The halogeno-(4-halogenophenyl)arsanyl cations Y–C₆H₄As⁺X (X = F, Cl, Br, I) are abundant in the 70 eV EI mass spectra of dihalogeno-(4-halogenophenyl)arsanes Y–C₆H₄AsX₂. This has been used to study the unimolecular reactions of ions Y–C₆H₄As⁺X by techniques of tandem mass spectrometry with respect to a possible rearrangement to nido-isomers, η⁴-(XYC₆H₄)As⁺, in which the As⁺ is capping a dihalogenobenzene moiety, YC₆H₄AX. The mass analyzed ion kinetic energy (MIKE) spectra of Y–C₆H₄As⁺X, 1a⁺–3a⁺ and 5a⁺ (X, Y = F, Cl, Br, see Scheme 2), display broad and even dish-topped peaks for the loss of HX and HY, respectively, indicating a large kinetic energy release (KER) during these reactions and the presence of a large reverse activation energy. The mean value of the KER, <T>, is specific for the type of the halogen substituent but independent of its original position and is identical to <T> found for the loss of HX or HY from metastable halogeno–phenylarsanyl cations, C₆H₅As^–X or C₆H₄As⁺Y. These results give strong evidence for a positional change of X and Y in Y–C₆H₄As⁺X before fragmentation, plausibly via intermediate η⁴- (XYC₆H₄)As⁺. However, the relative intensities of the signals for loss of HX and HY are different in the MIKE spectra of isomeric arsanyl cations and the spectra of the isomers obtained by collision-induced decomposition show that the metastable arsanyl cations represent a mixture of isomers with the original arsanyl cation prevailing. Clearly, interchange of halogen substituents in Y–C₆H₄Xas⁺ is slow compared with fragmentation and the intermediate nido-isomer, η⁴-(XYC₆H₄)As⁺, is separated from the isomers of the classical structure of a halogeno- (halogenophenyl)arsanyl cation by substantial activation barriers. Therefore, in the case of 4a⁺ and 6a⁺ (X, Y = Cl, I), any rearrangement is obstructed by the facile loss of the weakly bonded I substituent. This reaction model was verified by theoretical calculations of relevant stationary points of the minimum energy reaction pathways (MERP) of fragmentation and rearrangement at the level UBHLYP/6-311+g(2d,p)//UBHLYP/6-31+g(d). In particular, the calculations show that the nido isomers, η⁴-(XYC₆H₄)As⁺, are stable species but higher in energy than their “classical” isomers.

Keywords: elementorganic compound, heavy main group element, fragmentation mechanism, rearrangement, tandem mass spectrometry, metastable ion, KER, DFT calculations