BINUCLEAR PALLADIUM(II), PLATINUM(II) AND PLATINUM(IV) COMPLEXES CONTAINING 1,2-BIS(DIPBENYLPHOSPHINO)ACETYLENE - DIFFERENT ORIENTATIONS OFTHE DIPHOSPHINE-BRIDGES DUE TO METAL-PHOSPHORUS D-PI-D-PI BACK BONDING

Several novel Pd(II), Pt(II) and Pt(IV) dimers containing 1,2-bis(diph enylphosphino)acetylene (dppa) as bridging ligand have been prepared a nd characterized by X-ray diffraction methods, NMR spectroscopy (Pt-19 5{H-1}, P-31{H-1}, IR spectroscopy, elemental analyses and melting poi nts. The X-ray structures of the former reported complexes [Pd2Cl4(dpp a)(2)] (1) and [Pt2Cl4(dppa)(2)] (2) are given for the first time: 1: monoclinic, P2(1)/n, a=14.401(3), b=12.100(2), c=15.880(3) Angstrom, b eta=91.36(3)degrees, Z=2, R=0.039 for 6432 observed reflections (I>3 s igma(I)); 2: orthorhombic, Pbcn, a=13.995(3), b=17.669(4), c=21.960(4) Angstrom, Z=4, R=0.052 for 2332 observed reflections (I>3 sigma(I)). These two X-ray structures are discussed with respect to the farmer gi ven ambiguous interpretation of the X-ray structure of [PtPdCl4(dppa)( 2)]. Replacement of the chlorides in 1 or 2 by iodides leads to [Pd2I4 (dppa)(2)] (3) or [Pt2I4(dppa)(2)] (4). 4 is also fully characterized by an X-ray structure analysis for the first time: 4: monoclinic, P2(1 )/c, a=10.586(2), b=30.526(6), c=18.383(4) Angstrom, beta=100.09(3)deg rees, Z=4, R=0.061 for 4698 observed reflections (I>3 sigma(I)). The X -ray structures of 1, 2 and 4 reveal different orientations of the dpp a bridges due to metal-phosphorus d pi-d pi back bonding leading to di fferent angles between the square-planar coordination units, respectiv ely. Replacement of the chlorides in 1 by NO2- leads to [Pd-2(NO2)(4)( dppa)(2)] (5), whereas in 2 only one chloride can be substituted by NO 2- and [Pt2Cl3(NO2)(dppa)(2)] (6) is formed. A similar reaction of 2 w ith 2,2'-bipyridine (bipy) produces [Pt2Cl2(dppa)(2)(bipy)](BF4)(2) (7 ). Oxidation of 2 by chlorine gives the Pt(IV) dimer [Pt2Cl8(dppa)(2)] (8). An equimolar amount of PdCl2 reacts with one triple bond of 2 le ading to [Pt2PdCl6(dppa)(2)] (9). In an analogous manner [Pd4Cl8(dppa) (2)] (10) is formed from 1 and two equimolar amounts of PdCl2. The sta bility and reactivity of complexes 1-10 are discussed with respect to the X-ray structures of 1, 2, 4 and [PtPdCl4(dppa)(2)].