Isomeric Al2R4, Mg2R2 Species and Oligomerisation Products: Investigation of Al-Al and Mg-Mg σ Bonding

Initiated by ongoing experimental work in the field of sub-valent all-Iminium compounds and by recent results on isoelectronic Mg2R2, (s(1) states) species, detailed theoretical work was necessary to explain the unusual bonding situation in some Al2R4 isomers (R = PtBu2), of a dimeric Al4R'(8) (R' = OtBu) as well as Of SLibhalides Al2F4, Al2Cl4, Mg2Cl2 and some of their oligomers. These results are presented in this contribution. Especially the different isomers of the four-membered heterocyclic RAlR(2)AIR molecule needed careful theoretical investigation, since on the way from monomeric AIR, radicals to the Most Stable RAlR(2)AIR isomer with a butterfly shape, intermediates with different Al-Al interactions exhibiting diradical as well as diradicaloid character - have to be considered. The presented results on the unusual bonding in these AlP2Al four-membered heterocyclic systems are an important contribution to a better understanding of the bonding Situations in many inorganic molecules containing, for example, B2P2, C2P2, Ge2N2, Sn2N2 and Si-4 entities, which recently have drawn considerable interest.. In analogy to tire Al-II compounds some isoelectronic Mg-1 species have been investigated. Regarding the obtained results, special attention is drawn to MgCl oligomers, since we found unexpectedly high oligomerisation energies in our calculations. This high stabilisation is based on direct, Mg-Mg bonding as well as on Mg-Cl-Mg bridging bonds. These mixed bonding motifs may also he present in the so far unknown solid MgCl. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)