Carbonyl complexes are most stable when there are 18 valence electrons around the central transition metal. The CO ligand will intetact through its sigma and pi orbitals with all valence orbitals -- one s, three p and five d orbitals -- hence the 18 electron rule.
#Fe(CO)_5# has eight valence electrons on the iron and the CO molecules each add two more through the sigma orbitals, so we get up to 18. #Fe_2(CO)_9# gets 18 electrons around both iron atoms by sharing a pair of electrons in a metal-metal bond, and similarly for #Mn_2(CO)_{10}#. But with #Mn(CO)_5# can't get past 17 valence electrons (7 from the Mn, 10 from the CO molecules and no metal-metal bond) unless it takes one from something else. Hence -- an oxidizing agent.
