300ChemInform Abstract The preparation of the parent diolefin complexes (IV) and (VIII) is shown in the scheme. On protonation of the complexes (IV) with equimolar amounts of HBF4 monoprotonated cationic complexes could be precipitated with EtO2 in almost quantitative yields. All these compounds are fluxional at room temp. on the NMR time scale. Depending on R and M three different ground states are observed for these cations in the NMR spectra at low temperatures. While M=Ir a classical metal-hydride structure (XI) is to be seen the Co and Rh complexes show ground states (X) with "agostic" H-bridges (M-H-C). Free enthalpies of activation for methyl rotation and 1,4-H shift in (X) are obtained from total line-shape analysis. Only for R=H and M=Rh and in the presence of traces of Lewis bases L' (H2O, Py or acetone) variable amounts of coordinatively saturated allyl complexes of type (XV) (with L' instead of CO) competing with the agostic species are observable. More than equimolar amounts of basic solvents lead to irreversible deprotonation and recovery of the starting compounds. Stable compounds (XII) are formed with HCL, while protonation with HBF4 in the presence of CO affords high yields (numerical values not given) of (XIV) and (XV) in the case of (IVc). All the other complexes (IV) give only one of these isomers. The analogous protonation of the complexes (VIII) gives also protonated cationic compounds which are fluxional at room temp. and show different ground states similar to (X) and (XI) at low temperatures.