We have investigated the chemical dynamics of Mg+-acetaldehyde bimolecular complexes following Mg+-centered (3p <-- 3s) excitation. Non-reactive dissociation to Mg+ is the dominant decay channel. However, excitation in the 3p sigma (A ') <-- 3s sigma (A ') band also results in reactive dissociation to MgH+, MgCHO+, and MgCH3+ products, showing a clear sigma -like electronic orbital alignment preference for chemical quenching. Isotope substitution shows reaction involves a direct attack on the aldehydic C-H or C-C bonds; the relative product branching indicates that the probabilities for C-H and C-C bond cleavage are comparable.