Cobalt reaction entropy

The less favorable entropies of the polyamine ligand reactions are attributed to the greater loss of rotational freedom about the C N bonds of the aliphatic bases upon metal-ion coordination. It was observed that the coordinated polyamine of a cobalt dioxygen complex undergoes oxidative dehydrogenation under anaerobic conditions to form an imine with the double bond conjugated to the heterocyclic ligand.32... 

As examples of series of related reactions, compensation effects have been described [53] for the thermal decompositions of [CoXj (aromatic amine)2] type complexes (7 reactions) and also for a series of cobalt (III) and chromium (III) complexes (22 compounds studied in which two compensation trends were identified). Later work [54] examined the dehydrations and deamminations of dioximine complexes (two compensation trends identified), and [Co(NCS)2(ammine)2]-type complexes (three compensation trends identified). The systems involving larger entropy changes required less energy for activation [53]. Separate compensation plots for the dehydrations and the decompositions of eleven alkali and alkaline-earth metal dithionates were described by Zsako et al. [55]. 

The activation entropy for thermal decomposition of /ra j -[MnX(CO)3(PPh3)2] indicates that manganese-phosphorus bond breaking is rate-determining. The rate law for the decomposition of CoH(CO)4 to Co2(CO)8 plus hydrogen is second-order in cobalt compound. While this decomposition has previously been studied in the gas phase, this is the first report of decomposition kinetics in the liquid phase. The first step in reaction of Co2(CO)s with organomercury halides is solvent-induced disproportionation of the dimeric carbonyl. ... 

Rate laws, rate constants, activation parameters, and enthalpies and entropies of reaction have been determined for hydroxide attack at the bis-tptz complexes of cobalt(ii) and of nickel(n) (P. A. Williams, Transition Met. Chem., 1979,4,24). 

Table I lists some rate constants at room temperature for the chromium(Il) reduction of pentaammine cobalt (III) complexes containing monocarboxylic ligands. The tetraammineglycinate is a complex with a minimum of steric hindrance. The rate of reaction is very similar to that of the acetato complex, indicating that inductomeric effects are of little importance in these redox processes, so that the observed decreases in the reaction rates may be related to the steric hindrance of the ligand. As the bulk of the carbon ring increases the rate decreases, although this decrease is accompanied by an increase in activation entropy (4) (from -38 e,u, for the cyclopropane complex to -23...

This reaction is characterized by the activation parameters = 13.8 kcal mol" and tsS — 12 cal mol. Base hydrolysis by the Nlcb pathway to give the [Co(NH3)sOH] + ion as a minor product is found to have a much higher activation energy A/f = 28kcalmol and = + 35 cal mol-. Co-ordination of DMF to cobalt(m) increases the rate of base hydrolysis by more than a factor of 10, largely because of a more favourable entropy of activation. 

Cobalt(in) oxidizes 2-mercaptoethylamine (HMea) in [Co(en)2(Mea)] to the corresponding co-ordinated disulphide complex by pathways involving Co + and [CoOH] +. An outer-sphere mechanism is suggested by the activation entropy (—3.1 cal K mol ) for reaction with Co and the reaction with [CoOH] + is substitution controlled. Redox proceeds by formation of a co-ordinated radical complex [Co(en)2(Mea)] +,... 

It has been known for a few years that anation of /m -[Co(dmgH)2-(Me)(OHa)] is fast, since the methyl group has a powerful trans labilizing influence on the co-ordinated water. Recently activation parameters have been determined for replacement of this water by pyridine and by thiocyanate. Activation enthalpies are low by cobalt(m) standards 20.5 kcal mol for reaction with pyridine and 17.5 kcal mol for thiocyanate. However, these values are comparable with those for anation of the analogous, but much less reactive, complex /rans -[Co(dmgH)2(N02)-(0H2)] in fact it is the difference in activation entropies which determines the different reaction rates for these methyl and nitro-complexes. The... 

Rates and activation enthalpies for base hydrolysis of the Second-order rate constants at 25 °C are 88.5 and 1081 mol s respectively, and the activation enthalpies are 9.6 1.9 and 10.1 2.5 kcal mol. The predominant reaction of the 5 -thiocyanato-complex rra j-[Co(en)2(NH3)(SCN)] in basic solution is base hydrolysis, with less than 10% isomerization to the iV-thiocyanato-isomers. Base hydrolysis of [Co(NH3)5(SCN)] + is accompanied by much more isomerization. 5-Thiocyanato is approximately as good a leaving group as chloride from cobalt(m) in basic solution. The activation enthalpy for base hydrolysis of [Co(NH3)g(S203)]+ is 30.4 kcal mol the activation entropy is 25.7 cal deg mol". In dilute alkali, base hydrolysis of [Co(en)2(ox)]+ gives 100% m-[Co(en)2(OH2)2] +, but in 4M alkali, the product distribution is... 

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