Tetrakis(trifluoromethyl) cyclotetraarsane (F 3 CAs) 4 (2) was used to repeat the UV initiated [4ϩ2]-cycloaddition reaction of the diarsene F 3 CAsϭAsCF 3 (1) with cyclohexa-1,3-diene (CHD) and to isolate single crystals of the cycloadduct 4 for a X-ray diffraction analysis. 4 crystallizes in the space group P1 and contains the diarsene group in its E-configuration. 2 was also applied for [2ϩ2]-cycloaddition reactions of 1 with tBuCϵP and MeCϵC-N i Pr 2 , but in contrast to positive results with (F 3 CP) 4 the products were too labile for isolation. However, 2 was successfully used at room temperature as precursor for coordinating 1 as π-donor ligand to the Pd(PPh 3 ) 2 complex fragment yielding η 2 -bis(trifluoromethyl)diarsene-bis(triphenylphosphane)-palladium(0) 5, which was characterized by X-ray diffraction of single crystals and by spectroscopic investigations (NMR, IR, MS). Attempts to prove 599 the existence of the diarsene 1, generated by different methods, by spectroscopic studies very probably failed due to its extreme reactivity, not allowing the necessary concentrations for detection. Quantum chemical calculations of the stability of 1 with respect to dimerization, the stability of the [2ϩ2]-cycloadduct with 1-di(isopropyl)aminopropyne and the energy difference between 4 and the 2,3-dimethyl-1,3-butadiene cycloadduct of 1 were performed to understand the considerable differences between 1 and the related diphosphene F 3 CPϭPCF 3 .

Neue Phospha-und Arsa-Heterocyclen aus Cyclophosphanen bzw. -arsanen (ECF3)" (E = P, As; n = 4, 5) und 1,3-Dienen Novel Phospha-and Arsa-Heterocycles from Cyclophosphanes or -arsanes (ECF3)" (E = P, As; n = 4, 5) and 1,3-Dienes Andreas Karst, Burkhardt Broschk, Joseph Grobe*, Duc Le Van