Coupling constants alkynes

Table 2.4. Typical HH coupling constants (Hz) of some units in alicycles, alkenes and alkynes ...

Figure 27-8 Observed (upper) and calculated (lower) nmr spectra of 4-deuterio-1-buten-3-yne (CH2=CH—C=CD) at 60 MHz. The calculated spectrum is based on chemical shifts of 300, 297, and 283 Hz and coupling constants of 18.0, 11.5, and 2.0 Hz. The deuterium substitution was made to simplify the spectrum by eliminating small long-range couplings involving the double-bond hydrogens and the alkyne hydrogen.

Chiral Metal Atoms in Optically Active Organo-Transition-Metal Compounds, 18, 151 13C NMR Chemical Shifts and Coupling Constants of Organometallic Compounds, 12, 135 Compounds Derived from Alkynes and Carbonyl Complexes of Cobalt, 12, 323 Conjugate Addition of Grignard Reagents to Aromatic Systems, I, 221 Coordination of Unsaturated Molecules to Transition Metals, 14, 33 Cyclobutadiene Metal Complexes, 4, 95 Cyclopentadienyl Metal Compounds, 2, 365... 

Single bond JCH coupling constants for bound alkynes also offer insight into the bonding of terminal alkynes to metals. For M(CO)(HC=CR)-(S2CNEt2)2 complexes ch values well below the free alkyne value of 250 Hz but still above 200 Hz are observed (58). Since /ch is related to the fraction of s character in the C—H bond it addresses hybridization directly and thus crudely quantifies perturbation of the alkyne from sp hybridization on binding to the metal. 

Spin-spin coupling constants involving, 3C nuclei are usually dominated by the Fermi contact term (72) and therefore depend in part upon the hybridization of the interacting nuclei. The 13C- H coupling constants, /(CH), are a useful indication of the hybridization of a given carbon atom. Typical ranges for hydrocarbons are approximately 125 Hz for sp3 carbon atoms in alkanes, 155-160 Hz for sp2 carbon atoms in alkenes and arenes, and approximately 250 Hz for alkynes (8). Applications to structural determinations are mentioned throughout this review. 

The observation of 13C-13C coupling constants in fi2- and pA-t]2-alkyne cobalt clusters has also shed some light on the nature of bonding in these complexes (382). There is a drastic decrease in the (CC) coupling constant on going from the free alkyne to complexes where the ligand is bonded to two or four cobalt atoms. In Co4(CO)10(/i4->/2-HCCH), the 1 J(CC) constant is 21 Hz, compared to that of 171.5 Hz in HCCH, and theoretical calculations indicate that this is consistent with a rehybridization of the formally sp hybridized acetylenic C atoms toward sp3 hybridization. 

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. 

A similarity is seen to exist between the formal double bond of cyclopropene and the carbon-carbon bond in an alkyne in this context the value of Jqh 129.0 Hz for the coupling constant within the methyl group of 152 (see above) is slightly enhanced in the corresponding coupling, 132 Hz, in propyne. Indeed, the prediction was made by Walsh and noted by Closs, that the properties of cyclopropenes would be intermediate between those of unstrained alkenes and alkynes. 

Hydride complexes of platinum have received considerable study since the preparation of PtHCl(PEt3)2- Spectroscopic studies by NMR techniques have been widely used because of the structural information which can be obtained from coupling constant data to Pt and other nuclei. Platinum is widely used as a heterogeneous catalyst, and vibrational studies on platinum hydride complexes have been useful for comparison of a hydrogen atom bonded to a single platinum with that bonded to a surface. Complexes of platinum have been used to catalyze hydrogenation, hydrosilylation and isomerization reactions with alkenes and alkynes, as well as H/D exchange reactions on alkanes. Hydride complexes are frequently proposed as intermediates in these reactions, and the pathways related to the known chemistry of hydride complexes. 

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