In this thesis the results of several crystal structure determinations are discussed in terms of their multi-centre inter- and intra-molecular bonding interactions. In Section 1 the structures reported form part of a systematic investigation into the importance of secondary bonding in determining the crystal packing and influencing primary geometry in compounds of the non-metals. In pursuance of this aim, the following crystal structures have been determined: - (1) The diethyl tin dihalides, Et2SnX2 (X = Cl, Br, I). (2) The solid solvate SeOCl2-Dioxane and the compound lodoxybenzene. (3) The compounds iodobenzene diacetate and iodobenzene bis- (dichloroacetate). (4) cis -3. 5-Dibromo -4-oxo -2, 2,6,6 -tetramethylpiperidin -1 - yloxy, a derived bis nitroxide and (Z, Z)2, 4-Dibromo-1,5- diphen y lpen ta -3 -one. (5) The novel pseudo polyhalide anion [Me2N(ICI)21ˉas its tetramethylammonium salt. Each of these structures is discussed in relation to similar examples. In some cases no secondary bonds were found, and possible reasons for this are given. In others, a three-centre overlap of a o* orbital with two lone pairs is proposed as a viable alternative to the principal linear arrangement X-A...Y of primary and secondary bonds found in most examples. A discussion of the significance of these structures in relation to current stereochemical and bonding models, including charge transfer descriptions of the interactions, and possible methods to quantify the strengths of these Interactions are given in Chapter VI. In Section 2 the topological descriptions of the structures of the boron hydrides and related compounds are discussed in terms of their multi-centre bonding interactions, and the classification of the geometries of the boron polyhedra as closo, nido, arachno and hypho. The hypho class is by far the least well-known, and the crystal structures have been determined of three new examples isoelectronic with the hypothetical hypho B5H11 2ˉ, ion. viz. B5H9 . L [L = (Ph2P)2CH2, (Ph2PCH2)2 and (Me2 NCH2)2] The geometries of these compounds are significantly different, illustrating the diversity possible within such a related series. The structural relationships between these compounds are discussed in terms of the involvement of phosphorus 3d orbitals in the bonding of the phosphine adducts, and the contribution of vacant orbital valence structures to the structure of the tetramethylene diamine adduct.