In considering the carbene as a neutral ligand, it has one filled bond!!). orbital (sp2 hybrid) that donates its lone pair to an empty orbital on the metal in a typical ligand fashion. But it also has one empty orbital (pure character) that wants to interact with a lone pair of electrons in order to p form a stabilizing bonding interaction. This is a singlet state carbene formalism and the possible orbital interactions are shown below: If the metal is electron deficient (perhaps due to all the good -acceptor CO ligands) then it very well to the carbene. Thus we can’t -donate end up with a M-C single bond (even though we draw a double bond!) and some multiple bond character between the carbene carbon and the groups attached to it (like a NR2, OR, SR, Ph, etc). -donor Most Fischer Carbenes have metal configurations (assuming that we d6 electron count the carbene ligand as a neutral 2 e- donor), but and d4 d8 systems are known. The bond strength in Fischer Carbenes depends on several factors: Weak M=C Strong M=C Metal Electron-deficient Electron-rich (electron withdrawing ligands (electron donating ligands, 3rd like CO, NO, 1st row metal, row metal) electronegative metal) Carbene groups Good donating groups that can Simple sigma donors like H or to the carbene (like CH3 that can’t to the -bond -donate NR2, SR, OR, Ph); more than Carbene carbon atom. one donating group really weakens the M-C bond!! Note that most Fischer Carbenes favor the weak bonding situation, where the metal has a configuration (counting the carbene as neutral d6 ligand), CO ligands, and the carbene has groups. The -donating d6
bond!!). In considering the carbene as a neutral ligand, it has one filled orbital (sp2 hybrid) that donates its lone pair to an empty orbital on the metal in a typical ligand fashion. But it also has one empty orbital (pure p character) that wants to interact with a lone pair of electrons in order to form a stabilizing bonding interaction. This is a singlet state carbene formalism and the possible orbital interactions are shown below: If the metal is electron deficient (perhaps due to all the good -acceptor CO ligands) then it can’t -donate very well to the carbene. Thus we end up with a M-C single bond (even though we draw a double bond!) and some multiple bond character between the carbene carbon and the -donor groups attached to it (like a NR2, OR, SR, Ph, etc). Most Fischer Carbenes have d6 metal configurations (assuming that we electron count the carbene ligand as a neutral 2 e- donor), but d4 and d8 systems are known. The bond strength in Fischer Carbenes depends on several factors: Weak M=C Strong M=C Metal Electron-deficient (electron withdrawing ligands like CO, NO, 1st row metal, electronegative metal) Electron-rich (electron donating ligands, 3rd row metal) Carbene groups Good donating groups that can -bond to the carbene (like NR2, SR, OR, Ph); more than one donating group really weakens the M-C bond!! Simple sigma donors like H or CH3 that can’t -donate to the Carbene carbon atom. Note that most Fischer Carbenes favor the weak bonding situation, where the metal has a d6 configuration (counting the carbene as neutral ligand), CO ligands, and the carbene has -donating groups. The d6
chap06-alkyls.docx