The bis(bidentate) phosphine cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane (dppcb) has been used for the synthesis of a series of novel heterodimetallic complexes starting from [Ru(bpy)(2)(dppcb)]X(2) (1; X = PF(6), SbF(6)), so-called dyads, showing surprising photochemical reactivity. They consist of [Ru(bpy)(2)](2+)"antenna" sites absorbing light combined with reactive square-planar metal centres. Thus, irradiating [Ru(bpy)(2)(dppcb)MCl(2)]X(2) (M = Pt, 2; Pd, 3; X = PF(6), SbF(6)) dissolved in CH(3)CN with visible light, produces the unique heterodimetallic compounds [Ru(bpy)(CH(3)CN)(2)(dppcb)MCl(2)]X(2) (M = Pt, 7; Pd, 8; X = PF(6), SbF(6)). In an analogous reaction the separable diastereoisomers (ΔΛ/ΛΔ)- and (ΔΔ/ΛΛ)-[Ru(bpy)(2)(dppcb)Os(bpy)(2)](PF(6))(4) (5/6) lead to [Ru(bpy)(CH(3)CN)(2)(dppcb)Os(bpy)(2)](PF(6))(4) (9), where only the RuP(2)N(4) moiety of 5/6 is photochemically reactive. By contrast, in the case of [Ru(bpy)(2)(dppcb)NiCl(2)]X(2) (4; X = PF(6), SbF(6)) no clean photoreaction is observed. Interestingly, this difference in photochemical behaviour is completely in line with the related photophysical parameters, where 2, 3, and 5/6, but not 4, show long-lived excited states at ambient temperature necessary for this type of photoreaction. Furthermore, the photochemical as well as the photophysical properties of 2-4 are also in accordance with their single crystal X-ray structures presented in this work. It seems likely that differences in "steric pressure" play a major role for these properties. The unique complexes 7-9 are also fully characterized by single-crystal X-ray structure analyses, clearly showing that the stretching vibration modes of the ligand CH(3)CN, present only in 7-9, cannot be directly influenced by "steric pressure". This has dramatic consequences for their photophysical parameters. The trans-[Ru(bpy)(CH(3)CN)(2)](2+) chromophore of 9 acts as efficient "antenna" for visible light-driven energy transfer to the Os-centred "trap" site, resulting in k(en) ≥ 2 × 10(9) s(-1) for the energy transfer. Since electron transfer is made possible by the use of this intervening energy transfer, in dyads like 2-4 highly reactive M(0) species (M = Pt, Pd, Ni) could be generated. These species are not stable in water and M(II) hydride intermediates are usually formed, further reacting with H(+) to give H(2). Thus, derivatives of 3, namely [M(bpy)(2)(dppcb)Pd(bpy)](PF(6))(4) (M = Os, Ru) dissolved in 1:1 (v/v) H(2)O-CH(3)CN produce H(2) during photolysis with visible light.

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The bis(bidentate) phosphine cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane (dppcb) has been regioselectively oxidized leading to novel, hemilabile ligands. [Co2Cl4(dppcb)] (1a) is transformed via cobalt(II) mediated dioxygen activation into [Co2Cl4(2,3-trans-dppcbO2)] (2a) in excellent yield, where 2,3-trans-dppcbO2 is cis,trans,cis-2,3-bis(diphenylphosphinoyl)-1,4-bis(diphenylphosphino)-cyclobutane. By contrast, the in situ presence of dioxygen during the synthesis of Co2Br4(dppcb)] (1b) produces both [Co2Br4(2,3-trans-dppcbO2)] (2b) and [Co2Br4(1,3-trans-dppcbO2)] (3), where 1,3-trans-dppcbO2 is cis,trans,cis-1,3-bis(diphenylphosphinoyl)-2,4-bis(diphenylphosphino)-cyclobutane. The new compounds 2a, 2b and 3 have been obtained as pure, crystalline solids and all three X-ray structure analyses have been performed showing folded cyclobutane rings. Interestingly, the corresponding reaction using [Co2I4(dppcb)] (1c) proceeds chemoselectively. Thus, [Co2I4(dppcbO3)] (4), where dppcbO3 is cis,trans,cis-1,2,3-tris(diphenylphosphinoyl)-4-diphenylphosphinocyclobutane, is formed in excellent yield and also fully characterized by an X-ray structure analysis showing two different conformations of 4. However, [Co2(NO3)4(dppcb)] (1d) shows no dioxygen activation at all. Therefore, in order to reveal the mechanism of this oxidation [Co2I4(DMF)2(dppcb)] (5) has been prepared and its X-ray structure is presented. The synthesis of [Co2I4(PMe2Ph)2(dppcb)] (6) proves that this is a common reaction pathway. Furthermore, because the product distribution of the oxidation strongly depends on the kind of halides present, the whole series Co2X4(dppcbO4)] (X = Cl, 7a; Br, 7b; I, 7c) has been prepared, where dppcbO4 is cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphinoyl)-cyclobutane, and all three X-ray structures are given, also showing folded cyclobutane rings. It seems likely that coordination of dppcb to cobalt(II) is essential to form the regio- and chemoselectively oxygenated molecules.

The tetraphosphane all trans tetrakis-(di(2-methoxyphenyl)phosphanyl)cyclobutane) (o-MeO-dppcb) has been employed to coordinate metal dichlorides (metal = Ni(II), Pd(II) and Pt(II)), stereoselectively yielding the dinuclear complexes [Ni(2)Cl(4)(micro-(kappaP(1):kappaP(2):kappaP(3):kappaP(4)-o-MeO-dppcb))] and [Pt(2)Cl(4)(micro-(kappaP(1),kappaP(2):kappaP(3),kappaP(4)-o-MeO-dppcb))], characterized by two six and two five-membered metallacycles, respectively. Conversely, the reaction with PdCl(2) led, under comparable synthetic conditions, to the formation of the linkage-isomeric pair [Pd(2)Cl(4)(micro-(kappaP(1),kappaP(2):kappaP(3),kappaP(4)-o-MeO-dppcb))] and [Pd(2)Cl(4)(micro-(kappaP(1):kappaP(2):kappaP(3):kappaP(4)-o-MeO-dppcb))] in a ca. 4 : 1 ratio. The compounds obtained have been characterized in solution by multinuclear NMR spectroscopy and in the solid state by CP-MAS NMR spectroscopy, XRPD and single crystal X-ray diffraction. Compounds and have been tested as catalyst precursors for the CO-ethene-propene co-and terpolymerization in water-acetic acid mixtures. Their catalytic performance has been compared to that of [PdCl(2)(o-MeO-dppe)] (o-MeO-dppe = 1,2-(bis(di(2-methoxyphenyl)phosphanyl))ethane) and of [PdCl(2)(o-MeO-dppp)] (o-MeO-dppp = 1,3-bis(di(2-methoxyphenyl)phosphanyl)propane). The most striking result that emerged from the CO-ethene copolymerization study was that was three times more productive than , outperforming, under identical catalytic conditions, even 1b and 1c, that are classified amongst the most active catalysts for the CO-ethene copolymerization reaction.

P{ 1 H}), mass spectrometry, IR spectroscopy, elemental analyses and melting points. Furthermore, the solid-state structures of seven of these new compounds were fully determined by single-crystal X-ray diffraction analyses to study the influence of steric pressure. The precursor complex [Rh 2 (η 4 -cod) 2 (dppcb)]X 2 (1), X -= BF 4 -, PF 6 -, SbF 6 -, completely characterized by its X-ray structure, smoothly reacts with mono-or bidentate ligands containing phosphorus or nitrogen donor atoms. Thus, monophosphanes and monophosphites produce compounds of the structure type [Rh 2 L 4 (dppcb)](SbF 6 ) 2 [L = PMe 2 Ph, 2; PMePh 2 , 3; P(OMe) 3 , 5; P(OPh) 3 , 6]. The X-ray structures of 3 and 6 show that PMePh 2 and P(OPh) 3 are capable of compensating steric interactions. The treatment of 1 with diphosphanes leads to the structure type [Rh 2 L 2 (dppcb)](SbF 6 ) 2 [L = bis(diphosphanyl)-methane, dppm, 7; bis(diphenylphosphanyl)amine, dppam, 8; 1,2-bis(diphenylphosphanyl)ethane, dppe, 9; cis-1,2-