Institut de Química Computacional	UdG Universitat de Girona

The carbene carbon in Fischer-type complexes is considered to have a trigonal planar geometry, carbon hybridization being approximately sp². The distances between this carbon atom and its substituents are expected to be short relative to single bonds. As can be seen from Table 2, the bond length between the carbene C₆ atom and its alkenyl substituent (C₇) is midway between that of a C-C single bond (1.54 Å) and that of a C=C double bond (1.34 Å). Also the bond distance between C₆ and O₆ is shorter than a typical C-O single bond (1.43 Å). However, causes are different; electron delocalization along conjugated bonds is the major factor responsible for the shortening of the C₆-C₇ bond, whereas in the case of the C₆-O₆ bond electrostatic effects largely account for the shortening. Contributions of the [(CO)₅Cr^--C(OH)=C(H)-C⁺(H₂)] (and slightly [(CO)₅Cr^--C=O⁺(H)(C₂H₃)]) valence-bond resonance forms, respectively, are thereby implied.