Institut de Química Computacional	UdG Universitat de Girona

Regarding the C-O bonding in Cr(CO)₆ compared to the carbonyl bonding in 1, additional information about the s -donor/p -acceptor ability of CO and the carbene ligands may be provided by inspection of the C-O distance. Backdonation from metal to carbonyl takes place to a further extent in 1 than in Cr(CO)₆ as indicated by a shorter C-O bond length in the latter (calc.: 1.164 Å; exp. [39]: 1.141 Å) than in the former (Table 2). This is atributted to the aforementioned weaker p -acceptor character of the carbene as compared to CO. In fact, a shorter C-O distance comes from smaller backdonation to the CO orbital of maricel@stark.udg.es