The Nature of the Idealized Triple Bonds Between Principal Elements and the σ Origins of Trans-Bent Geometries-A Valence Bond Study

We describe herein a valence bond (VB) study of 27 triply bonded molecules of the general type X Y, where X and Y are main element atoms/fragments from groups 13-15 in the periodic table. The following conclusions were derived from the computational data: (a) Single pi-bond and double pi-bond energies for the entire set correlate with the molecular electronegativity, which is the sum of the X and Y electronegativites for X Y, The correlation with the molecular electronegativity establishes a simple rule of periodicity: pi-bonding strength generally increases from left to right in a period and decreases down a column in the periodic table. (b) The sigma frame invariably prefers trans bending, while pi-bonding gets destabilized and opposes the trans distortion. In HC CH, the pi-bonding destabilization overrides the propensity of the sigma frame to distort, while in the higher row molecules, the sigma frame wins out and establishes trans-bent molecules with 21/2 bonds, in accord with recent experimental evidence based on solid state Si-29 NMR of the Sekiguchi compound. Thus, in the trans-bent molecules less bonds pay more. (c) All of the pi bonds show significant bonding contributions from the resonance energy due to covalent-ionic mixing. This quantity is shown to correlate linearly with the corresponding molecular electronegativity and to reflect the mechanism required to satisfy the equilibrium condition for the bond. The pi bonds for molecules possessing high molecular electronegativity are charge-shift bonds, wherein bonding is dominated by the resonance energy of the covalent and ionic forms, rather than by either form by itself.