This thesis focuses on the experimental and theoretical investigation of small molecules containing phosphorus, nitrogen and sulfur. The first part of this work presents synthesis and characterisation of molecules bearing a P-N-N-group: N,N´,N´-[tris(trimethylsilyl)]-hydrazino-diphenylphosphane, (TMS)2N-(TMS)N-PPh2, and N,N´,N´-[tris(trimethylsilyl)]-hydrazino-phenyl(chloro)phosphane, (TMS)2N-(TMS)N-P(Cl)Ph, were obtained in the reaction of bis-[lithiumtris(trimethylsilyl)hydrazide] with PhnPCl3 - n (n = 1, 2). The structure and bonding of both species are discussed on the basis of experimentally observed (X-ray, Raman, NMR, and MS) and theoretically obtained data (B3LYP/6-31G(d,p), NBO analysis). Oxidation with sulfur and selenium results in the formation of (TMS)2N-(TMS)N-P(S)-Ph2, (TMS)2N-(TMS)N-P(Se)Ph2, (TMS)2N-(TMS)N-P(S)Ph(Cl) and (TMS)2N-(TMS)N-P(Se)Ph(Cl). Moreover, the thermal decomposition of N,N´,N´-[tris(trimethylsilyl)]hydrazine-dichlorophosphane, (TMS)2N-(TMS)N-PCl2 and the reaction with magnesium have been investigated. The formation and molecular structure of the novel MgCl2(THF)2* 2Mg[(TMS)NP(O)2N(TMS)2](THF) salt containing the hitherto unknown (TMS)NP-(O)2N(TMS)22- anion are discussed. DFT calculations (B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p)) are used to evaluate the bonding, ground-state structures, and energy landscape for the different isomers of (TMS)2N-(TMS)N-PCl2: the thermodynamics and kinetics of the successive elimination of chlorotrimethylsilane (TMS-Cl) resulting in the formation of covalent azide analogues such as TMS-PNN or TMS-NNP are discussed. While investigating the reaction of (TMS)2N-(TMS)N-PCl2 with Lewis acides a galliumtrichloride-adduct of 4-bis[trimethylsilylamino]-1,2,4,3,5-triazadiphosphole was discovered. This hitherto unknown triazadiphosphole represents one of the few examples of a five-membered ring-molecule containing only phosphorus and nitrogen. This ring system was unequivocally characterized by X-Ray, Raman, NMR and MS analysis. Furthermore the Staudinger-type reaction of N,N´,N´-[tris(trimethylsilyl)]-hydrazino-diphenylphosphane with SP(N3)3 and and C3N12 in different stoichiometries were investigated. It is possible to substitute only one azide group in SP(N3)3 and up to two azide groups in C3N12 with (TMS)2N-(TMS)N-PPh2, resulting in the expected products. Upon heating of a solution of (TMS)2N-(TMS)N-PPh2 and SP(N3)3 a new eight-membered ring containing phosphorus and nitrogen in alternating order was obtained. The molecule contains a long pnicogene-chain (eleven atoms in total, three phosphorus- and eight nitrogen-atoms). The reactions of (NSCl)3 and NSCl2- with AgX salts (X = CN, SCN, OCN), as well as the reaction of (NSCl)3 with [PPh4]X (X = CN, SCN, OCN) and HgX (X = CN, SCN) have been investigated in a combined experimental and theoretical study. Additionally, the reaction of NSCl2- salts with hydride-transferring-agents like NaBH4 or NaBH3CN was studied. The thermodynamics as well as structure and bonding of the formation of NSX, NSX2- and NSXY- (X, Y = Cl, H, CN, SCN, OCN) have been studied. Unfortunately the preparation of these species did not suceed. The substitution of sulfur by selenium with SeO2, as reported by Rawson, was investigated on the salts of NSCl2- and S2N3+. These reactions resulted polymeric products and selenium; the formation of a nitrogen-selenium-species could not be confirmed.