A detailed theoretical study of the carbonyl insertion reaction Mn(CO)(5)CH3 + CO --> Mn(CO)(5)-(COCH3) is presented using gradient corrected density functional theory. As has been well-documented experimentally, this reaction proceeds through a two-step mechanism. In the first step, a stereochemically well-defined intermediate is formed via migration of the methyl group to a cis carbonyl. In the second step, the incoming nucleophile attacks the intermediate to form product. Two stable intermediates have been located on the potential energy surface. Both are formally Mn(CO)(4)(COCH3); however, in one case the intermediate is stabilized by a strong agostic interaction between a methyl group hydrogen and the metal, and in the second case the acyl group distorts to form an Mn-O bond and thus acts as an eta(2) (dihapto) ligand. The transition states between the reactant and the intermediates have been located. In addition, the transition states for CO attack of each intermediate have also been characterized. A detailed kinetic analysis of two possible reaction channels demonstrates that the solvent unassisted mechanism proceeds via CO attack on the agostic intermediate, even though the dihapto intermediate is lower in energy. Our calculated energetics (both activation energy and overall exothermicity) are in excellent agreement with experiment. We have also investigated some aspects of the photodecarbonylation of Mn(CO)(5)(COCH3) to yield Mn(CO)(5)CH3. This reaction has been proposed to proceed via the dihapto intermediate, and we confirm this result on the basis of a comparison of calculated vs observed CO stretching frequencies of the experimentally characterized intermediate. Therefore, the thermal carbonylation of Mn(CO)(5)CH3 and the photodecarbonyIation of Mn(CO)(5)(COCH3) proceed along different reaction channels. Some additional comments on the role of solvent in the photodecarbonylation of Mn(CO)(5)-(COCH3) are included. Specifically, we fmd (again on the basis of a comparison of calculated vs experimentally observed vibrational frequencies) that the dihapto intermediate in the photodecarbonylation experiments is unsolvated, even in coordinating solvents such as THF.