Part I of this dissertation deals with the synthesis and reactions of transition metal complexes of 2,3-dihydrothiophene (2,3-DHT). Reaction of 2,3-DHT with HPt(acetone) (PEt[subscript]3)[subscript]2[superscript]+ results in the formation of the sulfur-coordinated 2,3-DHT compound HPt(2,3-DHT)(PEt[subscript]3)[subscript]2[superscript]+. Subsequent rearrangement by slow insertion of the 2,3-DHT olefin into the Pt-H bond gives Pt(PEt[subscript]3)[subscript]2(DHT·H)[superscript]+, which was characterized by an x-ray diffraction study. A tungsten analog of the latter compound, CpW(CO)[subscript]2(DHT·H), is prepared from the reaction of CpW(CO)[subscript]3[superscript]- with 2-chlorotetrahydrothiophene followed by photolytic decarbonylation. The reaction of CpW(CO)[subscript]2(DHT·H) with HCl yields an unstable tetrahydrothiophene (THT) complex, CpW(CO)[subscript]2(THT)Cl. In addition, several phosphine-containing tungsten carbonyl complexes of this partly hydrogenated thiophene have been prepared as a model for its adsorption on hydrodesulfurization (HDS) catalyst surfaces. These compounds are of the general form fac-W(CO)[subscript]3(L-L)(2,3-DHT) (L-L = dppe, dmpe) and cis-W(CO)[subscript]4L(2,3-DHT) (L = PPh[subscript]3, PMe[subscript]3, PMe[subscript]2Ph, PMePh[subscript]2);Part II of the dissertation involves the study of dithiocarbene complexes. The metallathiacyclopropane, Cp(CO)[overline]Fe[C(SMe)(SMe)[subscript]2] was obtained by photolysis of Cp(CO)[subscript]2Fe (C(SMe)[subscript]3) and reaction of Cp(CO)(MeCN)Fe (= C(SMe)[subscript]2) PF[subscript]6 with NaSMe. Temperature dependent NMR studies of Cp(CO)[overline]Fe[C(SMe)(SMe)[subscript]2] suggest that it is in rapid equilibrium with a carbene-mercaptide form Cp(CO)Fe(SMe) (= C(SMe)[subscript]2). The existence of the latter compound is supported by the reaction of the metallathiocyclopropane with two equivalents of the phosphines, PMe[subscript]3 and PMePh[subscript]2, to give CpFe(CO)(SMe)(PR[subscript]3) and the ylide (MeS)[subscript]2C = PR[subscript]3. Reactions of Cp(CO)[subscript]2Fe(=[overline]COCH[subscript]2CH[subscript]2O)[superscript]+, Cp(CO)[subscript]2Fe (= C(SMe)[subscript]2) [superscript]+ and related carbene complexes with reducing agents is described in the last section of the dissertation. The Cp(CO)[subscript]2Fe([overline]COCH[subscript]2CH[subscript]2O)[superscript]+ is readily reduced by sodium naphthalinide (NaNp) or Na/Hg to yield (Cp(CO)[subscript]2Fe) [subscript]2, C[subscript]2H[subscript]4 and CO[subscript]2. The same reduction of Cp(CO)[subscript]2Fe (= C(SMe)[subscript]2) [superscript]+ yields (Cp(CO)[subscript]2Fe) [subscript]2, (Cp(CO)Fe(SMe)) [subscript]2 Cp(CO)[subscript]2Fe (C(SMe)[subscript]3) and the metallathiacyclopropane, Cp(CO)[overline]Fe[C(SMe)(SMe)[subscript]2). Small amounts (0.1 equivalent) of NaNp catalyze reactions of Cp(CO)[subscript]2Fe([overline]COCH[subscript]2CH[subscript]2O)[superscript]+ and Cp(CO)[subscript]2Fe (= C(SMe)[subscript]2][superscript]+ with phosphines to give a mixture of CO- and carbene-substituted products Cp(CO)Fe(L)(=[overline]COCH[subscript]2CH[subscript]2O)[superscript]+, Cp(CO)[subscript]2Fe(L)[superscript]+ Cp(CO)Fe(L)[subscript]2[superscript]+ and Cp(CO)Fe(L) (= C(SMe)[subscript]2][superscript]+ (L = PMePh[subscript]2). A mechanism involving radical intermediates is proposed for the reactions.