Chapter one gives an introduction into heteroborane chemistry focussing on supraicosahedral metallacarboranes. Chapter two describes the first examples of untethered 4,1,2-MC2B10 13-vertex metallacarboranes prepared by polyhedral expansion of a tethered carborane precursor. C,C'-dimethyl 4,1,6-MC2B10 compounds were afforded as co-products of the reactions. In contrast to the analogous 1,2- to 1,6- carborane rearrangement, experimental observations and DFT calculations reveal that 4,1,2- to 4,1,6- metallacarborane isomerisation does not occur under standard laboratory conditions. Chapter three constitutes the most extensive examination to date of the relationship between ligand orientational preference and bond strength in supraicosahedral molecules. A structural study of nine 13-vertex indenyl cobaltacarboranes supplemented by DFT calculations on models of related naphthalene ferracarboranes provided information on the relative trans influences of facial cage atoms. Chapter four describes the synthesis and characterisation of 1,14,2,9- and 1,14,2,10-M2C2B10 14-vertex cobaltacarboranes. The first unambiguous confirmation of 1,14,2,9-M2C2B10 architecture from crystallographic studies and supporting spectroscopic data is presented. A series of six asymmetric, 14-vertex, bicapped hexagonal antiprismatic bimetallacarboranes with 1,8/13,2,x-Co2C2B10 atom arrangements, thought to be the results of direct electrophilic insertion reactions, are reported in chapter five. A detailed analysis of crystallographic data resulted in the identification of cage C atom positions in all six isomers. Chapter six contains experimental procedures and characterisation details for all the new compounds reported herein. Crystallographic data is listed in appendices A and B (CD) along with structure solution and refinement details.