Alkynyl complexes backbonding

One possible explanation for the generally longer C=C bonds of metal-alkynyl complexes relative to organic alkynes is that they are the result of M —> CCR 7r-backbonding [or RCC —> M 7r-bonding, as could be... 

A] 169) and mcr-Mn(CO)3 P(OPh)3 P(OPh)2(OC6H4-2-CH2) [2.207(4) A] 170), respectively, gives Arf(Mn-C) = 0.20 A for both pairs of complexes (Table 11). An unfortunate complication with this comparison is that the Mn-C bond distances of the alkyl reference compounds may be lengthened by ring strain, which would inflate Ac7(Mn-C). There is no independent evidence of M CCR 77-backbonding from spectroscopic studies of these alkynyl complexes that would lead one to expect short Mn-C bonds for them. 

A potentially important probe of the bonding in alkynyl complexes, and, in particular, the extent of M—> CCR 7r-backbonding, is to determine how the M—C=C—R fragment is structurally perturbed by the oxidation or reduction of the complex. The M—C=0 bond lengths of metal-carbonyl complexes are extremely sensitive to oxidation state, with M-C bonds lengthening and C=0 bonds contracting as the oxidation state of the metal increases (and, concomitantly, M - CO rr-backbonding decreases) (154). 

TT-backbonding, yet its CC-Si bond length [1.812(2) A] is not significantly shorter than that of the mean distance for all trimethylsilylethynyl metal complexes. Despite these uncertainties, the CC-Si bond distance bears close examination in future studies, particularly in conjunction with Si-NMR chemical shifts and MCC-Si nuclear spin-spin coupling constants, which have been reported to correlate with the extent of C-Si rr-bonding (184), because of the preliminary indication that it is shorter for metal-alkynyl complexes than in organic alkynes. 

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... 

---