Treatment of CpMoCl2(eta-C4H6) (1) with 1 equiv of allylmagnesium bromide yields the dinuclear complexes [CPMo(eta-C4H6)(mu-Br)](2) (2, major) and Cp2Mo2(eta-C4H6)(2)(mu-Br)(mu-Cl) (3, minor). A solid solution of compounds 2 and 3 adopts an anti geometry in the solid state, as shown by X-ray crystallography, whereas both anti and syn isomers are observed in benzene solution by H-1-NMR spectroscopy. The reaction of 1 with 2 equiv of allylmagnesium bromide yields [CpMo(eta-C3H5)(eta-C4H6)] as an equilibrium mixture of a major (98%, 4a) and a minor (2%, 4b) isomer. NOE-NMR studies indicate the CpMo(prone-C3H5)(supine-C4H6) orientation for the major isomer 4a, which is also found in the solid state by X-ray crystallography. The orientation of 4b is suggested by the ’H-NMR chemical shifts as CpMo(supine-C3H5)(supine-C4H6). Oxidation of 4a/b by ferrocenium hexafluorophosphate in dichloromethane gives the 17-electron compounds [CpMo(eta-C3H5)(eta-C4H6)][PF6] (5a/b). The green compound 5a converts into the more stable red-violet 5b with an estimated half-life of <20 s in THF. It can be observed, however at low temperature by EPR spectroscopy. The [CpMo(supine-eta-C3H5)(supine-eta-C4H6)][PF6] configuration for 5b has been confirmed by X-ray diffraction methods. Upon reduction with cobaltocene, 5b is converted selectively to 4b, followed by slow equilibration (t(1/2) = 6.5 h) with 4a. Refluxing or photolyzing a solution of 4a/b in benzene generates a third isomer, 4c, which adopts a CpMo(supine-C3K5)(s-trans-C4H6) configuration as confirmed by an X-ray analysis. The distribution of 4a and 4c at equilibrium is approximately 1:1 by starting either from 4a/b or from pure 4c and independent of the equilibration method (thermal/photochemical). Oxidation of 4c generates the corresponding 1-electron oxidation product 5c, which rapidly isomerizes to 5b. It can be observed, however, by EPR in THF together with a fourth isomer, 5d (ca. 1:1), believed to differ from 5c only in the orientation of the allyl ligand. Equilibrium, rate, and electrochemical data allow most of the thermodynamic and kinetic parameters related to the transformation of the different compounds to be sorted out. The faster s-trans- to s-cis-butadiene isomerization for 5 relative to 4 indicates the easier accessibility of the unsaturated 15-electron vs 16-electron intermediate. Possible reasons for this trend are analyzed.