Bond length acetylene

Compound Average M-M bond length Acetylenic C-C length Reference... 

Many of the reactions in which acetylene participates, as well as many properties of acetylene, can be understood in terms of the stmcture and bonding of acetylene. Acetylene is a linear molecule in which two of the atomic orbitals on the carbon are sp hybridized and two are involved in 7T bonds. The lengths and energies of the C—H O bonds and C=C<7 + 27t bonds are as follows ... 

Figure 8.1 The structure of acetylene, H —C=C—H. The H--0 C bond angles are 180°, and the C=C bond length is 120 pm. The electrostatic potential map shows that the r bonds create a negative (red) belt around the molecule.

The length of the carbon-carbon triple bond in acetylene is 120 pm, and the strength is approximately 835 kj/mol (200 kcal/mol), making itthe shortest and strongest known carbon-carbon bond. Measurements show that approximately 318 kj/mol (76 kcal/mol) is needed to break a rbond in acetylene, a value some 50 kj/mol larger than the 268 kj/mol needed to break an alkene n bond. 

The most remarkable structural features of the planar cyclic mixed trisethyne-monobutadiyne pentaacetylenes 181 and 62 are their drastically bowed diyne moieties [18]. The acetylenic carbon atoms deviate from linearity by an average of 11.7° in 181 and even 13.4° in 62. The internal C-C-C angle is smaller for 181 (103.8°) than for 62 (109.2°). The disparity of the C-C bond lengths in the spirocyclopropane moieties of 62 is analogous to those observed in the expanded [n]rotanes 165-168. 

It is interesting to note that the C-C triple bond character for the acetylenediide inside the silver(l) cages is retained in most of the examples due to the close resemblance of their C=C bond lengths ( 1.09-1.28 A) with that observed in free acetylene (1.205 A).212 The Ag-C bond distances, on the other hand, span a fairly wide range ( 2.01-3.53 A) due to the presence of both a- and 7r-bonding interactions in these systems. The observation of short Ag-Ag contacts of 2.71-3.37A, compared to that in silver metal (2.89 A)213 and the sum of van der Waals radii for silver ( 3.4 A), 1 was suggestive of weak argentophilic interactions associated with these complexes. 

We also note that there are significant differences in bond lengths for single, donble, and triple bonds. The carbon atoms in ethane are further apart (1.54 A) than in ethylene (1.34 A), and those in acetylene are even closer together (1.20 A) A refers to the Angstrom unit, 10 ° m. This is primarily a consequence of the different nature of the a bonds joining the two... 

Ni(azobenzene)]had been postulated as containing a ti-N=N bond a structural study confirms this. The structure is shown in (118) the N—N bond length of 1.385(5) A is longer than in the free ligand and very close to the single-bond length. The structure of [Ni(PhC=CPh)(CNBu )2] has also been reported and the acetylene ligand is also bonded side-on to nickel. The co-ordination... 

These covalent bond lengths are reasonably constant among molecules, as the paraffin C—C bond usually has a length of 154 pm, the olefin C=C double bond has a length of 134 pm, and the acetylenic triple bond has a length of 120 pm. The C—H bond is 109 pm in a paraffin and 105 pm in an acetylene. 

As expected, the formal C—C triple bond in benzyne is significantly weaker than in unstrained alkynes, the C=C stretching vibrations of which usually fall in the region 2150 cm. Nevertheless, o-benzyne is better described as a strained alkyne rather than a biradical, which is evident from the large singlet-triplet splitting of 37.5 0.3 kcal/mol as well as the alkyne-like reactivity (e.g., in Diels-Alder reactions). The enthalpy of formation of 4 was determined to be 106.6 3.0 kcal/mol by Wenthold and Squires. For the C=C bond length a value of 124 2 pm was found experimentally, " which comes closer to a typical C C triple bond (120.3 pm in acetylene) rather than a C C double bond (133.9 pm in ethylene). 

The production of two moles of carbon monoxide and the 18-electron rule lead us to predict that the acetylene molecule is acting as a four-electron donor. In fact this is just one of many complexes in which alkynes bind in this fashion.81 For example, the structure of the diphenylacetylene complex in Fig. 15.26 shows that the positions of the two rhodium atoms are such as to allow overlap with both tr orbitals in the carbon-carbon triple bond.82 The extent of back donation into the antibondirg orbitals determines the lengthening of the C—C bond and the extent to which the C—H bonds are bent away from the complex. Bond length values vary greatly from system to... 

Fig. 19. Ru6C(CO),5(PhCsCH) (45, 46). The six ruthenium atoms define a slightly distorted octahedron, with the carbide carbon at the center (mean Ru-C — 2.04(2) A). The phenylacetylene ligand is bound to one triangular face of the cluster. Ru-Ru bond lengths fall into two groups those on the face bearing the acetylene average 2.80(4) A, and the remaining metal-metal bonds average 2.92(5) A.

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