The unimolecular decompositions of 5-methyl-7-trimethylsilylheptanenitrile (1) and 6-methyl-8-trimethylsilyloctanenitrile (2) complexed to "bare" Co+ cations have been investigated by means of tandem mass spectrometry. The Co+-mediated bond activations display a high degree of selectivity in that the major (>90%) neutral product formed is trimethylsilane. Regio- and diastereospecific labeling experiments were conducted to gain further insight into the mechanistic details, in particular, the unique regio- and stereoselectivity of the losses of trimethylsilane from 1/Co+ and 2/Co+, respectively. The reaction follows a clean 1,2-elimination process, in which the building blocks of the neutral HSiMe(3) stem exclusively from activation of a C-H bond in the (omega - 1) position and the Si-C bond. The observed regiospecificity can only be accounted for by invoking a metal-ion-mediated cooperation of the SiMe(3) and the CN groups in the course of the multistep reaction sequence. The examination of diastereospecifically labeled isotopologues of 1 and 2 reveals a remarkably high diastereoselectivity for the desilylation, and the steric effects (SE) clearly exceed the kinetic isotope effects (KIE) associated with C-H and C-D bond activation, respectively; i.e., KIE = 1.6 +/- 0.1 and SE = 4.1 +/- 0.2 for 1/Co+, and KIE = 1.5 +/- 0.1 and SE = 3.0 +/- 0.2 for 2/Co+. These unprecedented examples for stereoselective C-Si bond activations by Co+ in the gas phase can be rationalized in terms of steric interactions in the different conformations of the metallacyclic intermediates.