[Mo2Cp2(μ-SMe)3(CH3CN) 2](BF4) (1) reacts with H[BF4] in acetonitrile at room temperature to give the tetrakis (acetonitrile) compound [Mo2Cp2(μ-SMe)2(CH3CN) 4](BF4)2 (3) through replacement of one SMe bridge by two terminal CH3CN ligands. In sharp contrast, the carbonyl analogue [Mo2Cp2(μ-SMe)3(CO)2]+ (2) fails to undergo a similar reaction. This suggests that the electronic properties of the ancillary ligands L in the [Mo2Cp2(μ-SMe)3L2]+ moiety govern the activation of a sulfur atom in protonation reactions. An X-ray diffraction study shows that 3 contains two CpMo(CH3CN)2 units bridged by two SMe groups and linked by a long Mo-Mo bond of 3.000(1) Å.