PALLADIUM(II) AND PLATINUM(II) COMPLEXES OF CIS-1,2-BIS(DIPHENYLPHOSPHINO) ETHENE - COMPLETELY PLANAR STRUCTURES DUE TO PI-BONDING IN THE CASE OF PLATINUM(II)

Several novel Pd(II) and Pt(II) complexes containing the diphos ligand cis-1,2-bis(diphenylphosphino)ethene (cis-dppen) 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 p oints. In contrast to the former reported and directly available compo unds [PdCl2(cis-dppen)] (1) and [Pd(cis-dppen)(2)] (BPh(4))(2) (3), an intermediate complex is formed in the synthesis of [PtCl2(cis-dppen)] (2). It is shown, that this intermediate is of the type [Pt(cis-dppen )(2)](2+). Both 2 and [Pt(cis-dppen)(2)] (BPh(4))(2) (4) are fully cha racterized by X-ray structure analyses:2: monoclinic, P2(1)/m, a=8.507 (2), b=18.236(4), c=11.205(2) Angstrom, beta=105.25(3)degrees, R=0.052 for 3070 observed reflections (I>3 sigma(I)); 4: monoclinic, P2(1)/c, a=10.718(2), b=17.283(3), c=24.549(5) Angstrom, beta=99.25(3)degrees, R=0.028 for 5588 observed reflections (I>3 sigma(I)). In 2 and 4 the Pt atoms show square-planar coordinations, where in both cases the eth ene bridges are strictly coplanar with the coordination planes. This g ives strong evidence of a pi bonding interaction including the ethene bridges. Replacement of the chlorides in 1 and 2 by 2,2'-bipyridine (b ipy) or 1,10-phenanthroline (phen) leads to the square-planar compound s [Pd(cis-dppen) (bipy)](BPh(4))(2) (5), {Pt(cis-dppen) (bipy)](PF6)(2 ) (6), [Pd(cis-dppen) (phen)] (BPh(4))(2) (7) and [Pt(cis-dppen) (phen )] (BF4)(2) (8). Similar reactions with NO2- produce [Pd(NO2)(2)(cis-d ppen)] (9), [Pt(NO2)(2)(cis-dppen)] (10) and [PtCl(NO2) (cis-dppen)] ( 11). From Pd(CN)(2) and cis-dppen [Pd(CN)(2)(cis-dppen)] (12) is forme d. The stability and reactivity of complexes 1-12 are discussed with r espect to the known non-planar X-ray structures of [PtCl2(dppe)], wher e dppe is 1,2-bis(diphenylphosphino)ethane, and [Ni(cis-dppen)(2)](ClO 4)(2).