| Summary: | <i>Chapter 1</i> reviews the development of the ideas now used in the discussion of the electronic structures of cluster compounds. The structure and reactivity of <i>nido</i>-B<SUB>10</SUB>H<SUB>14</SUB> (I) is described. The problem of the description of the structures of MB<SUB>10</SUB>H<SUB>12</SUB> metallaboranes is highlighted. A review of the electrochemical studies undertaken on boron-containing systems is presented. <i>Chapter 2</i> describes the r.m.s. misfit method. This is shown to distinguish between <i>nido</i> and <i>arachno</i> (B<SUB>10</SUB>) fragments in boranes. A new labelling scheme [B(a) to B(j)] is introduced. The structures of three MB<SUB>10</SUB>H<SUB>12</SUB> metallaboranes, [(CO)<SUB>3</SUB>CoB<SUB>10</SUB>H<SUB>12</SUB>]-, 9, [Pd(B<SUB>10</SUB>H<SUB>12</SUB>)<SUB>2</SUB>]<SUP>2-</SUP>, 11 and [Pt(B<SUB>10</SUB>H<SUB>12</SUB>)<SUB>2<SUP>2-</SUB></SUP>, 10, are then presented and discussed in terms of the results of r.m.s. misfit calculations between the [B<SUB>10</SUB>] fragments of these species with the [B<SUB>10</SUB>] fragments of 1 and <i>nido</i>-[B<SUB>11</SUB>H<SUB>13</SUB>]<SUP>2-</SUP>. The metal verticity is introduced to quantify the degree of metal-borane interaction. For 9, the borane fragment is found to be <i>arachno</i>-[B<SUB>10</SUB>H<SUB>12</SUB>]<SUP>4-</SUP> and the metal verticity 69.1% implying a Co<SUP>III</SUP> metal centre, a result later supported by a XANES study on a series of cobalt-containing species. The structure of the two <i>bis</i>borane systems and their nickel analogue are compared <i>via</i> the r.m.s. misfit method. The borane fragments in these systems are found to be of intermediate structure, between <i>nido</i> and <i>arachno</i>. <i>Chapter 3</i> gives the results of EHMO-FMO calculations on metallaborane systems. These show that the results of the r.m.s. misfit calculations can be understood in terms of the number of orbitals which the metal fragment uses in its interaction with the borane (the orbital contribution of the metal fragment). A continuum of metallaborane structures is seen, with the extremes typified by (PMe<SUB>2</SUB>Ph)<SUB>3</SUB>PtB<SUB>10</SUB>H<SUB>12</SUB> (fragment orbital contribution-3, verticity = 73.1% ) and [Cy<SUB>3</SUB>PAuB<SUB>10</SUB>H<SUB>12</SUB>]<SUP>-</SUP> (fragment orbital contribution = 1, verticity = 13.3% ). Variation in the B(a)..B(b), B(c)-B(d) and B(h)-B(i) distances in MB<SUB>10</SUB>H<SUB>12</SUB> metallaboranes can be best understood in terms of the occupancies of the frontier molecular orbitals of the [B<SUB>10</SUB>H<SUB>12</SUB>]<SUP>2-</SUP> fragments of these species. The B(a)..B(b) distance most closely follows the trend in metal verticities and, of the three criteria above, is the best reflection of the overall nature of the metallaboranes analysed.
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