Thiocyanato complexes linkage isomerism

Thallium, tetrakis(acetylacetone)-stereochemistry, 94 Thallium, triaquatrinitrato-structure, 97 Thermochromism, 470 Thiocyanato complexes linkage isomerism, 184 Thiocyanic acid metal extraction from, 540 Thioethers complexes chirality, 199 formation... 

An interesting study with BATO compounds concerns the linkage isomerization of coordinated [NCS] in [Tc(NCS)(cdoh)2(cdo)(BMe)] (440). Both a red, N-bond isothiocyanato complex [Tc(NCS)(cdoh)2(cdo)(BMe)] and the brown, S-bond thiocyanato complex [Tc(SCN)(dmg)3(BR)] (441) were isolated from the direct synthesis from [Tc04] . The N-bound (440) was found to be thermodynamically more stable than the S-bound isomer (441) and the conversion was monitored elegantly by spectroscopic methods. The X-ray crystal structure of (440) was determined. The NCS and SCN -substituted analogues exhibited r c N stretch at 2,114-2,124 cm and at 2,055-2,079 cm , respectively. " (Scheme 59). 

Tracer studies have shown that the isomerization of /ra/zj-[Co(en)2(NH3)-(SCN)] to the A-thiocyanato-linkage isomer in basic aqueous solution is intramolecular. The detailed mechanism may involve an intimate ion pair which does not exchange its thiocyanate with the thiocyanate in solution, a species in which the thiocyanate is Ji-bonded to the cobalt, or intramolecular attack of a deprotonated amine nitrogen at the thiocyanate carbon (Scheme 5). Aquation of [Co(NH3)5(sulphamate)] - is preceded by isomerization of the starting complex to an equilibrium mixture of N- and 0-bonded isomers. The specific rate constant for approach to equilibrium is l.lxlO s at 25 °C composite activation parameters are AH = 24.7 kcal mol and AS = 10.9 cal deg mol . Inability to estimate the equilibrium constant satisfactorily precludes the separation of these kinetic parameters into their forward and backward components. ... 

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