a covalent chemical bond produced by three pairs of electrons located in the field of two atomic nuclei. A triple bond consists of one σ-bond and two π-bonds; the resultant distribution of electron density between the two bound nuclei is axially symmetrical, as in the case of a single bond.
In the structural formulas of chemical compounds, triple bonds between atoms are indicated by three bond lines—for example, N≡N, C≡C, and C≡N. If one or both π-components of a triple bond, for example, between atoms A and B, form as a result of a donor-acceptor interaction, such triple bonds (A 
B or A B) are usually called quasi-triple bonds. (In the “normal” triple bond, A≡B, the σ-component and the two π-components form as a consequence of the interaction of unpaired electrons.) Examples of quasi-triple bonds are the bond of a terminal oxygen atom with the atom of a transition metal (O M, called an ylic bond)—for example, in K2[ReOCl5] and the bond in the BF molecule (B 
F). As a result of the high bond energy of triple bonds and quasi-triple bonds, the corresponding molecules (for example, N2 and CO) or molecular fragments (N≡M or O M) are thermodynamically stable and usually chemically inert. Thus, the fixation of molecular nitrogen—that is, the activation of nitrogen for chemical conversions—is impossible without its prior coordination as a Iigand in certain complexes of transition metals, leading to a significant reduction in the multiplicity (strength) of the nitrogen-nitrogen bond.
Whereas triple bonds between light atoms (carbon and nitrogen) were mainly studied in the past, greater attention is now being devoted to triple bonds involving heavy atoms, primarily atoms of the transition metals. Such triple bonds are found in new types of complexes. A large number of complexes of transition metals have already been studied with nitrido- (N≡), oxo-(O ), imino- (RN ), and carbino- (RC≡) groups as ligands with high chemical bond strength. An important feature of such ligands is their strong frans-effect (that is, the significant reduction in the metal-ligand bond strength upon transposition), which dictates the reactivity and many of the properties of these compounds. Dimeric clusters of transition metals with metal-metal triple bonds, such as Mo2[N(CH3)2]6, are also known. This molecule has a centrosymmetric inverted configuration (as in ethane, C2H6), which results from the axial symmetry of the electron density distribution in the M=M bond, where M is a metal.
REFERENCE
Shustorovich, E. M. Khimicheskaia sviaz’. Moscow, 1973.E. M. SHUSTOROVICH
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