Substitution inert labile 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. 

However, beeause of the kinetie lability of eobalt(ll), heterogenized catalysts based on Co are suseeptible to metal leaehing during liquid phase reaetions and thus repeated use of such catalysts is not practical from a chemical point of view. In order to avoid this problem, use of catalysts based on cobalt(lll), whieh is substitutionally inert, may be expected to show more attraetive eatalytic properties for the same reaetions. As expeeted, substitutionally inert cobalt(lll) eomplexes have been shown to be eatalytieally very aetive, and henee attractive, for alkylaromatie oxidation [26]. Also, as we shall see later, a series of tetramerie eobalt(lll) complexes eapable of cycling oxidation states between 111 and IV has also been foimd to be effective as eatalysts for the oxidation of alkyl aromaties, aleohols and alkenes [5,15,27]. 

Consequently, reduction of cobalt(III) ammines in basic solution is not favorable. A variety of reducing agents has been used to effect reaction (11). The fortunate coincidences that cobalt(III) complexes are substitution inert while cobalt(II) systems are labile and that cobalt(II) is resistant to oxidation or further reduction in acid solution offer many advantages in the study of redox processes. Not surprisingly, work with cobalt(III) complexes forms the basis for much of the present understanding of oxidation-reduction reactions. 

According to Taube , complexes can be divided into two classes i.e. inert and labile depending on the rate at which the substitution reaction occurs. The author defines labile complexes as those which take part in substitution reaction without any delay ( 1 min) on coming into contact with other reagents under ordinary conditions. These conditions are room temperature and concentration of the reagents of about 0.1 M. In contrast, inert complexes are slower to react. In this article, the author discuss the reasons for the difference in the kinetic behaviour of various inorganic complexes and also attempts to classify them in terms of their lability. Another detailed x ount on the same topic can be found in the book by Basolo and Pearson... 

OCTAHEDRAL SUBSTITUTION REACTIONS. LABILE AND INERT COMPLEXES... 

Low-spin complexes in which the metal configuration is d3, d4, d5, or d6 would require either a ligand to leave before substitution could occur or the utilization of an outer d orbital to form a seven-bonded transition state. Such complexes frequently undergo substitution by a dissociative process because expanding the coordination number of the metal is made difficult by the lack of a vacant orbital of suitable energy. In either the SN1 or the SN2 case, the substitution should be much slower than it is for labile complexes. Accordingly, complexes of V2+, Cri+, Mn4+ (all of which are d3 ions), low-spin complexes of Co3+, Fe2+, Ru2+, Rh3+, Ir3+, Pd4+, and Pt4+ (all of which are dt ions) and low-spin complexes of Mn3+, Re3+, and Ru4+ are classified as inert. Inert does not mean that substitution does not occur, but rather that it occurs much more slowly than it does in labile complexes. 

The rate constants for substitution reactions of labile complexes are frequently in the order of 101 to 106 M-1 s whereas those for inert complexes are as low as 10-5 to 10-8 s-1. Certainly the difference in electronic structures is one factor contributing to this enormous variation in rates of substitution. Other reasons will be discussed later in this chapter. 

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. 

Szilard-Chalmers reactions are applicable to elements existing in different stable oxidation states or forming substitution-inert complexes. Exchange reactions between the oxidation states or with the complexes should not take place during irradiation and chemical separation, because they would cause a decrease of the specific activity. Therefore, substitution-labile complexes are not suitable. 

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