Multiple Bonding of Heavy Main-Group Atoms

The triple bonds in group-15 molecules E2 (E=N-Bi), and in the group-14 tetrylynes HEEH (E=C-Pb), which are archetypical examples for multiple bonds between main-group atoms, are analysed with the help of an energy decomposition method. There is no particular weakening of π-bonding in the heavier homologues P2-Bi2 relative to N2. On the contrary, the relative contribution of π-bonding to the total bond strength is slightly higher in the heavier systems P2-Bi2 than in N2. The drastic differences between the chemical bonding of atoms of the first, and higher octal rows is discussed by comparing the dimerization energies of N2, and P2 yielding the tetrahedral species N4, and P4. The higher stability of P4 (Td), and the lower stability of N4 (Td) with regard to the diatomic species comes from the much longer, and weaker N-N σ-bonds in the tetrahedral species compared with the P-P σ-bonds. The N-N bonds in N4 (Td) are 32.8% longer than in N2 whereas the P-P bonds in P4 are only 16.2% longer than in P2. The N-N, and P-P bond lengths are determined by PPauli repulsion, which is identified as the origin for the stability of N2, and the instability of the heavier homologues P2-Bi2 with regard to the tetrahedral species. The unusual equilibrium geometries of the heavier group-14 homologues HEEH (E=Si-Pb), which deviate strongly from the linear geometry of HC≡CH, are explained in terms of interactions between the EH moieties. Chemical bonding in the heavier systems HEEH takes place between EH in the (X2Π) electronic ground state, whereas HC≡CH is bound through interactions between CH in the a4Σ- excited state. Bonding between two (X2Π) fragments of the heavier EH hydrides is favoured over the bonding in the a4Σ- excited state because the X2Π → a4Σ- excitation energy of SiH-PbH is significantly higher than for CH. Most of the low-lying energy minima of HEEH feature E-E triple bonds, but the bonding components come from donor-acceptor interactions that involve E-H donor orbitals or lone-pair orbitals rather than electron-sharing σ- and π-bonds.