Protons piano stool" complexes

Solution IR of -containing complexes such as 1 also indicate the simultaneous existence of H2 and dihydride forms by showing distinct vco absorptions for each tautomer [Eq. (9.4)],11 even for unstable photogenerated CpNb(CO)3(H2) and CpNb(H)2(CO)3 in liquid Xe, where NMR studies would be difficult.14 The H2 complex is thought to have a square-based-piano-stool geometry but the geometry of the dihydride is unknown. More recently a similar cationic system [Cp Os(CO)2(H2)] + (7) prepared by protonation of the corresponding monohydride showed equilibrium exclusively with the trans dihydride isomer (8) (87% at -80°C).15... 

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