Quadruple chemical bonding in the diatomic anions TcN- , RuC- , RhB- , and PdBe.

Quadruple bonding is uncommon for main group elements and the identification of species forming such bonds is remarkably interesting particularly in diatomic anions for which there is a lack of information. Here, it is found that the MX- anions, TcN- , RuC- , RhB- , and PdBe- , present quadruple bonding, as do the corresponding MX neutrals, even though a different type of σ2 bond is involved in ∑+ states of neutral and anions. Specifically, the ground states (X2 Δ or X2 ∑+ ) of the four anions and their first excited states (A2 ∑+ or A2 Δ) of TcN- , RuC- , and RhB- present quadruple bonds consisting of two σ and two π bonds: (4dz2  - 2pz )2 , 5pz 0  ← 2s2 , (4dxz  - 2px )2 , and (4dyz  - 2py )2 . Bond lengths, dissociation energies, spectroscopic data and electron affinities were calculated via high-level multireference and coupled-cluster methodology using the aug-cc-pV5ZX (-PP)M basis set. Strong bonding results in short bond lengths ranging from 1.602 (TcN- ) to 1.944 (PdBe- ) Å. Adiabatic (diabatic) binding energies reach up to 139 (184) kcal/mol. Electron affinities (EA) were calculated at 1.368 (TcN), 1.242 (RuC), 0.873 (RhB), 0.743 (PdBe) eV. Only for RhB has EA been measured experimentally at 0.961 eV, in good agreement with the value reported here.