Bonding and Structure of Metal-Alkyne Complexes

Metal-alkyne bonding generally is considered to be dominated by two orbital interactions (Figs, la and lb) which act synergistically (Dewar, 1951 Chatt and Duncanson, 1953 Maitlis, 1971a,c Nelson and Jonassen, 1971). 

Either picture qualitatively rationalizes two important structural features of the coordinated alkyne (1) the C=C bond order is decreased relative to that in the free ligand, and (2) its geometry is cis, bent. Analogy has been drawn between the structures of the coordinated alkyne and the excited state of the free ligand (Blizzard and Santry, 1968). Carbon-carbon bond lengths in coordinated alkynes range from 1.24 to 1.40 A, and the deformation angle 

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