Abstract The synthesis of the first 1-metalla-2,4-diphospha-3-silacyclobutanes [(dppe)M(PH) 2Si t-Bu(Is)] 1a (M=Ni, Is=2,4,6-triisopropylphenyl, dppe=1,2-bis(diphenylphosphino)ethane), 1c (M=Pd), 1d (M=Pt) and the analogous 1-nickela-2,4-diarsa-3-silacyclobutane [(dppe)Ni(AsH) 2Si t-Bu(Is)] ( 1b) is reported, which was achieved by salt metathesis reaction of t-Bu(Is)Si(EHLi) 2 ( 2a, E=P; 2b, E=As) with the respective metal chlorides [(dppe)MCl 2] in toluene at −70 °C. They were characterized by means of multinuclear NMR spectroscopy, mass spectrometry and combustion analysis. Their variable temperature 1H- and 31P-NMR spectra are in accordance with fast ring inversion processes and inversion of configuration at the E atoms (E=P, As), suggesting the presence of different diastereomeric forms in solution. The structures of the main diastereomeric form of 1a (triclinic, P1̄) and of the isotypic derivatives 1c and 1d (monoclinic, P2 1/c), respectively, were determined by X-ray diffraction analyses. The latter revealed all cis oriented EH and t-BuSi bonds and puckered ME 2Si rings. Upon heating of 1a and 1b in toluene, the first 1-metalla-2,4-diphospha-3-silabicyclo[1.1.0]butane 3a and its arsenic analogue 3b was formed through intramolecular dehydrogenation reaction and were isolated in the form of orange crystals in 60 and 85% yield, respectively. The latter exist in the two diastereomeric forms ( t-Bu)endo- 3a, ( t-Bu)exo- 3a (molar ratio 4:1) and ( t-Bu)endo- 3b, ( t-Bu)exo- 3b, respectively. Their structures were established by X-ray diffraction analysis which revealed normal PP (2.231(1) Å) and AsAs single bond distances (2.451(1) Å), suggesting that the Ni atom offsets most of the ring strain effects caused by the Si ring atom. In contrast to 1a and 1b, heating of 1c (M=Pd) in toluene at 80 °C affords, under evolution of H 2, merely the first dipalladiphosphasilabicyclo[1.1.1]pentane ( 4a) which was isolated in the form of black crystals in 65% yield. The analogous nickel compound 4b is accessible by the salt metathesis reaction of 1,3-dilithiated [(dppe)Ni(PLi) 2Si t-Bu(Is)] with [(dppe)NiCl 2] and was isolated in the form of red crystals in 79% yield. Both complexes 4a and 4b were structurally characterized by X-ray diffraction. Surprisingly, the Pt complex 1d remains unchanged even after prolonged heating in boiling toluene.