamagnetic cluster Pd4(dppm)4(H)]+ ( 2 ). The evolution of this starting material 1 , has been monitored using NMR (1H and 31P), UV‐vis and ESR spectroscopy. This new 56‐electron Pd cluster has been characterized from X‐ray crystallography, and consists of a cyclic species exhibiting an approximate puckered square structure. The Pd2 bond distances are 2.7367(10) and 2.7495(11)Å and indicate the presence of weak bonding. The diagonal Pd···Pd separations are 3.646(10) and 3.590(10)Å indicating that the square is relatively symmetric. Such a structure is unprecedented for “Pdx(dppm)x” species. Although not formally observed from the X‐ray data, the hydride is assumed to be fluxional as found in 1 . The cyclic voltammogram for 2 exhibits an irreversible reduction wave at —1.65V vs SCE which is greater than that found for 1 , and corroborates the lower oxidation state for Pd (+1/2). The Pd‐H bonding scheme and MO symmetry for a model cluster where the hydride has been placed at the center of the Pd4 frame, have been addressed qualitatively using the EHMO model. These calculations demonstrate clearly that the Pd‐H bonding is strong. |