Similarities and Differences on the Electronic Structure and Chemical Bonding of Isoelectronic Molecules: X ₂ NH and HNX ₂ NH, where X = N and C ^–

Identifying similarities and differences among isoelectronic moieties is crucial for understanding chemical bonding, stability, and reactivity. Here, the molecular and electronic structures, chemical bonding, and binding energies of the N₂NH, HNN₂NH, [C₂NH]^-/2-, and [HNC₂NH]^-/2- moieties are calculated via DFT, CCSD-(T), QCISD-(T), and MRCISD-(+Q) methodologies. Although N₂ and [C₂]^2- are isoelectronic and both exhibit triple bonds in their ground <i>X</i> ¹Σ_g ⁺ state and double bonds in their first excited <i>A</i> ³Σ_u ⁺ state, their binding energies and stabilities differ. These variations, along with the metastable nature of [C₂]^2-, affect the bonding and energetics of the studied species. Thus, the N₃H and N₄H₂ molecules have singlet ground states, while the [C₂NH]^2- and [C₂N₂H₂]^2- anions present triplet ground states. The NN-NH is formed via a dative bond from N₂(<i>X</i> ¹Σ_g ⁺) to NH-(<i>a</i> ¹Δ), while adding another NH, the bonding changes to HNN-NNH; their CCSD-(T) formation energies are -2.59 eV (N₂NH) and -11.93 eV (HNN₂NH). On the contrary, in both [C₂NH]^2- and [HNC₂NH]^2- anions, electron charge is transferred to NH, i.e., the in situ diatomic moieties are excited states of [C₂]^- and C₂ + NH^- (X²Π). Their formation energies are -1.14 eV ([C₂NH]^2-) and -8.57 eV ([HNC₂NH]^2-). Electron transfer from metastable [C₂]^2- to one or two NH stabilizes both [C₂NH]^2- and [HNC₂NH]^2-, suggesting their possible experimental detection.