Several novel Pd(II) and Pt(II) complexes containing the diphos ligand cis-1,2-bis(diphenylphosphino)ethene (cis-dppen) have been prepared and characterized by X-ray diffraction methods, NMR spectroscopy ( 1 9 5 Pt{ 1 H}, 3 1 P{ 1 H}), IR spectroscopy, elemental analyses and melting points. In contrast to the former reported and directly available compounds [PdCl 2 (cis-dppen)] (1) and [Pd(cis-dppen) 2 ] (BPh 4 ) 2 (3), an intermediate complex is formed in the synthesis of [PtCl 2 (cis-dppen)] (2). It is shown, that this intermediate is of the type [Pt(cis-dppen) 2 ] 2 + . Both2 and [Pt(cis-dppen) 2 ](BPh 4 )(4) 2 (4) are fully characterized by X-ray structure analyses: 2: monoclinic, P2 1 /m, a = 8.507(2),b = 18.236(4), c = 11.205(2) , β = 105.25(3)°, R = 0.052 for 3070 observed reflections (I > 3σ(I)); 4: monoclinic, P2 1 /c, a = 10.718(2),b = 17.283(3), c = 24.549(5) , β = 99.25(3)°, R = 0.028 for 5588 observed reflections (I > 3σ(I)). In 2 and 4 the Pt atoms show square-planar coordinations, where in both cases the ethene bridges are strictly coplanar with the coordination planes. This gives strong evidence of a π bonding interaction including the ethene bridges. Replacement of the chlorides in 1 and 2 by 2,2 -bipyridine (bipy) or 1,10-phenanthroline (phen) leads to the square-planar compounds [Pd(cis-dppen)(bipy)](BPh 4 ) 2 (5), {Pt(cis-dppen)(bipy)](PF 6 ) 2 (6,, [Pd(cis-dppen)(phen)](BPh 4 ) 2 (7) and [Pt(cis-dppen)(phen)](BF 4 ) 2 (8). Similar reactions with NO 2 - produce [Pd(NO 2 ) 2 (cis-dppen)] (9), [Pt(NO 2 ) 2 (cis-dppen)] (10) and [PtCl(NO 2 )(cis-dppen)] (11). From Pd(CN) 2 and cis-dppen [Pd(CN) 2 (cis-dppen)] (12) is formed. The stability and reactivity of complexes 1-12 are discussed with respect to the known non-planar X-ray structures of [PtCl 2 (dppe)], where dppe is 1,2-bis(diphenylphosphino)ethane, and [Ni(cis-dppen) 2 ](ClO 4 ) 2 .