This thesis reports the synthesis and characterisation of eight new platinum thiourea complexes. Thiourea dianion complexes of the type (Ph₃P)₂PtSC(=NR)NPh, R=Me, Et, Prⁿ, Prⁱ, Buⁿ and (Ph₃P)₂PtSC(=NPh)NR R=Buͭ , p-tol, and the thiourea monoanion complex of the type [(Ph₃P)₂PtSC(=NEtH)NPh]⁺ were synthesised. This research confirms the prediction made by Henderson et al that asymmetrically substituted thiourea dianion complexes could form isomers. NMR studies show that the kinetically favoured isomer is formed initially, and then undergoes a solution phase isomerisation process to form the thermodynamically favoured isomer. Evidence that this isomerisation is affected by difference in substituent size and the energy difference between the two structures was obtained both experimentally, by use of NMR spectroscopy techniques, and theoretically using DFT calculations. Theoretically optimised geometries of each isomer allowed the calculation of ΔG(GPtrem-GPtadj) for each pair. NMR techniques were used to monitor the isomerisation process, which was clearly visible in both ¹H and ³¹P spectra. The relative substituent size, difference in energy between the two structures and rate of isomerisation appear to be proportional to each other. The greater the substituent size difference, the greater the difference in energy and faster the rate of isomerisation from the kinetically favoured isomer to the thermodynamically favoured isomer.