Student Number : 0009404H - PhD thesis - School of Chemistry - Faculty of Science / Six-coordinate ruthenium complexes of the type ttt-RuX2(RNC)2(PPh3)2 (X = Cl ; R = tBu, 2,6-xylyl, benzyl, 2-OMe-4-Clphenyl and iPr; X = Br; R = = tBu, 2,6-xylyl, benzyl, iPr ) were synthesized and fully characterised by IR, NMR spectroscopy and elemental analysis. These complexes were all found to undergo solid-state isomerisation from the ttt-isomer to the cct-isomer. It was found that solid state isomerisation occurred before melting i.e. in the solid state for all the ttt-RuX2(RNC)2(PPh3)2 complexes investigated . The thermal solid state isomerisation process was monitored by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), thermomicroscopy and X-ray powder diffraction techniques. The results revealed that the isomerisation process occurred exothermally with darkening of the crystal. A mechanism which involved rotation of small ligands was proposed to explain the solid-state isomerisation process. Kinetic studies of the solid-state isomerisation of some of the ttt-RuCl2(RNC)2(PPh3)2 complexes were carried out and monitored by NMR spectroscopy and compared with kinetic studies performed by XRD methods. The results are similar and also suggest that the isomerisation reaction follows first order kinetics. The crystal and molecular structure of ttt-RuCl2(2,6-xylylNC)2(PPh3)2 was determined by X-ray single crystal diffraction techniques and its structure was compared with related ttt- RuX2(RNC)2(PPh3)2 complexes. Inter-molecular and intra-molecular interactions in these molecules in the solid state were analysed and used to rationalise the solid-state isomerisation results. The results obtained for the ruthenium complexes suggested that other six-coordinate complexes could undergo solid state isomerisation reactions and this study was extended to molybdenum complexes of the type cis-Mo(CO)4L2. These complexes undergo cis to trans isomerisation in solution. The solid state reactivity of these complexes was investigated for the first time. The complexes with L = PPh2Me and PPh2Et isomerised in the melt whist complexes with L = PPh3 amd P(m-tolyl)3 were indeed found to undergo solid state isomerisation reactions. Structural studies and thermomicroscopic investigations were done on cis- Mo(CO)4(PPh3)2 and Mo(CO)4{P(m-tolyl)3)2 and the results were used to rationalize the solid state reactivity of these complexes.