The activation volumes were obtained for the aquation reactions of [M(RNH(2))(5)L](3+) (M = Cr, R = Me, L = Me(2)SO, HCONMe(2) or MeCONMe(2) M = Rh, R = H or Me, L = Me(2)SO or HCONMe(2)); they are in accord with the existence of a dissociative shift in the substitution mechanism; not only with the size of the substituent R, but also with the size of the leaving ligands L.