This thesis reports a study of metal complexes of trimethyl-phosphine and trimethylarsine oxides. X-ray crystal structure determinations and other physical studies, notably infrared and nmr, were used to assess the factors that influence the stereo-chemistries of these complexes. It was observed that the compounds, (Mg(Me₃AsO)₅)(ClO₄)₂, (Ni(Me₃AsO)₅)(ClO₄)₂ and (Mg(Me₃PO)₅)(ClO₄)₂, readily adopt the square-pyramidal geometry. Two unusual features have been found in these complexes. The axial metal-oxygen bonds are consistently shorter than the corresponding basal metal-oxygen bonds, and the penultimate atoms (phosphorus and arsenic) of the basal ligands are coplanar with the square-base of the pyramid. This latter feature produces a large space in the region of the vacant sixth coordination site. The shorter axial metal-oxygen bond is discussed in terms of a stronger π interaction in the axial direction while the stability of the square-pyramidal geometry is explained in terms of electrostatic interactions between adjacent basal oxygen and arsenic atoms. Further evidence of this type of interaction was obtained from coulombic calculations. The crystal structures of (Mg(Me₃PO)₅)(ClO₄)₂ and (Mg(Me₃PO)₅H₂O)(ClO₄)₂ enable the influence of a sixth ligand on the coordination geometry to be assessed. The significance of the formation of the five-coordinate square-pyramidal geometry and the stereochemical effects of the binding of a sixth ligand are discussed in relation to the possible importance of these geometrical features in the biological function of calcium and magnesium ions. Comparisons between the donor powers of the phosphine oxide and arsine oxide are made. While arsine oxide has a greater σ donor strength, phosphine oxide is capable of a stronger π interaction. An interesting dinuclear structure was determined for a calcium arsine oxide complex. This complex, (Ca₂(Me₃AsO)₉)(ClO₄)₄, was found to be bridged by arsine oxide ligands. The different donor capacities of the terminal and bridging ligands are discussed in relation to X-ray and nmr results.