Borabenzene metal complexes

Several earlier reviews have treated borabenzene metal complexes in various contexts and in varying depth (2-5). This article places emphasis on recent results and perspectives while still being comprehensive. 

In mass spectra of borabenzene metal complexes, ions of type [M(C5H5B)]+ are commonly observed [Co(CsH5B)]+ decays via loss of an uncharged C5H5B fragment as evidenced by the observation of a metastable ion peak (7). 

All borabenzene-metal complexes investigated structurally so far show very similar patterns for the ligand geometry (Table I) and for the metal-ligand bonding (Table II) only the cobalt complex 6 deserves separate consideration (see below). 

The bonding situation in borabenzene-metal complexes has been treated by several authors and with varying methods and intentions (38-41). The complexes are partitioned in boratabenzene ions and positive complex fragments or metal ions. The tt MO s of the C5H5BR ligand (Fig. 2) may... 

Borabenzene metal complexes, like their cyclopentadienyl counterparts, are not readily amenable to breaking of the metal-ligand bond. Such bond breaking will occur more easily in complexes with antibonding electrons. By fortuitous chance the cobaltocene route to borabenzene chemistry (Section V,A) provided 19-e complexes with the useful property of inherently weakened metal-ligand bonds. Thus most of this section centers on the reactivity of Co(C5H5BMe)2 (7) and Co(CsH5BPh)2 (13). 

In contrast to the highly reactive organoboranes, borabenzene metal complexes are surprisingly inert toward nucleophiles. However, cationic complexes may undergo nucleophilic addition reactions, and nucleophilic substitution has been observed with compounds having a hydrogen or an electronegative substituent at boron. 

In the 16 years since the discovery of the first borabenzene derivative, the chemistry of borabenzene metal complexes has lost its pioneering... 

The chemistry of borabenzene metal complexes has largely remained the domain of a single research group. We hope that this review will provide stimuli and orientation, especially to those who intend to contribute to the further development of this area. 

An information source-book on organometallic congiounds of boron based on the primary literature up to mid 1983 has been published. Reviews of the literature concerned with the use of boranes in organic synthesis (work published in 1983), the use of complexes of diborane and organoboranes on a laboratory and industrial scale and a comprehensive overview of selective hydroboration and the synthetic utility of the organoboranes so obtained have appeared. More specifically, there have been two accounts of isotope incorporation using organoboranes and a report on borabenzene metal complexes. ... 

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