Specrrochinu'ca Acta, Vol. 49A, No. 9, pp. 1307-1314,1993 0584-8539/93 $6.00+0.00 Printedin GreatBritain ~) 1993 PergamonPressLtd IH and ]3C NMR study of cyclopentadienyi metal carbonyls in the solid state S. AIME,* L. CORDERO and R. GOBEITO Dipartimento di Chimica Inorganica Chimica Fisica e Chimica dei Materiali, Universit,~t di Torino, via P. Giuria 7, 10125, Torino, Italy and G. SZALONTAI University of Veszprem, NMR Laboratory, H-8 200, Veszprem Pf 158, Hungary (Received 28 April 1992; accepted 11 June 1992) Abstract--In this paper we deal with some structural and dynamic properties of Cp2W2(CO)6 (1) and Cp2Ru2(CO)4 (11) as shown by solid state 13C and ]H NMR experiments. The IR and 13C CPMAS spectra of a polycrystalline sample of I show that this compound possesses the anti rotameric structure found in a previously reported X-ray diffraction study. The analysis of the spinning side-band manifold in the ~3CCPMAS spectrum of I allows us to assess a different semi-bridging character between two CO-groups not seen from the X-ray results. The spectral features of compound H are fully consistent with the X-ray and solution structures previously reported. In both compounds the cyclopentadienyl iigands are involved in fast reorientation motions which modulate the magnetic interactions responsible for the relaxation of t3C resonances. The activation energies (Ej) associated with this reorientation process of the Cp ring along their Cs coordination axis have been determined to be 15.5 and 10.2 kJ tool -~ for I and II respectively on the basis of mH Tt measurements at different temperatures. Furthermore, we show that an empirical relationship relates Ea values and Tmm (the temperature at which proton relaxation is more efficient) in a related series of cyclopentadienyl compounds. INTRODUCTION ALTHOUGH the number of papers dealing with the application of high resolution 13C CPMAS (cross polarization-magic angle spinning) NMR spectroscopy to organometallic compounds is large [1], there is still a need for a better understanding of the various features involved in the solid state spectra to fully exploit the powerful potential of such an approach. On going from solution to solid state 13C NMR spectra, the main differences commonly found involve: (i) a higher number of signals and (ii) longer longitudinal relaxation times. The former feature is dependent upon the site symmetry, which may be lower than the molecular symmetry, resulting in different signals. In solution, internal vibrations and collisions with the solvent molecules reduce the number of observed resonances to that expected on the basis of a given molecular symmetry [2]. The relaxation behaviour of the 13C resonances in the solid state are dependent upon the dynamic properties of the system under study, as in solution. However, while in solution we deal mainly with molecular tumbling (internal motions superimposed on the overall molecular motion may be important only in limited cases), in the solid state the modulation of the magnetic interactions is provided in most cases by local motions associated with highly mobile moieties such as methyls, aliphatic chains, cyclopenta- dienyls (Cp), etc. [2]. The ability of a Cp ring to reorient rapidly about its (25 coordination axis was recognized in several metallocenes by the changes detected in the second moment measurement of tH wide-line spectra at different temperatures [3]. An improvement was then brought about by the measurement of tH longitudinal relaxation time T~ at different temperatures [4, 7], which allows the exploitation of a larger temperature range than does second moment measurement. The extent of rotation of a Cp ring may be quite different from system to system and it is mainly determined by the strength of the * Author to whom correspondence should be addressed. s~A) 1:9-M 1307