The molecular structures of nontrigonal pnictogen compounds reveal the steric strain in these molecules, and significantly differing bond angles at the pnictogen atoms indicate a considerable distortion of the coordination spheres.
In particular, the geometry at the central part of these compounds deviate strongly from traditional pnictogen compounds, and indicate molecular strain with an approach to a T-type molecular configuration. With different ligand motifs, the bond angles at pnictogen atoms can vary from 100˚ to almost 180˚. The flattened geometry of these molecules influences the relatively low energetic barriers for inversion of the configuration via planar coordinated pnictogen atoms in the transition state. These low barriers are in accordance with the dynamic behavior and fast equilibration processes observed in ambient temperature NMR.
Results of quantum chemical calculations confirm that in these compounds, the lone pair of electrons at the pnictogen atoms is localized in orbitals with relatively high s-character. From these results, only weak nucleophilicity was derived in accordance with some experimental observations such as the inertness towards benzyl bromide. The LUMO is delocalized but has important contributions from pnictogen empty p orbitals, which should favor a nucleophilic attack of substrates at this position in accordance with experimental findings. The pnictogen atom forms a three-center-four-electron bond with the two flanking nitrogen atoms, which is manifested by the HOMO-2.
For nontrigonal bismuth compounds, a Bi(I) electronic structure could be shown to be most appropriate. Natural bond orbital (NBO) analysis reveals an s-type lone pair and a p-type lone pair at the metal, with the remaining two p orbitals being involved in one two-center-two-electron bond and one three-center-two-electron bond. The p-type lone pair NBO has less than 2 electron occupancy as it is delocalized over the ligand frame. Although considerable Bi(I) character is indicated for the Bi compound, it exhibits reactivity similar to Bi(III) electrophiles, and expresses either a vacant or a filled p orbital at Bi.
From these results, two types of resonance structures can be drawn, one with a filled s-orbital and a vacant p orbital at the pnictogen center, the other one with negative charge on pnictogen, arising from the redox-non-innocent nature of the ligand. This is evident by shorter C-N bond lengths in nontrigonal pnictogen compounds than C-N single bonds in the corresponding ligands. These structures may reflect the specific bonding situation in these strained molecular systems.