H-1 AND C-13 NMR-STUDY OF CYCLOPENTADIENYL METAL-CARBONYLS IN THE SOLID-STATE

In this paper we deal with some structural and dynamic properties of C p2W2(CO)6 (I) and Cp2Ru2(CO)4 (II) as shown by solid state C-13 and H- 1 NMR experiments. The IR and C-13 CPMAS spectra of a polycrystalline sample of I show that this compound possesses the anti rotameric struc ture found in a previously reported X-ray diffraction study. The analy sis of the spinning side-band manifold in the C-13 CPMAS spectrum of I allows us to assess a different semi-bridging character between two C O-groups not seen from the X-ray results. The spectral features of com pound II are fully consistent with the X-ray and solution structures p reviously reported. In both compounds the cyclopentadienyl ligands are involved in fast reorientation motions which modulate the magnetic in teractions responsible for the relaxation of C-13 resonances. The acti vation energies (E(a)) associated with this reorientation process of t he Cp ring along their C5 coordination axis have been determined to be 15.5 and 10.2 kJ mol-1 for I and II respectively on the basis of H-1 T1 measurements at different temperatures. Furthermore, we show that a n empirical relationship relates E(a) values and T(min) (the temperatu re at which proton relaxation is more efficient) in a related series o f cyclopentadienyl compounds.