This thesis presents studies on the metallacyclic chemistry of thiourea dianion and monoanion ligands. The versatility of thioureas and their relatives (thiosemicarbazones and selenoureas) has been attributed to the propensity to functionalise the imine protons with a variety of functional groups. Asymmetrically substituted thioureas have been known to form coordination complexes with a variety of metal precursors including platinum, palladium, nickel, ruthenium rhodium, iridium, technetium and molybdenum. This thesis investigates the effect of varying the functionality or steric properties of the ligands on the physical, chemical, structural, and geometric properties of the resulting complexes. The thesis is organised into seven chapters. Chapter one is the review of the literature of the chemistry of thioureas, their properties, method of synthesis, applications and reactivity towards metals. A number of coordination complexes of thioureas with various metal precursors are also reviewed and the structural, chemical and biological properties of these complexes are discussed. The coordination chemistry of other related ligands including the selenoureas and thiosemicarbazones are also reviewed. Chapter two details the synthesis and reactivity of pyridyl substituted thiourea ligands towards cis-[PtCl₂(PPh₃)₂] (PPh₃ = triphenylphosphine). In this chapter the synthesis of a series of pyridyl-substituted thiourea dianion and monoanion complexes of the type [Pt{SC(=NR¹)NR²}(PPh₃)₂] and [Pt{SC(=NR¹)NHR²}(PPh₃)₂]⁺ respectively (where R¹ = phenyl, p-nitrophenyl, and p-methoxyphenyl; R² = Py(CH₂)ₙ n = 0, 1, 2; Py = 2-pyridyl) was explored. The complexes were characterised by ESI-MS, ¹H NMR, ³¹P NMR, FTIR and single crystal X-ray crystallography. Initial NMR investigation indicated isomerism in the complexes. Theoretical Gibbs free energy calculations were performed on the DFT optimised geometries of the complexes and the difference in electronic Gibbs free energy ΔG (GPtdistal – GPtproximal) between the two possible isomers of the dianion complexes were evaluated. A positive difference in Gibbs free energy values predicted a kinetically favoured proximal isomeric configuration for the thiourea dianion complexes (where the pyridyl functional group is bonded to the platinum adjacent nitrogen) The monoanion complex on the other hand gave a negative calculated Gibbs free energy difference, indicating a kinetically favoured distal isomeric configuration for the complex (where the pyridyl functional group is bonded to the nitrogen remote from the platinum metal). ³¹P{¹H} NMR investigations revealed the presence of an initial proximal isomer for the dianion complexes, which underwent solution phase isomerisation into the distal isomeric forms of the complexes and/or a mixture of the proximal and distal isomers. The opposite trend was recorded for the monoanionic complex, where the formation of an initial distal isomer and subsequent isomerisation into a 1:3 mixture of the proximal and distal isomeric forms of the complex was observed. Introducing an alkyl spacer between the pyridyl and the thiourea functional group did not have any significant effect on isomerism, however the steric nature and size of the R substituents in the complex affected the isomerisation process. X-ray crystallography was used to establish the NP₂S square planar geometry of the complexes. The dianion complexes are N,S-chelating so that each donor atom is opposite a phosphane-P atom; a trans-influence is evident on the Pt‒P bonds. In the monoanion complex, the anion coordinates in a similar fashion to that established in the neutral analogues and systematic variations in geometric parameters are evident. Chapter three investigates the coordination chemistry of organo-phosphorus palladium and nickel complexes of these asymmetrically substituted pyridyl thiourea ligands synthesised in Chapter two. The reaction of pyridyl-substituted thiourea ligands of the form R¹NHC(S)NHR² (where R¹ = Py(CH₂)ₙ; n = 0,1,2, R² = Ph, or p-C₆H₄NO₂) with [PdCl₂(dppe)], [PdCl₂(PPh₃)₂] or [NiCl₂(dppe)] resulted in cationic complexes of the type [M{SC(NR¹)HNR²}(L₂)]X, (M = Pd or Ni, L = PPh₃ or L₂ = dppe and X = BF₄ or BPh₄, dppe = bis-diphenylphosphinoethane) and a neutral complex [Pd{SC=(NPh)NC₆H₄NO₂}(dppe)]. The major outcome of this study is the synthesis of the six-membered ring cationic palladium complex 3a, where the palladium metal is coordinated to the pyridyl nitrogen. ³¹P{¹H} NMR indicated the possibility of E/Z isomerism in the complex. X-ray crystal structures of the complexes showed that introducing an alkyl spacer between the pyridyl and thiourea functionalities resulted in juxtaposition of the pyridyl functional group from the proximal to the distal position. Chapter four explores the synthesis and structure of half sandwich arene ruthenium, Cp* rhodium and iridium complexes of the pyridyl substituted thioureas (Cp* = pentamethylcyclopentadienyl) The complexes were formed from the metal dimers [C₆Me₆/cymene RuCl₂]₂ or [Cp*MCl₂]₂ (M = Rh, Ir) and a set of pyridyl substituted ligands to form complexes. The ruthenium complexes formed mononuclear bisthiourea complexes [(η⁶-C₆Me₆)Ru{SC(=NPy)NHPh}{SC(NHPy)NHPh] 4a and [(η⁶-C₆Me₆)Ru{SC(=NPy)NHC₆H₄OMe}{SC(NHPy)NHC₆H₄OMe] 4b with a three legged piano stool configuration where the C₆Me₆ arene ligand occupies three coordination sites on the ruthenium metal and the remaining three positions are occupied by the two thiourea ligands coordinated in a bidentate and monodentate fashion to a pseudo-octahedral geometry. Introduction of the alkyl spacer between the pyridyl and thiourea functional groups resulted in tridentate cationic mono thiourea complexes of the form [(η⁶-C₆Me₆)Ru{SC(=N(CH₂)₂Py)NHR]PF₆ 4c - 4d, (R = Ph, or p-C₆H₄NO₂). X-ray crystal structures of the complexes showed that the longer arm of the pyridyl functionality allows the ligand to fold around the metal centre resulting in tridentate coordination. Introduction of the bulky PPh₃ ligand into the coordination sphere of the ruthenium complex resulted in mono-cationic bidentate complexes of the form [(η⁶-C₆Me₆)Ru{SC(=N(CH₂)ₙPy)NHR}PPh₃]X (n = 0, 1, 2; R = Ph, p-C₆H₄OCH₃, p-C₆H₄NO₂; X = BF₄ or PF₆) 4e-4q. The pentamethylcyclopentadienyl rhodium and iridium complexes formed similar pseudo-octahedral three legged piano stool complexes with a Cl anion occupying one of the coordination sites along with N,S donor atoms of the cationic thiourea ligand to form neutral complexes of the type [Cp*M{SC(=NPy(CH₂)ₙ)NR}Cl] 4r-4w, (where M = Rh or Ir, n = 0, 1, 2 and R = Ph or, p-C₆H₄OCH₃). The chloride anion in the complexes were later substituted with the bulky PPh₃ to give monocationic complexes [Cp*M{SC(=NPy(CH₂)n)NR}PPh₃] 4x-4ze as BF₄ salts. In Chapter five, a series of asymmetrically substituted thioureas containing phosphonate, hydroxyalkyl and silatrane functional groups were synthesised and characterised. Supramolecular interactions in the crystal structure of the phosphonate substituted thioureas were analysed using the Non-covalent Interaction (NCI) program Bonder, a locally developed program, which provides numerically equivalent results to existing NCI codes such as NCIplot, NCImilano or Multifwn. The coordination chemistry of these thiourea ligands were explored by reacting them with platinum and palladium precursor complexes cis-[PtCl₂(PPh₃)₂] and [PdCl₂(dppe)]. The resulting complexes were characterised by ¹H, ³¹P NMR, ESI-MS and X-ray crystallography. The results show that the complexes formed mostly cationic complexes of the type [Pt{SC(NPh)NHR}(PPh₃)₂]BF₄ 5a-5e and [Pd{SC(NPh)NHR}dppe]BF₄ 5h-5k (R = phosphonate, hydroxyethyl, dihydroxyethyl and silatrane functional groups). The X-ray crystal structure of the complexes confirmed the square planar NP₂S geometry of the compounds. Chapter six introduces a new set of alkyl bridged bisthiourea ligands and their organo-phosphorus platinum group metal complexes of the type [{(PtSC(NPh)NHC₆H₅)₂CH₂}(PPh₃)₄] .2BPh₄ 6a, [M₂{(SC(NPh)NHC₆H₅)₂CH₂}(dppe)₂].2BPh₄ 6b-6c (where M = Pd, Ni), [Pt₂{SC(NPh)NH}₂(CH₂)n(PPh₃)₄].2BPh₄ 6d-6g (n = 4, 6, 8, 12), and [M₂{SC(NPh)NH}₂(CH₂)₄(dppe)₂].2BPh₄ 6h-6o (where M = Pd, Ni). The ESI-mass spectral analysis of the complexes showed the presence of the monochelated and dichelated species. Increasing the molar concentration of the metal precursor complexes and reaction time from 2 to 8 hours resulted in the disappearance of the monochelated species from the ESI-mass spectra of the platinum complexes, but not for the palladium and nickel complexes. ³¹P{¹H} NMR characterisation of the methylene bridged bisthiourea complexes 6a-6c, showed multiplets indicative of isomerism. These peaks however disappeared as the length of the bridging alkyl chain increased. X-ray crystallographic studies were used to confirm the coordination geometry of the complexes.