Recent advances in the chemistry of small cage Main Group metallacarboranes

This overview presents our significant recent findings in the area of small cage Main Group metallacarboranes during the past several years. The highlights of these findings are as follows. (1) The study on the C4B8-carborane led to the introduction of an unusual 'carbons-apart' carborane species that acts as a restricted electron-acceptor by removing only the valence electrons of a Group 1 or Group 2 metal thus forming the corresponding alkali and alkaline earth metal compounds. Further reactivity studies of these species in solvent extraction of radioactive cesium ion (Cs-137(+)) from nuclear wastes has rekindled our interest to explore new avenues in this area of Main Group metallocarboane research. (2) The syntheses and structures of fully sandwiched anionic lithia- and magnesacarboranes and half-sandwich lithia-, cesia- and magnesacarboranes have exemplified their rich structural and coordination chemistry beyond their existence as versatile synthons for the production of other metallacarboranes of d-block and f-block elements. (3) Synthesis and reactivity studies on half- and full-sandwich gallacarboranes opened up the possibility of using such compounds as precursors in the formation of conducting materials. (4) Even though two germanium atoms are present, the reactivity study on the mixed-valence digermacarboranes with Lewis bases shows no indication of exo-polyhedral Ge(IV) atoms being the Lewis acid site. On the contrary, the donor N atoms of the Lewis bases bond exclusively with the apical Ge(II) atom. (C) 1999 Elsevier Science S.A. All rights reserved.