The research described in this thesis concerns the synthesis and characterisation of manganese(II), iron(II), cobalt(II) and zinc(II) complexes bearing linear pentadentate ligands with N-, O- and S-donors. The complexes can be described by the general formula [LM(II)X2], where X is an anionic ligand such as chloride, bromide, triflate or hexafluoroantimonate, L is a linear pentadentate ligand and M is the metal(II) ion. Two aspects have been the main focus in this study: the coordination geometry of these complexes and their catalytic performance in the oxidation of hydrocarbons. The X-ray structure analysis of some of the complexes revealed an intriguing change in the coordination of the ligand around the metal centre from octahedral to pentagonal bipyramidal or trigonal bipyramidal depending on the ligand field strength. Furthermore, it was found that one of the iron(II) complexes in solution changes its coordination geometry from pentagonal bipyramidal to trigonal bipyramidal. This represents a novel temperature-dependent complex coordination equilibrium. The complexes were tested for catalytic hydrocarbon oxidation using hydrogen peroxide as primary oxidant. The manganese(II) and cobalt(II) complexes were found to be inactive, whereas the iron(II) complexes showed low to moderate activity and product selectivity. Furthermore, the reactivity of the iron(II) complexes towards other primary oxidants was investigated and it was found that when reacted with tert-butylhydroperoxide at low temperatures, a series of iron(II) complexes with pentadentate ligands X2Py3 (X = NH, NMe, S and O) form iron(III) tert-butylperoxo complexes. These reactive intermediates were characterised by several spectroscopic methods and it was concluded that in correlation with the ligand field strength, the complexes with ligands (NH)2Py3 and S2Py3 form low-spin species, whereas those with ligands (NMe)2Py3 and O2Py3 form high-spin iron(III) complexes.