Alkynyl vibrational structure

The purpose of this review is to attempt to discern, from consideration of the full range of data from these techniques, which metrical and vibrational-spectroscopic parameters may be meritoriously applied to the discussion of the electronic structures of metal-alkynyl complexes, and what conclusions may be drawn from them. Because this review aims to extract the intrinsic parameters that characterize M—C=C—R bonding, the discussion is limited in scope to compounds possessing terminal (rj ) alkynyl ligands bound to a single metal center (2) complexes, clusters, oligomers. 

The history of routinely interpreting the structures and spectra of metal-carbonyl and -cyano complexes from a rr-backbonding perspective tempts one to carry this approach over to metal-alkynyl complexes as well. But however successful this may appear to be for individual or limited series of metal-alkynyl compounds, consideration of the whole body of metrical and vibrational-frequency data associated with the M—C=C—R linkage makes it clear that, rather than a particular datum or series of data demonstrating the presence of 7r-backbonding, they are... 

The electronic structure of compounds 53-59 has been studied in some detail, using vibrational spectroscopy, crystallographic data and EHMO calculations. It was thus concluded that for these electron-deficient (15-electron) species, the extent of retrodonation from the metal to the alkynyl Ti-system is not significant, and is in fact negligible when R = silyl alkyl, aryl. A small contribution is noted where R comprises the electron-withdrawing carbonyl moiety. 

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