The chelation dynamics of [Mn{eta(5)-C5H4C(O)R}(CO)(3)] complexes 1 (R=CH2(SCH3)), 2 (R=CH(SCH3)(2)), and 3 (R=C(SCH3)(3)) in room-temperature acetonitrile solution have been investigated on the picosecond time scale by UV-pump IR-probe transient absorption spectroscopy. Similar to the previously observed behavior in n-heptane solution, irradiation of 3 in acetonitrile at 289 nm induces CO loss to exclusively yield a Mn-S chelated dicarbonyl product. Unlike the behavior of 1 and 2 in n-hexane and n-heptane solutions, UV excitation of either 1 or 2 in acetonitrile solution induces CO loss to also exclusively yield the chelated products, with no evidence of a competing solvation pathway. All three complexes exhibit ultrafast chelation in < 13 ps. Faster vibrational cooling in acetonitrile vs alkane solutions suggests stronger solute-solvent interaction, perhaps via hydrogen bonding. Ring-opening resulting from continuous irradiation of the pendant sulfide's chelates, [Mn{eta(5)-C5H4C(O)CH(SCH3)(2)-kappa S}(CO)(2)] (4) and [Mn{eta(5)-C5H4C(O)C(SCH3)(3)-kappa S}(CO)(2)] (5), is also discussed.