This thesis is divided into two main sections. Section I is concerned with oxygen and sulphur ligand complexes of the divalent first row transition metals, and oxygen and nitrogen ligand complexes of the Group VB trihalides are described in Section II. Section I. (1). The preparation of new tertiary phosphine sulphide and arsine sulphide complexes of cobalt (II) is reported. Comparative spectral and magnetic data are presented for the tetrahedral oxo- and thio-ligand complexes of the types CoL₄₂ and CoL₂x₂ (X = Cl, Br and I). (L = Me₃PO and Me₃PS; Me₃As O and Me₃As S). In the perchlorate complexes corresponding oxo- and thio-ligands produce similar ligand field splitting energies, but lower β values of the sulphides and higher intensities of their spectral bands point to their greater covalent character. The considerably lower magnetic moments of CoL₄₂ thio-complexes as compared with their oxo-analogues is further evidence for differences in metal-oxygen and metal-sulphur bonding. Electronic spectra for the CoL₄₂ complexes have been compared with the spectra of related tetrahedral cobalt (II) complexes. The ⁴A₂→’⁴T₁(P) transition band shapes and intensity distributions are virtually identical for both the phosphine and arsine sulphides. In contrast the characteristic profiles produced by phosphine oxides are different from those produced by arsine oxides. Metal-ligand vibrations have been assigned for cobalt and corresponding zinc complexes. (2). Spectral and magnetic results show that trimethylarsine sulphide continues to give tetrahedral [M(Me₃AsS)₄] (ClO₄)₂ and M(Me₃AsS) ₂ x₂ (X= Cl, Br) complexes with iron (II) and nickel (II). The low β value and magnetic moment of [Ni(Me₃AS S)₄] (ClO₄)₂ again demonstrates the high metal-ligand covalency produced by thio-ligands. Metal-ligand vibrations fall in the same frequency range as observed for cobalt (II) complexes. By contrast trimethylarsine oxide reacts with the dihalides of iron, cobalt and nickel to give bridged octahedral M(Me₃As O)₂x₂ (X = Cl, Br) complexes thus providing an example of tetrahedral bridged octahedral isomerism involving oxo-ligands. Their structures have been elucidated using infra-red and electronic spectral, and magnetic data. The low As- O stretching frequencies suggest that polymerisation is being achieved via oxo- rather than halogen bridges. Further evidence is obtained from the infra-red spectra of the analogous M(Me₃As O)₂ (NCS)₂ thiocyanate complexes. (3). The preparation of high-spin five-co-ordinate [ML₅]²⁺ and [ML₄(ClO₄)]⁺ complexes with trimethylarsine oxide is described for a number of first-row, divalent, transition metal ions. The electronic spectra of these complexes more closely resemble the spectra obtained for other complexes known, from X-ray analysis, to have a basically square pyramidal structure rather than those of the trigonal bipyramidal class. Five co-ordination is considered to be stabilised by the steric requirements of the ligande Metal-ligand stretching vibrations are assigned and their frequencies compared with values for related tetrahedral complexes. Section II. Metal-ligand stretching frequencies have been assigned for MX₃ dipy and MX₃L₂ ( L = py, pyO and Ph₃As O) complexes of arsenic, antimony and bismuth. For the arsenic and antimony MX₃L₂ compounds υ(M - L) values fall in the same region as for the corresponding germanium and tiny MX₄L₂ compounds. However, υ(Sb -X) values are significantly lower than υ(Sn - X). Terpyridyl complexes are varied. Their compositions and constitutions were elucidated by spectral and conductivity studies. Antimony complexes are formulated as [SbX₂terpy] 2⁺ SbX² ²- and arsenic complexes as [As Cl₂ terpy]⁺ AsCl₄- and [As Br₂ terpy] ⁺ Br - . The bismuth complex [Bi Cl₃ terpy] is non-ionic.