The preparation of two new and unusual cationic Pd(I) dimeric complexes, containing bridging PBu2t and terminal PHBu2t ligands, is described. One of these, [Pd2(mu-PBu2t)(mu-PHBu2t)(PHBu2t)2]+, [2]+, contains a Pd-H-P bridging unit, the first example of this type of bonding. A second, [Pd2(mu-PBu2t) (CO) 2(PHBu2t)2]+, [3]+, represents a rare example of a Pd(I) complex with terminal CO ligands. Electrochemical, ESR, one-and two-dimensional multinuclear NMR, and X-ray crystallographic studies have been carried out on both [2]+ and [3]+. Relevant crystallographic parameters for the two triflate complexes: compound [2a]+ triclinic space group P1, a = 10.245(1) angstrom, b = 10.739(1) angstrom, c = 22.624(3) angstrom, a = 79.816(9)-degrees, beta = 87.876(10)-degrees, gamma = 68.266(8)-degrees, V = 2274.7(5) angstrom3, z = 2; compound [3a]+ orthorhombic, space group Pbca, a = 21.910(7) angstrom, b = 21.120(3) angstrom, c = 16.625(8) angstrom, V = 7707(4) angstrom3, Z = 8

Palladium(II) bis(tricyclohexylphosphine) cationic hydrides have been obtained by oxidative addition of the strong acids [H3O+][BF3OH-] or [H3O+][BF4-] to (PCy3)2Pd, 1; by this route, the compounds trans-[(Cy3P)2Pd(H)(H2O)]X (X = BF3OH, 2; X = BF4,4) have been isolated and characterized. A strong dependence of the stability of trans-[(Cy3P)2Pd(H)(L)]X complexes upon the L-X combinations was observed. When L = H2O, stable cationic hydrides are obtainable only with fluorine-containing anions (BF3OH-, BF4-, or PF6-), while immediate decomposition to 1 was observed when the metathesis of X- with BPh4- or B(n-Bu)4- was attempted. Stable complexes with X = BPh4 could be isolated when L = MeCn. No reaction was observed when 1 was reacted with [Et2OH+][BF4-]. These findings can be explained in principle by two hypotheses: (i) when the strength of the M-L bond is not sufficient to stabilize the [(R3P)2Pd(H)(L)]X complexes, further thermodynamic assistance is furnished by hydrogen-bond formation between the metal-bonded ligand L and the X counteranion, and/or (ii) depending upon the nature of L, the cationic hydrides are sufficiently acidic to decompose reactive anions such as BPh4- or B(n-Bu)4-. IR and crystallographic data on trans-[(Cy3P)2Pd(H)(H2O)]BF4 show the existence, in the solid state, of strong hydrogen bonds between the Pd-bound water molecule and the BF4- anion. A conductivity study on 4 and on the related hydride trans-[(Cy3P)2Pd(H)(MeCN)]BF4, 5, shows the existence of large concentrations of ions both in 4 and 5 solutions, though the values of LAMBDA-M (10-20% lower for 4 as compared to 5) could account for the presence in 4 of appreciable amounts of the hydrogen-bonded species. The aquohydride 4 has evidenced its acidic behavior by protonating strong and weak bases (Ph3C-, OH-, Et3N, and PCy3), and further information on its acid-base behavior has been obtained through the study of exchange reactions with D2O. The aquodeuteride trans-[(Cy3P)2Pd(D)(H2O)]BF4. 9, can in fact be obtained by reacting for a few minutes a solution of complex 4 with D2O while the d3 derivative trans-[(Cy3P)2Pd(D)(D2O)]BF4, 10, can only be obtained after prolonged reaction times. Complex 2 behaves similarly, and trans-[(Cy3P)2Pd(D)(H2O)]BF3OD, 11, or trans-[(Cy3P)2Pd(D)(D2O)]BF3OD, 12, can be prepared by controlling the reaction times with D2O. These data suggest that the acidity of the hydridic hydrogen is higher than the acidity of the hydrogens of the PdOH2 moiety. The last hypothesis is further confirmed by the reaction of trans-[(Cy3P)2Pd(D)(H2O)]BF4 with Ph3CLi, which yields a 70/30 Ph3C-d/Ph3C-h mixture. Crystals of 4 are monoclinic, C2/c, a = 30.587 (6) angstrom, b = 13.408 (4) angstrom, c = 19.121 (5) angstrom, beta = 99.23 (3)-degrees, V = 7740.2 angstrom 3, Z = 8, R = 0.040 for 3077 reflections with F(o) > 4-sigma-(F(o)

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The "complexes as metal/complexes as ligand" strategy has been used to prepare new polynuclear heterometallic complexes containing two, 3, 4, and ten metal units. The metal ions employed are Ru(2+), Os(2+), and Pt(2+); the bridging ligands are 2,3- and 2,5-dpp (dpp = bis(2-pyridyl)pyrazine), and the terminal ligands are Cl(-) and bpy (bpy = 2,2'-bipyridine). [Os(bpy)(2)(mu-2,3-dpp)Ru[(mu-2,3-dpp)PtCl(2)](2)](PF(6))(4) (OsRuPt(2)) and [Os[(mu-2,3-dpp)Ru[(mu-2,3-dpp)PtCl(2)](2)](3)](PF(6))(8) (OsRu(3)Pt(6)), which have a dendritic structure, are the first examples of polynuclear complexes based on the dpp bridging ligand simultaneously containing three different metals. All of the new compounds feature absorption spectra with intense ligand-centered transitions in the UV region and metal-to-ligand charge transfer (MLCT) bands in the visible region. The complexes are luminescent both at room temperature in acetonitrile solution and at 77 K in butyronitrile rigid matrix. OsRuPt(2) and OsRu(3)Pt(6) luminesce in the near-infrared spectral region at 975 and 875 nm, respectively, at room temperature. This indicates that the emitting state is a (formally triplet) MLCT involving Os(2+) and the bridging ligand mu-2,3-dpp and that all the absorbed energy is efficiently channeled to the single Os-based unit. OsRu(3)Pt(6) appears to be the first decanuclear species displaying such efficient antenna effect.

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