13C and 15N NMR mechanistic study of cyanide exchange on oxotetracyanometalate complexes of Re(V), Tc(V), W(IV), Mo(IV), and Os(VI)

A range of complexes with general formula [MO(X)(CN)(4)](n-) of W(IV), Mo(IV), Re(V), Tc(V), and Os(VI) were prepared and characterized by C-13, N-15, O-17, and Tc-99 NMR, utilizing C-13- and N-15-enriched cyano complexes. A correlation between M-O and M-CN bond strength from X-ray crystallographic data and \(1)J(W-183 - C-13)\ coupling is reported. The cyanide (HCN/CN-) exchange kinetics on the trans-dioxotetracyanometalate complexes and protonated/substituted ([MO(X)(CN)(4)](n-)) forms thereof were studied in aqueous medium. The dioxotetracyano complexes show a trend of reactivity M(IV) > M(V) > M(VI), which is in agreement with the increase in M-L bond strength (L = O2- or CN-) and a dissociative activation for the cyanide and the oxygen exchange in these complexes. Rate constants (X, k(Xc)) in s(-1) at 298 K for the [MO(X)(CN)(4)](n-) complexes are as follows for Mo(IV): O2-, >0.4; OH-, (1.7 +/- 0.1) x 10(-2); H2O, (1.5 +/- 0.1) x 10(-2); CN- , (9.6 +/- 0.8) x 10(-3). For W(IV): O2-, (4.4 +/- 0.4) x 10(-3) OH-, (9.6 +/- 0.9) x 10(-5); H2O, (1.1 +/- 0.1) x 10(-4); CN-, (1.1 +/- 0.1) x 10(-2); N-3(-), (3.1 +/- 0.2) x 10(-4); F-, (4.8 +/- 0.1) x 10(-5). For Tc(V): O2-, (4.8 +/- 0.4) x 10(-3); H2O, <4 x 10(-5); NCS-, <4 x 10(-5). For Re(V): O2-, (3.6 +/- 0.3) x 10(-6) and (1.2 +/- 0.1) x 10(-4) for CN- and HCN, respectively; OH2, <4 x 10(-8). For Os(VI): O2-, <4 x 10(-9) and (1.2 +/- 0.1) x 10(-4) for CN- and HCN, respectively. The cyanide exchange kinetics were: correlated with previously determined proton and oxygen exchange, spanning a kinetic domain of more than 12 orders of magnitude for the five metal centers studied.