Electronic Manifestations of Scandide Contraction: Theoretical Photoelectron Spectroscopy of Monovalent Group 13 Compounds

A wide-ranging density functional theory (DFT) survey of monovalent Group 13 complexes (trielylenes) has afforded detailed insights into periodic trends in these compounds. Five classes of neutral complexes were examined, based on β-diketiminate, bis(imino)carbazolate (pincer), hydrotrispyrazolylborate, cyclopentadienide, and monocoordinate aryl ligands. Also examined was a set of <i>N</i>,<i>N</i>'-diaryl-1,4-diazabutadiene dianion-coordinated triel(I) anions. In general, the ionization potential of the Ga-based lone pairs was found to be nearly 1 eV or more higher than that of Al-based lone pairs in similar species; the corresponding IPs for In were found to hover around the values calculated for Ga. This general trend may be thought of as an electronic manifestation of scandide contraction, which results in stabilization of the 4s and 4p subshells due to poor screening by the filled 3d subshell. Only for the aryl series was this effect found to be quite muted, apparently because these complexes do not harbor a genuine metal-based lone pair. Instead, the highest occupied molecular orbital (HOMO) consists of a metal-carbon σ-antibonding orbital, spread over multiple atoms. The calculations also underscore the major role of the supporting ligand in modulating the electronic properties of trielylenes and help explain why certain species such as Al(I) bis(imino)carbazolates might not be stable enough to permit isolation.