Substitution inert

The superb elegance of this demonstration lies in the choice of reactants which permits no alternative mechani.sm. Cr" (d ) and Co" (d ) species are known to be substitutionally labile whereas Cr" (d ) and Co " (low-spin d ) are substitutionally inert, Only if electron transfer is preceded by the formation of a bridged internrediate can the inert cobalt reactant be persuaded to release a Cl ligand and so allow the quantitative formation of the (then inert) chromium product. Corroboration that electron transfer does not occur by an outer-sphere mechanism followed by los.s of CP from the chromium is provided by the fact that, if Cl is added to the solution, none of it finds its way into the chromium product. 

Substitution-inert metal ions as probes of biological function. J. I. Legg, Coord. Chem. Rev., 1978, 25,103-132 (84). 

As regards intimate mechanism, electron transfer reactions of metal complexes are of two basic types. These have become known as outer-sphere and inner-sphere (see Chapter 4, Volume 2). In principle, an outer-sphere process occurs with substitution-inert reactants whose coordination shells remain intact in... 

A -4 being 9.8 x 10 P.mole sec under the same conditions. The substitution-inert CrCl " species is formed as a product. With CrCl as the Cr(III) species, the reduction... 

Beattie and Basolo have investigated the reactions of the substitution-inert octahedral complexes of Pt(IV) with tris(bipyridine)chromium(II). A rapidmixing, stopped-flow apparatus was made use of in the majority of experiments. 

The complex has been separated by ion exchange and characterised by direct analysis . The complex has a distinctive absorption spectrum (Fig. 11), quite unlike that of Np(V) and Cr(III). The rate coefficient for the first-order decomposition of the complex is 2.32 x 10 sec at 25 °C in 1.0 M HCIO. Sullivan has obtained a value for the equilibrium constant of the complex, K = [Np(V) Cr(III)]/[Np(V)][Cr(III)], of 2.62 + 0.48 at 25 °C by spectrophotometric experiments. The associated thermodynamic functions are AH = —3.3 kcal. mole" and AS = —9.0 cal.deg . mole . The rates of decay and aquation of the complex, measured at 992 m/t, were investigated in detail. The same complex is formed when Np(VI) is reduced by Cr(II), and it is concluded that the latter reaction proceeds through both inner- and outer-sphere paths. It is noteworthy that the substitution-inert Rh(lII), like Cr(III), forms a complex with Np(V) °. This bright-yellow Np(V) Rh(III) dimer has been separated by ion-exchange... 

Reactions (6), (7) and (9) occur with substitution-labile oxidants and (6), (7) and (8) with substitution-inert oxidants. It is postulated that the species (MS03)" involves metal-oxygen coordination for labile M", and metal-sulphur coordination for inert M". The latter mode is thought to prevent sulphur-sulphur bond formation. 

This oxidant, which has been utilised only recently , is almost substitution-inert, and any reaction proceeding via substitution would be expected to have an activation energy of the order of 30 kcal.mole . Those oxidations which have been examined have much lower activation energies. The reduction product is IrClg which is also solvolysed slowly. 

In the cases of substitution-inert complexes of Fe(ni) it is envisaged that R-forms a temporary bond with the ligand through which the electron transport takes place. 

Substitution-inert complexes have also recently been introduced into DNA as modified-base phosphoramidites. Interest here is generally focused on photo- and redox-active metal species for use as probes for sensing applications (165) and in studies on DNA-mediated electron... 

There appear to be no well-characterized Co111—Si bonded compounds. Interactions with silicates, particularly on surfaces, are through oxygen atoms. The use of the complex [Co(en)2(LL)]Cl3 (where LL = (MeO SiC C C NHCt Ct NTy permits bonding of the substitution-inert CoN6 species to a silica surface.549... 

Tc(acac)3) [18]. Tc(acac)3 is known to have the low-spin d4 configuration [19]. Metal ions with d4 configuration are classified as substitution-inert [2], The substitution-inert character of Tc(acac)3 has already been pointed out in the course of its nuclear synthesis by the 97Ru(acac)3(y, n) reaction [20]. 

The substitution-inert character of the metal(III) ion in the second transition series has already been discussed in 2.3. However, interesting behavior has been reported by Kasahara et al. [23], who found that a p-diketone coordinated to the central Ru(III) could easily be replaced by an acetonitrile with the aid of a strong acid. When the reaction was conducted in acetonitrile, its stoichiometry was confirmed by means of spectrophotometrie titration as follows ... 

The clusters [Re6Se8(PEt3)5(MeCN)]2+ and [Re6Se8(PEt3)5(dmso)]2+ are substitutionally inert, with MeCN and dmso exchange or... 

Most helicates have linear axes, though a few helicates with circular axes are known - indeed the chiral (D4) molecular squares formed from Zn2+ and 2,5 -bis(2,2 -bipyridin- 6 -yl)pyrazine, 22, may be regarded as circular helicates (450). The formation of circular or linear forms seems to depend on balances between kinetic and thermodynamic control iron(II)-poly-2,2/-diimine systems with their substitutionally-inert metal centers provide useful systems for disentangling thermodynamic and kinetic contributions. The mechanism of formation of circular helicates of this type is believed to entail a kinetically favored triple helicate intermediate (484). Self-assembly of chiral-twisted iron(III)-porphyrin dimers into extended polynuclear species takes place through the intermediacy... 

Such reactions are observed in electron transfer reactions of substitutionally inert complexes. The mechanism involves three steps. 

According to Taube, the inner sphere mechanism can takes place when both oxidizing and reducing agents are substitution inert and when ligand transfer from oxidant to reductant is accompanied by electron transfer. The inner sphere electron transfer mechanism may be represented by the scheme... 

In applying this principle to proteins, one would ideally like to modify a protein at one specific site with a number of related, substitution-inert, inorganic redox reagents, and then study the intramolecular electron transfer step as a function of a wide variety of variables (e.g., the redox potential and hydrophobicity of the redox reagent). Such a study is extremely difficult to carry out with large proteins, and none has been reported thus far. We have, however, found out that horseheart cytochrome c is amenable to modification at a single site by the... 

Kinetic and mechanistic studies of nucleophilic substitution at metal(IV) centers are fairly rare (263). Platinum(IV) has the substitution-inert low-spin d configuration, and presumably undergoes nucleophilic substitution by an associative mechanism thanks to its high charge and large size. However there are actually very few data, probably thanks to the tendency for platinum(IV) to oxidize ligands. Substitution kinetics at metal(IV) centers may be more conveniently studied for complexes of the type ML2X2, where M — e.g., Sn, Ti, V, or... 

Most of the studies of the reactivity of this oxidation state have been carried out with these types of complexes. The low spin d Ni(IV) resembles Co(III) in being diamagnetic, substitution inert and capable of resolution into optical isomers. Reductions are outer-sphere when Co(edta), ascorbate and Co(phen)3+ Ref. 269 are used, intermediate Ni(III) species are detected and characterized. Oxidation of Co(edta) by Ni (S-Me2L) + or Ni (S-MejL), L = 12, R = R]=CH3i R2 = H, produces a =10% excess of (+)-... 

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]. 

---