SPECIFIC CATION CATALYSIS IN THE THIOCYANATE PHOTOANATION OF ACIDOPENTACYANOCOBALTATE(III) COMPLEXES

The thermal anation of Co(CNBr3- by 2 M thiocyanate ion at 22 degrees C in 0.1 M NaOH proceeds by two pathways. Direct anation occurs with a pseudo-first-order rate constant of about 8 x 10(-6) s(-1), while ana tion of Co(CN)(5)(H2O)(2-) in equilibrium with Co(CN)(5)(OH)(3-) forme d by the concurrent base hydrolysis of Co(CNBr3- occurs with a pseudo- first-order rate constant of 2.15 x 10(-6) s(-1). The thiocyanate anat ion product obtained shows an overall S:N-bonded linkage isomer ratio of 4.5 +/- 0.2. Measurements of the product S:N linkage isomer ratio a nd of the ratio of aquation to anation were obtained for photosubstitu tion of Co(CN)(5)N-3(3-) at 5 degrees C and pH 13 in 2 M aqueous thioc yanate with various counterions. For the group of singly charged ions Li+, Na+, K+, and NH4+, the aquation:anation ratio ranged from 9.9 +/- 1.4 for Li+ to 2.5 +/- 0.5 for NH4+ while the range of S:N product ra tios was 12.7 +/- 1.1 to 10.6 +/- 2.6 over the same group. These resul ts reveal a linear relationship between ln(aquation/anation) and 1/hyd rated radius, while the S:N ratio is essentially constant. Compared wi th Li+, Ca2+ showed the increase in anation efficiency expected on the basis of its higher charge. A constant total quantum yield of 0.073 /- 0.005 was found for photosubstitution of Co(CNI3- in NaClO4/NaSCN m ixtures of total concentration 4 M, showing that the anation and aquat ion are competitive processes, The cation apparently assists anation b y helping overcome the Coulomb repulsion between the entering thiocyan ate and a coordinatively unsaturated intermediate of like charge. The higher S:N ratio for the photoprocess is not a result of the cation as sistance but is an intrinsic property of the intermediate, consistent with involvement of the tripler state five-coordinate species theoreti cally predicted.