Variable temperature and pressure study of the aquation reactions of cobalt(III) and chromium(III) penta- and tetra-amines

Preparation of a series of specific penta- and tetra-amine derivatives of C o-III and Cr-III with a neutral leaving ligand has been carried out in orde r to accomplish a fine tuning of the associativeness/dissociativeness of th eir substitution reactions. Spontaneous aquation reactions of the neutral l igands have been studied at variable temperature and pressure. Although rat e constants and thermal activation parameters show an important degree of s catter, the values determined for the activation volumes of the substitutio n process illustrate the mechanistic fine tuning that may be achieved for t hese reactions. In all cases, in the absence of important steric constraint s in the molecule, electronic inductive effects seem to be the most importa nt factor accounting for the dissociative shifts observed both for pentaami ne (i.e.Delta V double dagger=+4.0 or +14.0 cm(3) mol(-1) and +5.2 or +16.5 cm(3) mol(-1) for the aquation of cis- or trans-[Co(MeNH2)(NH3)(4)(DMF)](3 +) and cis- or trans-[CoL15(DMF)](3+) respectively, where L-15 represents a pentaamine macrocyclic ligand), and tetraamine systems (i.e.Delta V double dagger=+4.1 or +8.4 cm(3) mol(-1) and -10.8 or -7.4 cm(3) mol(-1) for the aquation of cis-[Co(NH3)(4)Cl(DMAC)](2+) (DMAC=dimethylacetamide) or cis-[C o(en)(2)Cl(DMAC)](2+) and cis-[Cr(NH3)(4)Cl(DMF)](2+) or cis -[Cr(en)(2)Cl( DMF)](2+)). From the results, clear evidence is obtained which indicates th at, only when the situation is borderline I-a/I-d, or the steric demands ar e increased dramatically, dissociative shifts are observed; in all other ca ses electronic inductive effects seem to be dominant for such a tuning of t he substitution process.