Triply bonded

Acetylene is linear and alkynes have a linear geometry of their X—C=C—Y units The carbon-carbon triple bond m alkynes is com posed of a CT and two tt components The triply bonded carbons are sp hybridized The ct component of the triple bond contains two electrons m an orbital generated by the overlap of sp hybndized orbitals on adja cent carbons Each of these carbons also has two 2p orbitals which over lap m parrs so as to give two tt orbitals each of which contains two electrons... 

Chemical Properties. Hydrogen cyanide is a weak acid its ionization constant is of the same magnitude as that of the natural amino acids (qv). Its stmcture is that of a linear, triply bonded molecule, HC=N. 

The relatively slower rearrangement of alkynyl and cyano substituents can be attributed to the reduced stability of the intermediate derived by cyclization of the triply bonded substituents ... 

The single-bond covalent radius of C can be taken as half the interatomic distance in diamond, i.e. r(C) = 77.2pm. The corresponding values for doubly-bonded and triply-bonded carbon atoms are usually taken to be 66.7 and 60.3 pm respectively though variations occur, depending on details of the bonding and the nature of the attached atom (see also p. 292). Despite these smaller perturbations the underlying trend is clear the covalent radius of the carbon atom becomes smaller the lower the coordination number and the higher the formal bond order. 

Another, simple form of elemental carbon would be chains formed from carbon atoms. As a prototype model a single>stranded chain is most suitable. If branching were to be considered, all intermediate forms up to and including the diamond and graphite like clusters would be included. For non branched chains, the two variants to choose from are a system of alternating singly and triply bonded carbon atoms (poly-ynes), and a system with all double bonds (cumulenes). Cumulene structures are assumed to be the preferred ones for odd membered chains, whereas the even ones may have some poly-yne character. Recent studies on linear Cg show that a cumulene-like structure is preferred, both at the SCF level and when correlation is accounted for(50). 

A well-known example of the effect of bond order on bond length is provided by the bonds in ethane, ethene, and ethyne, which have the lengths of 154, 134, and 120 pm, respectively. Covalent radii for doubly and triply bonded atoms can be obtained from double and triple bond lengths in the same way as for single bonds. Some values are given in Table 2.2. 

Figure 2.3 Lewis diagrams of some molecules of nitrogen, oxygen, and showing the increased separation of the lone pairs from the singly bonded, to the doubly bonded, and to the triply bonded molecules.

Terminal alkyne an alkyne with a hydrogen attached to a triply bonded carbon. 

G A. Ozin, University of Toronto In view of the current interest in electrically conducting, undoped and doped polyacetylene thin films, do you envisage any possibility of initiating a controlled polymerization of a metal-metal triply bonded organometallie complex to produce a species of the form ... 

The nitrogen atom is triply bonded to the oxygen atom and both atoms in the structure possess a lone pair of electrons. This gives each atom an octet and a positive formal charge appears on the oxygen atom. 

A corresponding Lewis-like picture can be developed for triply bonded metal alkylidynes (H MCH), such as the duodectet-rule-conforming examples... 

A large number of tests showed that a value of 0.25 for s was optimal. The mean error in the dissociation energies for 49 diatomic molecules was reduced from 0.2 eV to 0.1 eV. Using an average s was particularly impressive for triply bonded molecules The average error for N2, P2, and As2 was reduced from 0.45 eV to less than 0.15 eV. Similar absolute improvements were obtained for excitation and ionization energies.20... 

Logically, more bonds between atoms should mean a greater force of attraction and increased carbon density. But doubly bonded alkenes are more reactive, not less reactive, than singly bonded alkanes or triply bonded acetylenic compounds. Further, how could the force associated with a polyvalent atom be divided a priori to react to the atoms encountered, sometimes dividing itself in half, other times by thirds To assume that valences act across empty space, free of atoms, some chemists noted, "is only possible on paper or in a model where there are lines or wires but not forces. "89... 

The presence of the endothermic triply-bonded acetylene (ethyne) group confers explosive instability on a wide range of acetylenic compounds (notably when halogen is also present) and derivatives of metals (and especially of heavy metals) [1], Explosive properties of butadiyne, buten-3-yne, hexatriyne, propyne and propadiene have been reviewed, with 74 references [2], The tendency of higher acetylenes to explosive decomposition may be reduced by dilution with methanol [3], The class includes the separately treated groups ... 

As shown in this review, the synthesis, physical properties, and reactivity of doubly bonded germanium species are now well known. In contrast, triply bonded derivatives of group 14 elements —M = X (X = C, M, N, P) are still unknown, even though it seems possible, using convenient groups, to stabilize such species. Note, however, although it is not the topic of this... 

In conclusion we can predict that the next review on low coordinate germanium will describe some triply bonded germanium derivatives. 

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