The aqueous equilibrium and protonation kinetics of the protonated forms of the [MO(2)(CN)(4)]((n+2)-) complexes for the Mo(IV) and W(IV) systems (pH 6-14), and the Tc(V) and Re(V) systems (pH 0-8), have been quantitatively studied by C-13 and O-17 NMR at 25.0 degrees C and mu = 1.0-1.4 m (KNO3). The pK(a1) values for the [MO(OH2)(CN)(4)](2-) complexes for W(IV) and Mo(IV) were determined as 7.87(5) and 9.88(5). O-17 line broadening were simulated and proton exchange rate constants for the Mo(IV) and W(IV) systems were determined : (i) [MO(OH)(CN)(4)](3-) + OH-reversible arrow [MO(2)(CN)(4)](4-) + H2O(k(-2b), k(2b)); (ii) [MO(OH2)(CN)(4)](2-)+ [*MO(OH)(CN)(4)](3-)reversible arrow [MO(OH)(CN)(4)](3-) +[*MO(OH2)(CN)(4)](2-) (k(lex)). In strong basic medium (pH > 11) for the Mo(IV) and W(IV) systems, the deprotonation rate constants (k(-2b)) for the [MO(OH)(CN)(4)](4-) complexes were determined as 2(1) X 10(7) and 4(2) x 10(8) M(-1) s(-1), while protonation rate constants (k(2b)) for the [MO(2)(CN)(4)](3-) complexes of 1.3(2) x 10(9) and 7(2) x 10(8) s(-1) were obtained. In weak basic medium where hydrolysis of the aqua-ore complex is less important, the lower limit of the direct proton exchange constant, k(lex), was determined as 1 x 10(7) M(-1) s(-1) for both Mo(IV) and W(IV). Proton transfer in the Re(V) and Tc(V) systems can be described by (iii) [MO(OH2)(CN)(4)](-) reversible arrow [MO(OH)(CN)(4)](2-) + H+ (k(la), k(-la)); (iv) [MO(OH)(CN)(4)](2-) reversible arrow [MO(2)(CN)(4)](3-) + H+ (k(2a), k(-2a)); (V) [MO(OH)(CN)(4)](2-)+ [*MO(2)(CN)(4)](3-)reversible arrow [MO(2)(CN)(4)](3-) + [*MO(OH)(CN)(4)](2-) (k(2ex)). The values [Tc(V) results in square brackets] for the protonation and deprotonation rate constants, and the direct proton exchange rate constants in acidic medium (pH < 5) were determined as follows : k(la) greater than or equal to 1 x 10(8) s(-1) [greater than or equal to 1 x 10(7)], k(2a) = 1.1(2) x 10(7) s(-1) [1.1(5) X 10(7)], k(-2a), = 6(1) X 10(10) M(-1) s(-1) [similar to 1 X 10(11)],k(-1a) greater than or equal to 2 X 10(9) M(-1) s(-1) [greater than or equal to 8 X 10(9)], and k(2ex) less than or equal to 5 X 10(7) M(-1) s(-1) [greater than or equal to 5 x 10(8)].