10- and 14-Valence-Electron Triple Bond Systems: Isoelectronic Families Differing By a dδ4 Ring

Triple bonded A≡A’ 10-valence electrons (VE) main-group diatomics, such as N2, CO, CN- and NO+, have a strong σ2π4 triple bond plus a lone pair at each end. Adding four more valence electrons, as in CPt, yields 14-VE families which always have an additional δ4 ring.

A recent paper studied these occupied orbitals, which are formally non-bonding core orbitals but have the size and orbital energy of a typical valence orbital. The researchers investigated bond distances, molecular orbitals, vibrations, and nuclear quadrupole coupling constants (NQCC) of the triple bonds of diatomic A≡B molecules and polyatomic chains. The zeroth order regular approximation (ZORA) was applied to all calculations. The calculations were in good agreement with previous work and experiments.

We concluded that this δ4 ring, although a non-bonding core orbital, is energetically and radially in the valence range of the 5d element, and although its Coulomb attraction must be substantial, there is no contribution to covalent bonding. In this way, the δ4 ring resembles a σ2 lone electron pair.

Au CN2 orbitals
Highest occupied molecular orbitals of [Au(CN)2]

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Michiko Atsumi and Pekka Pyykkö, 10-Valence-Electron C≡O and the 14-VE C≡Pt: Two Triple-Bonded Isoelectronic Families Differing by a dδ4 Ring, Inorg. Chem. 2023 ASAP

Key concepts