SiNCO+ and SiNCS+ and their neutral counterparts

[Si,C,N,O]+ and [Si,C,N,S]+ cations formed upon electron ionization of trimethylsilyl isocyanate and isothiocyanate, respectively, are probed by tandem mass spectrometry. In the case of [Si,C,N,O]+, the experimental data suggest that the isocyanato group remains intact upon ionization such that silicon isocyanate SiNCO+ is formed. Neutralization-reionization mass spectrometry indicates that also neutral SiNCO is a viable molecule in the gas phase. Instead, partial isomerization to SiSCN+ is indicated for the sulfur analog. Because of this structural dichotomy, the nature of the [Si,C,N,S]+ species giving rise to the recovery signal in the neutralization-reionization experiments remains uncertain.