Reactions of [Mo(CO)(6)] under vacuum in alpha-cages of Na56Y zeolite fully loaded with chemisorbed PMe3 form cis-[Mo(CO)(4)(PMe3)(2)] but mixtures of [Mo(CO)(5)(PMe3)] and cis-[Mo(CO)(4)(PMe3)(2)] are formed under CO. Reactions under vacuum exhibit low enthalpic and very negative entropic factors (Delta H-double dagger = 71.4 +/- 3.5 kJ mol(-1) and Delta S-double dagger = -102 +/- 11 J K-1 mol(-1)) compared with much higher enthalpic and positive entropic factors for CO dissociative reactions with P(n-Bu)(3) in xylene observed elsewhere. Reaction at 66 degrees C under vacuum is ca. 10(3) times faster than "spontaneous" CO dissociative reactions in solution. Intrazeolite substitution is concluded to occur by a "zeolite assisted" mechanism in which two oxide ions in the cavity walls simultaneously displace two neighbouring CO ligands from the [Mo(CO)(6)]. This contrasts with even lower enthalpy, and more negative entropy values, for simultaneous displacement of three neighbouring CO ligands in thermal decarbonylation reactions. The alpha-cages behave as multidentate anionic "zeolate" ligands with varying numbers of O2- ions participating to create highly ordered transition states. These results emphasize the high degree to which such kinetic studies can reveal intimate details of the nature of these activating effects.195862871