Reaction of [MCl2(cod)] (M = Pd, Pt) with a tetraphosphine, meso-1,3-bis[(diphenylphosphinomethyl)phenylphosphino]propane (dpmppp), afforded the mononuclear complexes [MCl2(dpmppp)] (M = Pd (3a), Pt (3b)), in which the dpmppp ligand coordinated to the M ion by two inner phosphorus atoms to form a six-membered chelate ring with two outer phosphines uncoordinated. The pendant outer phosphines readily reacted with [RhCl(CO)2]2 to give the cationic heterotrinuclear complexes [MRh2(μ-Cl)3(μ-dpmppp)(CO)2]X (X = [RhCl2(CO)2], M = Pd (4a), Pt (4b); X = PF6, M = Pd (5a), Pt (5b)). The nickel analogue [NiRh2(μ-Cl)3(μ-dpmppp)(CO)2]PF6 (5c) was also prepared. A neutral homotrinuclear Rh3 complex, [Rh3(μ-Cl)3(μ-dpmppp)(CO)2] (6), was synthesized by the reaction of [RhCl(CO)2]2 with dpmppp and was further reacted with HgX2 (X = Cl, Br, I) to afford the Rh3Hg tetranuclear complexes [Rh3(HgX)(μ-Cl)2(μ-X)(μ-dpmppp)(CO)2]PF6 (X = Cl (7a), Br (7b), I (7c)), where the Rh3(μ-Cl)2(μ-X) cores act as tridentate ligands to form three donor–acceptor Rh→Hg interactions. The two CO ligands of 7a–c were replaced by XylNC to yield [Rh3(HgX)(μ-Cl)2(μ-X)(μ-dpmppp)(XylNC)2]PF6 (X = Cl (8a), Br (8b), I (8c)). The isocyanides had an appreciable influence on the three Rh→Hg interactions, which was monitored by the 2 J HgP values observed in the 31P{1H} NMR spectra and discussed on the basis of DFT calculations. Complex 6 also reacted with CuCl and HBF4 to give [Rh3(CuCl)(μ-Cl)3(μ-dpmppp)(CO)2] (9) and [Rh3(μ3-H)(μ-Cl)3(μ-dpmppp)(CO)2]BF4 (10), respectively. These results suggested that the new tetraphosphine dpmppm proved quite useful in constructing fine-tunable heterometallic frameworks.

Electron-deficient dinuclear rhodium complexes [Rh2Cl2(μ-dpmppp)(RNC)] (1), with the linear tetraphosphine ligand dpmppp, showed reversible binding of molecular oxygen to form asymmetric dirhodium η(2)-peroxo complexes [Rh2Cl2(O2)(μ-dpmppp)(RNC)] (2) stabilized by a Rh→Rh dative bond.