New Chemistry of Metallocarboranes and Metalloboranes*

Ten years after the discovery of metallocarboranes, this field of chemistry is still advancing at a rapid pace. While the polyhedral expansion of carboranes has proved to be a general method for the synthesis of metallocarboranes, polymetallic compounds may be synthesized by polyhedral expansion of monometallocarboranes, sometimes producing supraicosahedral complexes. The polyhedral contraction reaction, which produces lower metallocarborane compounds, may be used in conjunction with expansion techniques to regulate polyhedral size. Polyhedral subrogation allows the replacement of a polyhedral 81-fl vertex with a transition metal, and therefore complements the other two new synthetic tools. The synthesis of metallocarboranes by thermal metal transfer, while still in its early stages of development, may prove to be another valuable synthetic method. Recent studies on the chemistry of certain metallocarboranes have shown oxidative addition to a polyhedral {BH} vertex. In addition, specific catalytic hydrogen—deuterium exchange on boron hydrides, carboranes and metallocarboranes occurs with several transition metal complexes. A new rhodium metallocarborane has been shown to be an excellent catalyst for olefin hydrogenation, hydrogen—deuterium exchange on boron, olefin isomerization and hydrosilylation. 1. HISTORICAL PERSPECTIVE This year marks the tenth anniversary of the welding of two diverse areas of chemistry, those of the transition metals and of the boron hydrides, by the discovery of the first metallocarborane complex1. The rationale behind this combination of diverse specialities was developed from two major chemical advances: the synthesis2 and the molecular orbital description3 of ferrocene and the discovery4 of the degradation of icosahedral carboranes to produce anionic species. The realization that these carborane anions, when fully deprotonated to form C2B9HI dianions, should have orbitals of proper symmetry and energy to interact with transition metals, as was found with the cyclopentadienide ion, led to the synthesis of the carborane analogue of ferrocene, the bis(dicarbollyl)iron(II) dianion, [(l,2-C2B9H1 1)2Fe]2 *Contnbution No. 3321 from the Department of Chemistry, University of California, Los Angeles, California 90024.