Kinetically inert complexes

A further factor which must also be taken into consideration from the point of view of the analytical applications of complexes and of complex-formation reactions is the rate of reaction to be analytically useful it is usually required that the reaction be rapid. An important classification of complexes is based upon the rate at which they undergo substitution reactions, and leads to the two groups of labile and inert complexes. The term labile complex is applied to those cases where nucleophilic substitution is complete within the time required for mixing the reagents. Thus, for example, when excess of aqueous ammonia is added to an aqueous solution of copper(II) sulphate, the change in colour from pale to deep blue is instantaneous the rapid replacement of water molecules by ammonia indicates that the Cu(II) ion forms kinetically labile complexes. The term inert is applied to those complexes which undergo slow substitution reactions, i.e. reactions with half-times of the order of hours or even days at room temperature. Thus the Cr(III) ion forms kinetically inert complexes, so that the replacement of water molecules coordinated to Cr(III) by other ligands is a very slow process at room temperature. 

It will not have escaped the reader s attention that the kinetically inert complexes are those of (chromium(iii)) or low-spin d (cobalt(iii), rhodium(iii) or iridium(iii)). Attempts to rationalize this have been made in terms of ligand-field effects, as we now discuss. Note, however, that remarkably little is known about the nature of the transition state for most substitution reactions. Fortunately, the outcome of the approach we summarize is unchanged whether the mechanism is associative or dissociative. 

The first resolution of an octahedral complex into its enantiomers was achieved in 1911 by A. Werner, who got the Nobel Prize in 1913, with the complex [Co(ethylenediamine)(Cl)(NH3)] [10]. Obviously, resolution is to be considered only in the case of kinetically inert complexes whose enantiomers do not racemize quickly after separation. This is a very important remark since, as noted above, the interesting complexes are those containing exchangeable sites required for catalytic activity and thus more sensitive to racemization. We will not discuss here the very rare cases of spontaneous resolution during which a racemic mixture of complexes forms a conglomerate (the A and A enantiomers crystallize in separate crystals) [11,12]. 

The acrylato-bridged dicobalt(III) complex 97 (241) was selected in an orienting experiment (Scheme 9). Indeed, bromination of the kinetically inert complex proceeded smoothly and produced complex 98 as the sole product, as established by IR... 

Buckingham and Engelhardt200 have studied the hydrolysis of propionic anhydride in the presence of kinetically inert complexes of the type [M(NH3)5OH]n+. These reactions occur by nucleophilic attack of coordinated hydroxide on the anhydride (Scheme 32). For reactions of M-OHl" l,+ with propionic anhydride, the Bronsted plot of log kMOH versus the p.Ka of M—OH2k+ is a smooth curve if values for reaction with HzO and OH- are included. Although Icmoh for [(NH3)5CoOH]2+ (3 M-1 s-1) is about 103-fold less than fcoH. its reaction will compete favourably at neutral pH with base hydrolysis. Such effects are considered in more detail in Section 61.4.2.2.3. 

These steric interactions become more pronounced when we consider the introduction of an additional chelate ring in those complexes containing three didentate en ligands. The A XkX form of an [M(en)3]n+ cation is estimated to be 7-8 kJ mol-1 more stable than the A 888 diastereomer. This becomes particularly important when we consider kinetically inert complex cations, such as [Co(en)3]3+, where there is a significant barrier to the interconversion of the diastereomers. In practice, the conformation of the chelate rings in [Co(en)3]3+ salts depends upon the nature of both the anions and any additional solvent molecules in the lattice which can form hydrogen bonds with the en ligands. We will return to this topic in Chapter 7, where we discuss some reactions of [Co(en)3]3+ salts in which an extraordinary steric control is exerted. 

Figure 3-8. The hydrolysis of a kinetically inert complex containing a monodentate amino acid ester co-ordinated through nitrogen. The only effect of the metal is a long-range polarisation which slightly increases the electrophilic character of the carbonyl carbon atom.

E.s.r. data have been reported for the complexes [CrOX2 S2P(OEt)2 ] (X = Cl or NCS) and discussed in terms of the electronic structure of these complexes.233 Two new and kinetically inert complexes of chromium(v) have been reported. [CrO(iVN)-Cl3] (NN = bipy or phen) have been obtained by the dehydrochlorination of the corresponding (H2iViV)[CrOCl5] salt in a dry C02 atmosphere at 80 °C.234 Cr02Cl2 reacts with hexamethylmelamine (L) in anhydrous EtOAc to produce [Cr02ClL].235... 

Ligand Exchanges Ligand exchange mechanism may be associative (A), dissociative (D) or Interchange (Ia or Id). Kinetically inert complexes are formed by Cr3+ and Co3+ and by 4d- and... 

The existence of kinetically inert complexes is useful in mechanistic studies, and important for the separation of different isomers. 

Kinetically Inert Complexes of the Siderophores in Studies of Microbial Iron Transport... 

The most thermodynamically stable and kinetically inert complexes of the trivalent lanthanides are those of the ligand DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate) (42, 43). Our search for lanthanide macrocyclic complexes that would remain intact for longer time periods led us to examine derivatives of DOTA. There are two potential difficulties with the use of DOTA complexes of the trivalent lanthanides for RNA cleavage. First, the overall negative charge on the complex is not conducive to anion binding for example, Gd(DOTA)-does not bind hydroxide well (44). Second, DOTA complexes of the middle lanthanides Eu(III) and Gd(III) have only one available coordination site for catalysis. The previous lanthanide complexes that we used, e.g., Eu(L1)3+, were good catalysts and had at least two available coordination sites. 

In the reaction between Co111 and Cr2+, the kinetically inert complex [Co(NH3)5X]2+ reacts with the labile Cr11 aqua ion to give labile Co2+(aq) and a substitution-inert Cr111 chloro complex which must have been formed via a bridged intermediate. 

In marked contrast to Pd , Pt forms many thermally stable and kinetically inert complexes that are invariably octahedral. 

So far, we have generally assumed that the complexation reaction is fast and that equilibrium is attained. This is often, but not always, the case. The differentiation of the thermodynamic terms, stable and unstable, from the kinetic terms, labile and inert (or robust), should be made. The classical example of a kinetically inert complex is the hexamine cobalt(III) cation in acid solution ... 

Low-spin rf transition metal complexes are classical examples of kinetically inert complexes. When injected into mice, species such as [ColNHsle] , [Fe(l,10-phen)3]2+, [Ru(bipy)3]2+, and [Os(terpy)s] rapidly cause convulsions, paralysis, and death by respiratory failure. They produce a curariform block at the neuromuscular junction, consistent with inhibition of acetylcholine esterase. The d isomers of [Rulphen)] " and [Os(phen)] + are 1.5-2 times more potent than the I isomers (9,10). These inert complexes are excreted largely unchanged from the body. 

Late-transition metal salts have been utilized as catalysts to promote Friedel-Crafts acylation of arenes and heteroarenes with anhydrides. A mismatch between their soft metal center and the hard carbonyl oxygen atoms of the products avoids the formation of a kinetically inert complex and results in catalytic turnovers. Although late-transition metal salts exhibit, a priori, rather poor Lewis acidity, sufficient reactivity can be gained by rendering them cationic. The acylation of variously substituted... 

High-spin Fe complexes of the siderophores are kinetically labile. If Fe is exchanged for Cr, kinetically inert complexes are obtained which can be studied in solution as models for the Fe + complexes. 

Platinum. In marked contrast to PdIV, platinum(iv) forms many thermally stable and kinetically inert complexes. So far as is known, Ptiv complexes are invariably octahedral and, in fact, Ptlv has such a pronounced tendency to be six-coordinated that in some of its compounds quite unusual structures are adopted. An apparent exception to the rule is /i5-C5H5Pt(CH3)3 but, as with other /i5-CsHs complexes, the ring can be considered as occupying three positions of an octahedron. Several interesting examples of this tendency of Ptlv to be 6-coordinate exist where novel bonding is required for this to be achieved (see below). 

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