KINETIC AND STRUCTURAL STUDIES ON THE OXOTETRACYANOTECHNETATE(V) CORE - PROTONATION AND LIGATION OF DIOXOTETRACYANOTECHNETATE(V) IONS AND CRYSTAL-STRUCTURE OF 2,2'-BIPYRIDINIUM TRANS-OXOTHIOCYANATOTETRACYANOTECHNETATE(V)

The tetracyanodioxotechnetate(V) ion undergoes protonation reactions in the acidic aqueous solutions, forming the [TcO(OH)(CN)4]2- (1) and [TcO(OH2)(CN)4]- (2) complexes. A dimeric species, [Tc2O3(CN)8]4- (3), is rapidly formed from the hydroxo oxo complex, and it is therefore impossible to isolate complex 1. The coordinated water of [TcO(H2O)(CN)4]- can be displaced by monodentate ligands such as thiocyanate. The reaction with thiocyanate ions gives [TcO(NCS)(CN)4]2- (4), the composition and structure of which was confirmed by means of an X-ray crystal structure determination. 2,2'-Bipyridinium oxothio-cyanatotetracyanotechnetate(V) (C25H18N9OSTc) crystallizes in the monoclinic space group, C2/c, with a = 18.210 (4) angstrom, b = 19.473 (1) angstrom, c = 15.501 (3) angstrom, beta = 107.501 (14)-degrees, Z = 8, rho(calc) = 1.490 g cm-3. A final R value of 6.4% resulted from refinement of 1868 observed reflections. The first acid dissociation constant of 2 (at 15.0-degrees-C), the forward and reverse rate constants, and the equilibrium quotient for the reaction (at 25.0-degrees-C) trans-[TcO(H2O)(CN)4]- + NCS- reversible k-1 k1,k1 trans-]TcO(NCS)(CN)4)2- + H2O have been determined in 1.0 M (KNO3) ionic strength aqueous medium as 10(-2.90 (5)), 22.2 (3) M-1 s-1, 0.43 (4) s-1, and 4 (2) M-1, respectively.