Alkyne ligands, geometry

The chemistry of metal complexes featuring alkyne and alkynyl (acetylide) ligands has been an area of immense interest for decades. Even the simplest examples of these, the mononuclear metal acetylide complexes L MC=CR, are now so numerous and the extent of their reaction chemistry is so diverse as to defy efforts at a comprehensive review. " The utility of these complexes is well documented. Some metal alkynyl complexes have been used as intermediates in preparative organic chemistry and together with derived polymeric materials, many have useful physical properties including liquid crystallinity and nonlinear optical behaviour. The structural properties of the M—C=C moiety have been used in the construction of remarkable supramolecular architectures based upon squares, boxes, and other geometries. ... 

Homoleptic phenoxido complexes of the composition [Re(L)4] where L = 2,6-diisopropylphenoxide or 2,6-dimethylphenoxide have been prepared by the reaction of [ReCl4(THF)2] with the lithium salts of the ligands. The molecular geometry is square planar and the metal center is well protected from above and below the Re04 plane by the isopropyl groups and protects the complex from reactions with alkynes, whereas such a reaction and the formation of [Re(OC6H3-2,6-Me2)4(RC=CR)] adducts (R = Me, Eth, Ph) has been observed for the dimethyl derivative of the phenoxide. ... 

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. 

The ground state geometry produces a LUMO consisting of dyz destabilized by -rr donation the alkyne is a four-electron donor. Vacant 7r orbitals of the two single-faced 7r-acid C2 ligands stabilize dxy (77 ) and dxz (it ) which house the four metal electrons. 

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