Boron-metal clusters

As a result of the systematic application of coordination-chemistry principles, dozens of previously unsuspected stnicture types have been synthesized in which polyhedral boranes or their anions can be considered to act as ligands which donate electron density to metal centres, thereby forming novel metallaboranc elusters, ". Some 40 metals have been found to act as acceptors in this way (see also p. 178). The ideas have been particularly helpful m emphasizing the close interconnection between several previously separated branches of chemistry, notably boron hydride clu.ster chemistry, metallaboranc and metallacarbaborane chemistry (pp. 189-95). organometallic chemistry and metal-metal cluster chemistry. All are now seen to be parts of a coherent whole. 

The mles can readily be extended to isoelectronic anions and carbaboranes (BH=B =C) and also to metalloboranes (p. 174), metallocarbaboranes (p. 194) and even to metal clusters themselves, though they become less reliable the further one moves away from boron in atomic size, ionization energy, electronegativity, etc. 

Au-B bonds are also present in metal clusters with intersticial or peripheral boron atoms. An example is the cluster [Fe4(CO)12BH(AuPPh3)2], which was prepared by reaction of [AuCl(PPh3)] with the carbonyl iron dihydride. With the oxonium salt the reaction proceeds to the trinuclear gold derivative [Fe4(CO)12B(AuPPh3)3] (357).2063-2070 The ruthenium analogues and complexes with other ligands have been also synthesized as, for example, (358).2071-2079... 

The polyhedral boranes and carboranes discussed above may be regarded as boron clusters in which the single external orbital of each vertex atom helps to bind an external hydrogen or other monovalent atom or group. Post-transition main group elements are known to form clusters without external ligands bound to the vertex atoms. Such species are called bare metal clusters for convenience. Anionic bare metal clusters were first observed by Zintl and co-workers in the 1930s [2-5], The first evidence for anionic clusters of post-transition metals such as tin, lead, antimony, and bismuth was obtained by potentiometric titrations with alkali metals in liquid ammonia. Consequently, such anionic post-transition metal clusters are often called Zintl phases. 

Although legions of icosahedral carboranes 49 are known, the formation of supra-icosahedral boron-containing clusters was restricted to metal carboranes (MxC2B10 or MxC4Bg, x = 1,2) [62, 63]. 

The recent work of Grimes and his co-workers 14, 26, 84, 96-96b, 98, 119, 191, 192) has shown the smaller carboranes to be particularly fertile sources of metal-boron-carbon clusters, of which a few further examples are shown in Fig. 19. 

The boron hydrides, including the polyhedral boranes, heteroboranes, and their metaUa derivatives, encompass an amazingly diverse area of chemistry. This class contains the most extensive array of structurally characterized cluster compounds known. Included here are many novel clusters possessing idealized molecular geometries ranging over every point group symmetry from identity (C[) to icosahedral (I[). Because boron hydride clusters may be considered in some respects to be progenitorial models of metal clusters, their development has provided a framework for the development of cluster chemistry in... 

Boron Cage Compounds 789 Metal Clusters 807 Conclusion 819... 

B—C Bond Distances in Transition Metal Clusters Containing Boron Atoms"... 

There are still only a few examples of mixed metal-transition metal clusters containing boron atoms. The first to be reported was Cp2Co2Fe(CO)4B3H3 (92). This cluster results from the photolysis of 2-CpCoB4Hg [a basally substituted isolobal analog of pentaborane(9)] with Fe(CO)5. NMR spectroscopic data are consistent with the proposed structure shown in Fig. 17. 

The favored geometry in coordination compounds, boron hydrides, and metal clusters is the polyhedron that has all faces equilateral or near equilateral triangles... ... 

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