Alkyne ligands, four-electron description

Kreissl has pointed out that the rj -ketenyl ligand in CpW(CO)(Et2-NC=CMe)[i71-C(CO)C6H4Me] acts as a multiple electron donor with resonance form ii below an important contributor to the description of this ligand (71). The impact of this 7r-donor role for the V-ketenyl ligand on the cis-alkyne geometry is minimal as the aminoalkyne ligand structural parameters fall in the range typical of four-electron donor alkynes. 

Nearby elements also display chemistry reflecting alkyne 7rx donation to a vacant metal dir orbital. In Group V, Lippard s coupled carbonyl product Ta(Me3SiOC=COSiMe3)(dmpe)2Cl is a d4 Ta(I) alkyne monomer (116), similar in electron count to CpV(CO)2(RC=CR) complexes (231). The preparative route and physical properties of a series of Ta(CO)2-(RC=CR)(I)L2 d4 monomers are compatible with a four-electron donor description for the alkyne ligands (231a). The d2 configuration has also... 

A symmetry-based molecular orbital description of the unusual four-coordinate C3v W(RC=CR)3(CO) series of molecules was presented by King in 1968 (32). The tt orbitals of the three alkynes yield linear combinations of A2 and E symmetry. Since there is no metal orbital of A2 symmetry only the degenerate E combination of n orbitals finds metal orbital mates for bonding and antibonding combinations. The three alkyne 7T orbitals serve as er donors [(Ax + E) symmetry] as does the fourth ligand (Al symmetry). Thus the total metal electron count adheres to the effective atomic number rule [W(0)(6) + 37T j(6) + 2ir (4) + lo-(2) = 18 electrons]. 

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