Heavier Group 14 Analogs of Metal Vinylidene Complexes

Although metal vinylidene complexes are quite abundant [4-18], heavier group 14 analogs of metal vinylidene complexes containing M=C=E or M=E=C moiety (E = heavier group 14 elements) have not yet been found. A recent theoretical study of the stability of the heavier Group 14 analogs of vinylidene complexes 

In this chapter, we first analyzed the electronic structures of metal vinylidene and allenylidene complexes. The electronic structures allow us to understand the reactivities of these complexes. For metal vinylidene complexes of the Fischer-type, nucleophilic attack usually occurs at the a-carbon and electrophilic attack at the P-carbon. For the corresponding metal allenylidenes, electrophilic attack occurs at the P-carbon and/or the metal center. Then we briefly reviewed the theoretical study of the barriers ofrotation ofvinylidene ligands in various flve-coordinate complexes M (X) C1(=C=CHR)L2 (M = Os, Ru L = phosphine). The study showed that 7t-acceptor ligands (X), electron-withdrawing substituents and lighter metals gave smaller barriers. 

The dimerization process of [(C5H4Me)(dmpe)Mn=C=CHR]+ to [(C5H4Me) (dmpe)Mn=CCHRCHRC=Mn(dmpe)(C5H4Me)], studied both experimentally and theoretically, was briefly discussed. Density functional theory calculations showed that the dimerization process was favorable when R=H, Me, Ph and C6H4Me) but unfavorable when R = silyl. The calculations also showed that the LUMO of a given dimer consisted of Tc -antibonding character between Mn and Co, 

Another focus of this chapter is the alkynol cycloisomerization mediated by Group 6 metal complexes. Experimental and theoretical studies showed that both exo- and endo- cycloisomerization are feasible. The cycloisomerization involves not only alkyne-to-vinylidene tautomerization but alo proton transfer steps. Therefore, the theoretical studies demonstrated that the solvent effect played a crucial role in determining the regioselectivity of cycloisomerization products. [2 + 2] cycloaddition of the metal vinylidene C=C bond in a ruthenium complex with the C=C bond of a vinyl group, together with the implication in metathesis reactions, was discussed. In addition, [2 + 2] cycloaddition of titanocene vinylidene with different unsaturated molecules was also briefly discussed. 

Gompared with vinylidene complexes, allenylidne complexes are not well studied theoretically. In this chapter, theoretical calculations of an allenylidene-vinylvinyli-dene equilibrium and the Z/E isomeric interconversion of aminoallenylidene ruthenium complexes were summarized. 

---

Heavier group 14 analogs of metal vinylidene complexes... 

---