Institut de Química Computacional	UdG Universitat de Girona

A recent DFT investigation on Fischer carbenes [26] has shown that M=C bond lengths are very sensitive to the level of theory employed. M=C bond distances evaluated locally were found to be 0.04-0.06 Å shorter than their NLDA counterparts. Nonlocal corrections afford Cr=C distances that are closer to experimental data. Our geometrical parameters (Table 2) are in good agreement with those previously reported, also at NLDA level [15,25]. From experimental studies of various carbene complexes, it has been observed that the bond M=C(carbene) is significantly shorter than an M-C single bond, but longer than the bond M-C(CO) in metal carbonyls [37]. Values from Table 2 fit such a pattern, and reflect the importance of the Cr(dp ) ® C(p in carbene or p ^* in CO) backdonation. Thus, the reason of the larger elongation in the M=C(carbene) bond as compared to the M-C(CO) bond can be found in the weaker maricel@stark.udg.es