Alkyne ligands, four-electron

Ward BC, Templeton JL (1980) Nuclear magnetic resonance studies of alkynes as four-electron donor ligands in monomeric tungsten(n) complexes. J Am Chem Soc 102 1532-1538... 

Four-Electron Alkyne Ligands in Molybdenum(ll) and Tungsten(ll) Complexes... 

The term four-electron donor is sometimes used to describe the alkyne ligand in circumstances where alkyne ir donation supplements classic metal-olefin bonding. The utility of this scheme lies in its simplicity, and with some reluctance we shall rely on the four-electron donor terminology to suggest global properties of metal alkyne monomers. The general implications and specific hazards characterizing these descriptors... 

Section IV describes reactions which produce four-electron donor alkyne ligands by coupling two single carbon based ligands (CR, CO, CNR). Section VIII,A describes syntheses of i72-vinyl complexes (t 2-CR=CR2 ) by nucleophilic addition to four-electron donor alkyne ligands. In the products of the coupling reactions and in the reagents of the r 2-vinyl syntheses, the role of alkyne 7t donation is crucial. 

A cationic acetylene carbene complex has been prepared by alkylation of an acyl oxygen (77). The alkyne ligand remains intact and retains the spectral properties of a four-electron donor on conversion of the neutral acyl to the cationic heteroatom carbene [Eq. (25)]. Mayr has entered... 

The structures of Cp2Mo(PhC=CPh) (81) and Cp2Mo(CF3C=CF3) (128) have been included in Table I as standards for two-electron donor alkyne ligands in d4 monomers. The observed metal-carbon distances of 2.14 (R = Ph) and 2.13 A (R = CF3) exceed the four-electron average by 0.10 A, a substantial distance difference. 

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