Ruthenacyclobutane complexes (SiP3)(PMe3)Ru(CH2EMe2CH2) (SiP3 = MeSi(CH2PMe2)(3); 1, E = C; 2, E = Si) were synthesized from (SiP3)(PMe3)RuCl2 (3) and 2 equiv of the Grignard reagents, Me3ECH2MgCl. Metallacycle 1 was found to reversibly interconvert with the allyl complex (SiP3)Ru(Me)(eta(3)-CH2CMeCH2) (4) and PMe3 when heated above 75 degrees C. From the results of kinetic studies and thermolysis of labeled material, the interconversion is proposed to take place by reversible beta-methyl elimination/insertion. Conversion of 1 to 4 is an endothermic process (Delta H degrees = 14.3 +/- 1.1 kcal mol(-1)), but it is entropically favorable (Delta S degrees = 40.9 +/- 2.8 cal K-1 mol(-1)) due to the loss of the PMe3 ligand. Activation parameters for the beta-insertion were determined to be Delta H-double dagger = 26.0 +/- 1.2 kcal mol(-1) and Delta S double dagger = -10.5 +/- 0.9 cal K-1 mol(-1) Allyl complex 4 has been isolated as a mixture of isomers (7:1 endo:exo). The mechanism of interconversion of 4(endo) and 4(exo) was determined by H-1{P-31} NMR spectroscopy (EXSY) to be a process involving a stereochemically rigid, square-pyramidal eta(1)-intermediate. Thermolysis of 4 leads to loss of CH4 and the production of the trimethylenemethane complex (SiP3)Ru(eta(4)-C(CH2)(3)) (7). The solid state structures of 1 and 7 were determined by X-ray diffraction.