We describe theoretical insights into the mechanism of Hg-C bond protonolysis in methyl mercury coordinated by the tris(2-mercapto-1-tert-butylimidazolyl)hydroborato ligand, the structural and functional analogue of the organomercurial lyase MerB. Different cleavage pathways including both frontside and backside attack transition states were systematically studied by the hybrid density functional method B3LYP. Dependence of Hg-C bond activation on the primary sulfur coordination number of mercury was elaborated, and conceptual DFT indexes were suggested to be more appropriate than gross charge of atom sites in interpreting the dependence. Furthermore, absence of configurational inversion in MerB-catalyzed reactions was accounted for by examinations of the backside protonolysis pathways in the present system. Lastly, a rationalization was provided about the choice between different characteristics of transition states including both four-center and six-center ones.