Optimized synthetic approaches to [TcH 3 (PPh 3 ) 4 ] (1) and mer-trans-[TcH(CO) 3 (PPh 3 ) 2 ] (5) as key compounds of the technetium hydrido chemistry are reported. They give access to pure and stable samples of the complexes. The solid-state structures of the two title compounds have been determined. Reactions of [TcH 3 (PPh 3 ) 4 ] with monodentate basic phosphines such as PMe 2 Ph or PMe 3 lead to mixtures o f m i x e d -p h o s p h i n e c o m p l e x e s o f t h e c o m p o s i t i o n s [TcH 3 (PR 3 ) n (PPh 3 ) 4−n ] (n = 1−3), from which the mixed-phosphine trihydride complex [TcH 3 (PPh 3 ) 2 (PMe 3 ) 2 ] (2) could be isolated. Spectroscopic data and DFT calculations suggest a fluxional structure between a capped trigonal prism and a pentagonal bipyramid. Reactions of the monohydride mer-trans-[TcH(CO) 3 (PPh 3 ) 2 ] (5) with HX (X = Cl, Br, I) give [TcX(CO) 3 (PPh 3 ) 2 ] (6) complexes in good yields, while mer-trans-[Tc{η 1 -O(CR)O}(CO) 3 (PPh 3 ) 2 ] (7) or cis−trans-[Tc{η 2 -OO(CR)}(CO) 2 (PPh 3 ) 2 ] (8) complexes are formed with carboxylic acids depending on the substituent R and the conditions applied. Chelate formation of the formato ligand can also be obtained by thermal decarbonylation of the isolated mertrans-[Tc(η 1 -O(CH)O)(CO) 3 (PPh 3 ) 2 ] (7a) complex. The latter reaction is reversible, and the tricarbonyl compound is reformed when [Tc{η 2 -OO(CH)}(CO) 2 (PPh 3 ) 2 ] (8a) is exposed to CO gas. Reactions of [TcH(CO) 3 (PPh 3 ) 2 ] (5) with phenyl seleninic acid (PhSeOOH) in methanol give the tetranuclear cluster [{Tc(CO) 3 } 3 (μ 3 -OH)(μ 2 -O(SePh)O) 3 {Tc(CO) 3 }] (10) and a small amount of the bridged dinuclear oxalato complex [Tc 2 (CO) 6 (ox)(OPPh 3 ) 2 ] (11). The latter compound is the result of a complex reaction, which involves a metal-induced oxidation of methanol to formate and a subsequent C−C coupling of two formato ligands. The plausibility of the proposed mechanism is supported by the fact that the dimeric oxalato complex is much more efficiently formed when the formato complex [Tc{η 1 -O(CH)O}(CO) 3 (PPh 3 ) 2 ] (7a) is directly reacted with PhSeOOH. Exclusively the monomeric oxalato complex [Tc(η 2 -oxH)(CO) 2 (PPh 3 ) 2 ] ( 12) is formed during a reaction of mer-trans-[TcH(CO) 3 (PPh 3 ) 2 ] (5) with oxalic acid. The remaining proton of the Hox − ligand of [Tc(η 2 -oxH)(CO) 2 (PPh 3 ) 2 ] (12) can be removed by NEt 3 , and the ion pair (HNEt 3 )[Tc(η 2 -ox)(CO) 2 (PPh 3 ) 2 ] (13) is formed.