Multiple covalent bonds acetylene

Carbon can form multiple covalent bonds by sharing more than two electrons with a neighboring atom (Section 7.5). In ethylene, the two carbon atoms share four electrons in a double bond. In acetylene, the two carbons share six electrons in a triple bond ... 

Carbon atoms can also share more than one electron pair with another atom to form a multiple covalent bond. Consider the examples of a carbon-carbon double bond in ethene (ethylene) and a carbon-carbon triple bond in ethyne (acetylene). 

Carbon likes to form bonds so well with itself that it can form multiple bonds to satisfy its valence of four. When two carbon atoms are linked with a single bond and their other valencies (three each) are satisfied by hydrogens, the compound is ethane. When two carbons are linked by a double bond (two covalent bonds) and their other valencies (two each) are satisfied by hydrogens, the compound is ethylene. When two carbons are linked by a triple bond (three covalent bonds) and their other valencies (one each) are satisfied by hydrogens, the compound is acetylene. 

Several examples of single bonds consisting of a pair of shared electrons have just been seen. Two other types of covalent bonds are the double bond consisting of two shared pairs of electrons (4 electrons total) and the triple bond made up of three shared pairs of electrons (6 electrons total). These are both examples of multiple bonds. Examples of a double bond in ethylene and of a triple bond in acetylene are shown in Figure 4.13. 

Olefins and acetylenes can, potentially, bond to metal as either pi or o ligands. In the former case the metal atom is approximately equidistant from two carbon atoms linked by a multiple bond while in the latter case the metal atom is bonded to one particular carbon atom by a covalent cr-bond. 

The contribution of n bonding interactions in classical multiple bonds to the bond strength becomes obvious from the EDA results of the HB=BH, H2C=CH2, HN=NH, and HC=CH bonds, which are shown in Table 7. The covalent contributions to the attractive interactions are stronger than the electrostatic contributions in the above molecules. The relative contribution of AE r decreases for the double bonds in the order HB=BH > H2C=CH2 > HN=NH. The degenerate n bond in acetylene contributes 44.4% to the orbital interactions, which means that it is not much weaker than the a bond. Note that the absolute values of AEcr in acetylene (—215.5kcalmoH ) and ethylene (—212.2 kcal mol ) are nearly the same while AE r in acetylene (—172.4 kcal mol ) is more than twice as strong as in ethylene (—79.2 kcalmol ). 

There have been extensive experimental and theoretical studies devoted to the structural and bonding characterization of weakly bound van der Waals complexes of acetylene. Structures of these complexes can often be determinated experimentally by means of Fourier transform microwave and infrared spectroscopic techniques. On the theoretical side, advanced treatments are required to understand the complex nature of the weak bonding in terms of the relative contributions of polarization and dispersion interactions, interactions of multiple moments, and electrostatic interactions involved in these completes. To determine the interaction energy in a weak complex, it is necessary to use large basis sets with the inclusion of electron correlation interactions. Theoretical calculations have been reported for van der Waals complexes of acetylene with COj [160], CO [161, 162], AICI3 [163], NH3 [164], He [165], Ar [166], H2O [167], HCN [168], HF [169-172], HCl [173, 174], and acetylene itself in the forms of non-covalent dimer [175-180], trimer [175,181], tetramer [175, 182, 183], and pentamer [175]. These calculations are very useful for the determination of multiple isomeric forms of the complex. For example, calculations at the MP2/6-31G level along with IR spectra indicate that the HCN-acetylene complex exists in a linear form in addition to the T-shaped structure observed previously by microwave studies (see Fig. 1-5) [168]. 

---