In this thesis, detailed investigations were carried out on aromatic 5‐membered heterocycles containing a low‐coordinated phosphorus atom, such as triazaphosphospoles and the related triazaphospholenium salts. Initially, the focus was placed on 1,2,3,4‐triazaphospholes. Novel triazaphospholes with electron‐withdrawing arylsulfonyl substituents on the N(3) atom were synthesized. For the first time it could be shown that these electron‐withdrawing tosyl and mesitylsulfonyl groups have a major influence on the reactivity of the heterocycle. Upon coordination to a gold(I) center, an unprecedented N2 release and the subsequent formation of a N2P2 heterocycle were observed. This shows a previously unknown reactivity of these new triazaphospholes and presents a new route to the P2N2 heterocycles with the possibility of gaining access to previously inaccessible substitution patterns. Furthermore, it was shown that triazaphospholes react as dienes in [4+2]‐cycloaddition reactions with the electron‐poor hexafluoro‐2‐butyne giving the corresponding CF3‐substituted 2H‐1,2,3‐diazaphospholes in a concerted cycloaddition‐cyclorevision reaction under pivaloyl nitrile elimination. Both the coordination chemistry of the triazaphosphole and that of the diazaphosphole were investigated. The crystallographic characterization of the formed coordination compounds clearly confirms that the heterocycles coordinate to a W(CO)5 fragment via the phosphorus atom. The observed tungsten(0)‐pentacarbonyl complex of the triazaphosphole is the first example of a tungsten complex of a triazaphosphole and one of the few examples of a coordination compound of a triazaphosphole with a coordination via the phosphorus atom. A closer look was also taken at triazaphospholenium salts as this compound class has only recently been described. Triazaphospholenium salts could be synthesized by alkylation of triazaphospholes with Meerwein reagents. For the first time, triazaphospholenium salts with a TMS group in the 5‐position have been synthesized, which provided access to novel protodesilylated products. In addition, these [BF4]− salts of the TMS‐substituted triazaphospholenium cations form novel, previously unknown BF3 adducts by elimination of TMS‐F. These BF3 adducts were isolated and fully characterized, and a targeted synthesis of the related BEt3 adducts could also be shown. The BR3 adducts represent a new and interesting class of compounds, as they can be regarded as phosphorus analogues of tetrazol‐5‐ylidenes with an abnormal substitution pattern. The protodesilylated products are also of interest as the protodesilylating reaction is rarely observed in classical triazaphospholes and this is the first example of a protodesilylated triazaphospholenium salt. Finally, the reactivity of pyridylmethyl‐functionalised triazaphospholes was investigated in quaternization reactions with Meerwein reagents. Due to the different nucleophilicity of the nitrogen atoms, both a chemoselective and a stepwise alkylation of the triazaphosphole derivative can be observed. The choice and stoichiometry of the alkylation reagent plays a crucial role, and allowed access to a large number of mono‐ and di‐cationic species. The coordination chemistry of these charged triazaphospholes and the triazaphospholenium salts with Cu(I) halides shows a versatile coordination chemistry, which manifests itself in different coordination modes depending on the charge of the ligand and the type of Cu(I) halide. Only coordination via the phosphorus atom is observed. These compounds are some of the few examples in which coordination of the triazaphosphol and triazaphospholenium heterocycles occurs via the phosphorus atom.
