This thesis describes novel reactions of cyclomanganated compounds of the general form shown below. The ligand is bonded by a metal-carbon σ-bond, with X acting as a donor atom to form the chelate ring. The organic ligands were derived from aryl and heteroaryl ketones, chalcones, dienones and azabutadienes. A preliminary investigation has previously been undertaken into the reaction of carbon disulfide with some orthomanganated aryl ketones. In the present study these reactions were further investigated, with the isolation of new η2-S, S-dithiocarboxylate complexes, e.g. 46. In other cases the reaction of carbon disulfide with cyclomanganated compounds afforded the novel Mn4 species 50 in low yield. The reactions of orthomanganated acetyl-aryl compounds with organic isocyanides (C≡NR) afforded a number of novel compounds. For example η2-(2-acetylphenyl)-tetracarbonylmanganese (79) afforded four different types of products depending on the isocyanide; tert-butyl isocyanide produced 82, ethyl isocyanide produced 95 and 96, and 4-methoxyphenyl isocyanide produced 98 and 99. With orthomanganated 2-acetylthiophene, CNBut gave the quinone species 88 and 89,which showed high anti fungal and antitumor activity. Cyclomanganated dienones (e.g. 127) were previously known to react with alkynes to give either (η5-pyranyl)Mn(CO)3 (e.g. 130) or (η5-oxocycloheptadienyl)Mn(C0)3 (e.g.129) products. The factors influencing which product is formed were more systematically investigated and a deuterium-labelling experiment was used to probe the1,2-phenyl shift that occurs for the (η5-oxocycloheptadienyl)Mn(C0)3 products . A preliminary investigation of the reactions of 4-phenyl-1-(4-tolyl)-l-azabuta-1, 3-diene(TAD) with PhCH2Mn(CO)5 and unsaturated substrates including phenyl acetylene, methyl acrylate, PhNCO, ButNC and carbon disulfide is also described. A number of novel compounds were produced, including 169 from the reaction involving PhNCO. In contrast η2-(5-methoxy-2-acetylphenyl)tetracarbonylmanganese with ally1 isothiocyanate gave simply 2-allyl-4-methoxyacetophenone by allylation of the orthomanganated starting compound. Allyl bromide behaved similarly. However, allyl bromide and ethyl bromide when reacted with the cyclomanganated chalcone 54 gave furanone products (e.g. 190 and 191). An oxidative addition and reductive elimination mechanism was proposed to explain this reaction. Among others, the crystal structures of 50, 89 and 130 were determined.