Periodic Trends in Low-Valent Group 14 and Group 15 Compounds: Theoretical Photoelectron Spectroscopy of Tetrylenes, Tetrylones, and Singlet Pnictinidenes.

Scalar-relativistic DFT calculations have been used to calculate the lowest ionization potentials (IPs) of six series of low-valent Group 14 (tetrel) and Group 15 (pnictogen) compounds with a view to uncovering periodic trends, especially the impact of scandide contraction and the inert pair effect. The compounds studied include: (a, b) EF2 and ECl2 (where E is a tetrel), (c) N,N'-dimethylimidazol-2-ylidine and its higher tetrel analogues (tetrylenes), (d, e) a carbodiphosphorane and a carbodicarbene and their higher tetrel analogues (tetrylones), and (f) a series of singlet pnictinidenes based on a pincer ligand. For the three series of tetrylenes studied (a-c), the IPs of the tetrel-based lone pairs exhibit a distinct crest at germanium, reflecting an electronic manifestation of scandide contraction. For the two tetrylone series and the singlet pnictinidenes (d-f), the IPs of the two lone pairs exhibit divergent trends. The lower of the two IPs, corresponding to ionization of the π-type lone pair is found to decrease monotonically down the groups. The higher IP, corresponding to ionization of the σ-type lone pair, in contrast, increases down the groups, reflecting the combined influence of scandide contraction and the inert pair effect.