DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 965–970 965 Redox behavior of the molybdenum and tungsten metallafullerenes M( 2 -C 60 )(CO) 2 (phen)(dbm) (phen  1,10-phenanthroline; dbm  dibutyl maleate): (spectro)electrochemistry and theoretical considerations Piero Zanello,* a Franco Laschi, a Marco Fontani, a Carlo Mealli,* b Andrea Ienco, b Kaluo Tang, c Xianglin Jin * c and Lei Li c a Dipartimento di Chimica dell’Università di Siena, Pian dei Mantellini, 44, 53100 Siena, Italy b ISSECC-CNR, Via Nardi 39, 50132 Firenze, Italy c Institute of Physical Chemistry, Peking University, Beijing 100871, P.R. China Received 28th September 1998, Accepted 20th January 1999 Electrochemistry of M(η 2 -C 60 )(CO) 2 (phen)(dbm) (M = W 1, Mo 2; phen = 1,10-phenanthroline; dbm = dibutyl maleate) shows that the complexes undergo four sequential reduction processes. As with free C 60 , the first three electrons add reversibly (even if the relevant potentials are shifted ca. 0.15 V toward negative values), whereas the fourth reduction features chemical irreversibility. Cyclic voltammetry gives evidence that, as a consequence of the latter process, the metal fragment decomplexes and [C 60 ] 3- is released. In good agreement with this picture, a qualitative MO approach shows four close LUMOs for the ground state structure of the uncharged complexes. The first three levels are delocalized over C 60 (somewhat extended to the dmb π system), while the fourth one is metal– fullerene antibonding (back donation d π → π* C 60 ) and its occupation causes fulleride dissociation. The EPR spectra of the electrogenerated [1] - and [2] - monoanions are significantly different from that of [C 60 ] - and seem suggestive of metal character for these radical species. At present, this result is unexpected in that the unpaired electron in the anions [1] - and [2] - should be intuitively centered on the coordinated fullerene. Introduction The preparation and structural characterization of fullerene metal complexes, especially of C 60 1 and C 70 , 1j,2 constitutes a growing field of research. From the electrochemical viewpoint, multiple electron transfer processes which have been shown to occur reversibly in free fullerenes 3 have not equally encouraged exhaustive investigations on their transition metal adducts. In fact, the available data are still limited for C 60 complexes, 1f,4 while, aside from a brief report, 1f there are no data for the C 70 -analogs. The effects of coordination on the electrochemical behavior of fullerenes may be summarized as follows. A metal is almost invariably dihapto linked to one [6:6] bond of C 60 mainly owing to the significant back donation exerted by a filled d π orbital into the olefin-type C–C π* moiety which is locally exhibited by both the t 1u and t 1g low lying LUMOs of fullerene. 5 Even when the latter is uncoordinated, the high delocalization of the LUMOs induces any added electron density to spread over the carbon soccer ball so that multielectron reductions become progressively more difficult. 4 In the presence of a metal frag- ment, one t 1u level is destabilized and, although the remaining levels are not directly involved in bonding interactions, an inductive effect reduces their capability to accept electrons with a concomitant shift of the redox potentials toward more negative values with respect to free fullerene. Conversely, the reductive process seems facilitated for the adduct between the bimetallic species Fe 2 S 2 (CO) 6 and C 60 . 1f This case, which would require an ad hoc theoretical analysis, is intuitively different as the complexation at the [6:6] edge does not involve any metal atoms, rather the two sulfur bridges between them. Thus, the redox behavior of the coordinated fullerene requires a careful examination of the bonding capabilities of the metal fragment itself and the evaluation of the nature and energy of the frontier MOs. We present here an electrochemical investigation performed on two complexes of fullerene C 60 with metals of Group VI, the structures of which have been recently characterized. 1g For illustrative purposes, a drawing of the species M(η 2 -C 60 )(CO) 2 - (phen)(dbm) (M = W 1, Mo 2; phen = 1,10-phenanthroline; dbm = dibutyl maleate) is shown in Scheme 1. To the best of our knowledge, only a preliminary report on the electrochemical behavior of the W(0)-fullerene complex W(η 2 -C 60 )(CO) 3 (dppe) [dppe = 1,2-bis(diphenylphosphino)- ethane] has been reported. 4c We judged it essential to present a qualitative description of the MO structure of the complexed fullerenes in order to derive a useful picture of the effects occurring when one or more electrons are added to these systems. At this time, the large nuclearity of the latter and the complicated magnetic properties Scheme 1 Published on 01 January 1999. Downloaded on 21/07/2014 09:14:34. View Article Online / Journal Homepage / Table of Contents for this issue