Metallaborane types

In this section, monoboron clusters will be dealt with first, moving through the transition metals group by group with increasingly higher ratios of boron to metal. Many of these metallaborane species contain more than one type of metal and they are discussed in the order of the metal present in the starting material from which they are synthesized. 

There is a series of metallaboranes of the type [SBgHjoM], [S2B7H7M], etc., where M seems to contribute two cluster electrons giving a nido-electron count. Nevertheless, these metallaboranes must be designated as arachno-dusters. Though the principal structure of the 10-vertex nido- and arachno-skeleton is the same, they differ structurally by the position of the bridging H atoms (which are the gunwale positions 5-10 and 7-8 in the arachno-case) and, moreover, in their physical properties, particularly in the sequence of the 11B NMR shift values. We will discuss the ar-10 clusters with formal ni-10 electron count later. 

The synthesis of metallaboranes in quantity permits the examination of reaction chemistry and we have already described a considerable number of reactions of chroma-,27, 281 29, 30 molybda-,20 ruthena-,15, cobalta-,31 and rhodaboranes.13 This work constitutes a true study of the effects of the transition metal on chemistry as the metal ancillary ligand (Cp ) is unchanged. Space does not permit more than a single example that serves to illustrate the type of new chemistry that has been revealed by these new metallaboranes. 

Cp M fragment withdraws electrons from the skeletal bonding system effectively collapsing the normal closo structure into a highly capped closed cluster core of lower vertex number. These hypoelectronic34 metallaboranes constitute a new structure type in metallaborane chemistry and promise an interesting focus for future research. 

A new mode of metallaborane degradation was discovered in our laboratory recently and the preliminary details have been published.1 The metallahexaborane [2,2,2-(PPh3)2(CO)-nitfo-2-OsBsHJ, (I)2 reacts with the phosphines, PR to give products of the type [2,2,2-... 

Classify the following metallaboranes by structural type a. C2B7H9(CoCp)3 b. C2B4H6Ni(PPh3>2... 

As a result of the systematic application of coordination-chemistry principles, dozens of previously unsuspected structure 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 metallaborane clusters, - Some 40 metals have been found to act as acceptors in this way (see also p. 178). The ideas have been particularly helplul m emphasizing the close interconnection between several previously separated branches of chemistry, notably boron hydride cluster chemistry, metallaborane and metallacarbaborane chemistry (pp. 189-95), organometallic chemistry and metal-metal cluster chemistry. All are now seen to be parts of a coherent whole. 

Polyhedral isomerizations may be studied using either a microscopic or macroscopic approach. The microscopic approach uses details of polyhedral topology to elucidate possible single polyhedral isomerization steps, namely which types of isomerization steps are possible. Such isomerization steps consist most commonly of so-called diamond square-diamond processes or portions thereof. The microscopic approach to polyhedral isomerizations is relevant to understanding fluxional processes in borane and metallaborane polyhedra. 

Metallaborane and borylene complexes are related to some extent because both types possess direct metal-boron bonds however, the nature of these interactions is varied. In metallaborane clusters, the framework is made up by nonclassical, electron-deficient, multicenter two-electron bonds, while the borylene ligand (BR) is related to one or more transition metal centers by classicrJ, electron-precise, two-center two-electron (2c-2e) bonds [280 -283]. Since the discovery of various bridged and terminal borylene complexes by Braunschweig and Aldrige, the chemistry of this subarea of transition metal complexes of boron has received significant attention from both structure/bonding and... 

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