Several novel Pd(II), Pt(II) and Pt(IV) dimers containing 1,2-bis(diphenylphosphino)acetylene (dppa) as bridging ligand have been prepared and characterized by X-ray diffraction methods, NMR spectroscopy ( 195Pt{ 1H}, 31P{H 1H}), IR spectroscopy, elemental analyses and melting points. The X-ray structures of the former reported complexes [Pd 2Cl 4(dppa) 2] ( 1) and [Pt 2Cl 4(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) A ̊ , β = 91.36(3)°, Z = 2, R = 0.039 for 6432 observed reflections ( 1>3σ(1)); 2: orthorhombic, Pbcn, a = 13.995(3), b = 17.669(4), c== 21.960(4) A ̊ , Z = 4, R = 0.052 for 2332 observed reflections (1>3 σ(1)). These two X-ray structures are discussed with respect to the former given ambiguous interpretation of the X-ray structure of [PtPdCl 4(dppa) 2]. Replacement of the chlorides in 1 or 2 by iodides leads to [Pd 2I 4(dppa) 2] ( 3) or [Pt 2I 4(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) A ̊ , β = 100.09(3)°, Z = 4, R = 0.061 for 4698 observed reflections (1>3 σ(1)). The X-ray structures of 1,2 and 4 reveal different orientations of the dppa bridges due to metal-phosphorus dπ-dπ back bonding leading to different angles between the square-planar coordination units, respectively. Replacement of the chlorides in 1 by NO 2 − leads to [Pd 2(NO 2) 4(dppa) 2] (5), whereas in 2 only one chloride can be substituted by NO 2 − and [Pt 2Cl 3(NO 2) (dppa) 2] ( 6) is formed. A similar reaction of 2 with 2,2′-bipyridine (bipy) produces [Pt 2Cl 2(dppa) 2(bipy)](BF 4) 2 (7). Oxidation of 2 by chlorine gives the Pt(IV) dimer [Pt 2Cl 8(dppa) 2] ( 8). An equimolar amount of PdCl 2 reacts with one triple bond of 2 leading to [Pt 2PdCl 6(dppa) 2] ( 9). an analogous manner [Pd 4Cl 8(dppa) 2] ( 10) is formed from 1 and two equimolar amounts of PdCl 2. The stability and reactivity of complexes 1–10 are discussed with respect to the X-ray structures of 1,2,4 and [PtPdCl 4(dppa) 2].