The guest-host chemistry of the Pd3(dppm)3C02+ cluster (dppm = ( ( C~H S )~P ) ~C H~) has been investigated in some detail for a series of inorganic and organic substrates by X-ray crystallography (in two cases), UV-visible (and IR) spectroscopy, and molecular mechanics. The two X-ray crystallography characterized complexes are the [Pd3(dppm)3-CO](PF6)y(CH3)2CO and [Pd3(dppm)3CO](CF3C02)(PF6).2(CH3)2CO, which have been used to obtain model structures of the empty cavity (formed by the six upper dppm-phenyl groups) and filled cavity (by the CF3CO2anion), respectively. The latter was also used as a starting structure for the computations. The binding constants ( K 1 were measured spectroscopically using the Benesi-Hildebrand (B.-H.), Scatchard (Scat), and Scott (Scot) methods for about 20 different substrates. The stoichiometry of the association is found to be 1:l where the K I I values range from 0.07 to 10 000 M-I. The substrate-cluster associations are competitive and reversible for most studied systems. In some cases (nitro, cyano, and diazonium derivatives), very slow thermal reactions have been observed. Further molecular mechanic calculations on the cluster-aromatic associated complexes showed that agostic interactions are possible at the minimum computed energy configuration for the aromatic compounds. Overall, these studies show that the binding strength of a substrate (both organic and inorganic) into the bifunctional cavity (metallic center and hydrophobic section) is found to be related to a combination of parameters. These properties are the substrate charge and ligand strength, and the size and hydrophobic properties of the substrates. Crystal data: [Pd3(dppm)3CO](PF6)2, 173 K, triclinic (PI), a = 13.640(4) A, b = 14.0639(17) A, c = 22.4835(15) A, a = 104.291(7)', 0 = 105.143(17)', y = 99.301(18)', V = 3914.7(13) A3, Z = 2, R = 0.039, R, = 0.037; [Pd3(dppm)3CO](CF3C02)(PF6),293 K,monoclinic(P21),a= 11.0189(10)A,b= 26.6515(19)A,c= 14.4746(14) A, 0 = 99.786(9)', V = 4188.9(6) A3, Z = 2, R = 0.057, R, = 0.053. organic molecules and ions within the cavity of the P d 3 ( d~p m )~-COz+ cluster in a variety of solvents at 298 K. During the course of this work, the X-ray structures of [Pt3(dppm)3CO](X)-(Y).n(CH3)2CO ( x = Y = PF6-, n = 1; x = CF3C02-, Y = PFs-,

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The photoinduced addition of 0, onto the unsaturated cluster Pd,(dppm),C~~+ (as a CF,CO,-salt) in acetonitrile is reported. The final product ~d , ( d~~m ) , ( O , )~ (~( 0 , ) = 838 and 866 cm-') is formed in a multistep fashion in which two photochemical intermediates are observed (presumably ~d,(dppm),(0,)(~0)~+ and ~d , ( d~~m ) , ( 0 , )~+ ) .No X-ray structure could be obtained, but numerous spectroscopic findings demonstrate that 0, binds the Pd, center as a peroxo-02, and acts as a two-electron donor that triply bridges the metal atoms (forming a 44-electron cluster). The very small excited state lifetimes (between 25 and 35 ? 10 ps) obtained by picosecond flash photolysis indicate that the primary photoreaction is unimolecular, and demonstrate that the first photochemically added 0, molecule must be preassembled in the excited state prior to any photoinduced transformation. This ~d , ( d~~m ) , C 0~+ . . . 0 , ground state complex is responsible for the photoinduced production of the bisdioxygen compound and can be observed by UV-visible spectroscopy. The low efficiency of the photoreaction (quantum yield (0) = 0.033 ? 0.004) is explained by the very short excited state lifetime at 298 K, and the competition of 0, with solvent molecules to occupy the unsaturated site of the empty cavity in ~d , ( d~~m ) ,~0~+ (i.e., ground state guest-host chemistry). The binding constant for 0, with ~d , ( d~~m ) ,~0~+ is roughly estimated to range between 1 and 730 M-I in the ground state and is considered to be weak.