Reactivity coordination sphere

The dehydrogenative coupling of silanes does not stop at the stage of disilanes in the coordination sphere of early transition metals like Zr and Hf, but chain polymers of low molecular weight are formed. As reactive intermediates in this reaction, silylene complexes are also assumed. However, alternative mechanisms have been discussed (sect. 2.5.4). 

The introduction of redox activity through a Co11 center in place of redox-inactive Zn11 can be revealing. Carboxypeptidase B (another Zn enzyme) and its Co-substituted derivative were oxidized by the active-site-selective m-chloroperbenzoic acid.1209 In the Co-substituted oxidized (Co111) enzyme there was a decrease in both the peptidase and the esterase activities, whereas in the zinc enzyme only the peptidase activity decreased. Oxidation of the native enzyme resulted in modification of a methionine residue instead. These studies indicate that the two metal ions impose different structural and functional properties on the active site, leading to differing reactivities of specific amino acid residues. Replacement of zinc(II) in the methyltransferase enzyme MT2-A by cobalt(II) yields an enzyme with enhanced activity, where spectroscopy also indicates coordination by two thiolates and two histidines, supported by EXAFS analysis of the zinc coordination sphere.1210... 

An S4 coordination sphere of the type seen in the Ada protein, where four cysteines coordinate, was achieved with the tridentate tris(2-mercapto-l-phenylimidazolyl)hydroborato ligand. The zinc thiophenolate derivative showed reactivity of the thiolate linkage (Figure 8).506... 

In spite of the rich chemistry developed starting from the OsHCl(CO)(P Pr3)2 complex, the presence of a carbonyl group in its coordination sphere is probably a limitation for some subsequent developments. In this context it seems important to mention the encouraging reactivity of the related osmium(IV) complex, OsH2Cl2(P Pr3)2, that in methanol afford OsHCl(CO)(P Pr3)2. We believe that both interrelated osmium complexes present not only a rich chemistry but also a promising future as starting materials in organometallic chemistry. 

Transition metal centered bond activation reactions for obvious reasons require metal complexes ML, with an electron count below 18 ("electronic unsaturation") and with at least one open coordination site. Reactive 16-electron intermediates are often formed in situ by some form of (thermal, photochemical, electrochemical, etc.) ligand dissociation process, allowing a potential substrate to enter the coordination sphere and to become subject to a metal mediated transformation. The term "bond activation" as often here simply refers to an oxidative addition of a C-X bond to the metal atom as displayed for I and 2 in Scheme 1. 

This reaction has lent itself to the development of its asymmetric version (Scheme 88). The trick here is to remove the choride ligands from the coordination sphere of the platinum-chiral ligand complex. This makes the metal center more electrophilic, thus reactive reactions can be run at lower temperature. Interestingly, the best ligand was found to be the atropisomeric monophosphine (fJ)-Ph-BINEPINE.312 Enantiomeric excess up to 85% was observed. Very recently, enantioselectivity up to 94% ee has been achieved using [(AuCl)2(Tol-BINAP)] as pre-catalyst for the reaction of another enyne.313... 

We first consider outer sphere transfer (ET) reactions, e.g. D" + A -> D + A, a donor-acceptor electron transfer without significant coupled internal reorganization of the D and A species [27,29,30]. A hallmark of such reactions, which has been long appreciated [27], is that the reactive coordinate is itself a many-body collective solvent variable (and is not the coordinate of the electron itself)- In particular, if R and P stand for the reactant and product, then the reactive coordinate is... 

Upon comparison of the k< m exchange rate of the Tc(V) system with that of the Re(V), the significant increase in reactivity (ca. 3 orders of magnitude) is very prominent and not necessarily indicative of an associative activation. It is, however, possible that the Tc(V) hydroxo complex might be very reactive via an associative pathway, since it is known that the Tc(V) center much more readily accepts electron density than does the corresponding Re(V) complexes (55). The greater ease by which coordination sphere expansion can occur in third-row d-series transition elements such as W(IV) and Re(V) (not very easily... 

Weiss studied68a the reactivity of both new complexes, and found that a variety of phosphines and phosphites would also convert the vinylcarbene complex 139 into the corresponding vinylketene complex (140), capturing one of the carbonyl ligands from the coordination sphere of the metal to become the ketene carbonyl. Only in the case of triphenylphosphine was the dicarbonyl(phosphine)vinylcarbene complex (141) isolated, which then required addition of carbon monoxide to convert it to the dicarbonyl(triphe-nylphosphine)vinylketene complex 140.a. This interconversion was reversible and proceeded quantitatively. 

Rbo is a homodimeric protein, each subunit of which contains two distinct mononuclear nonheme iron centers in separate domains (Fig. 10.4) (Coehlo et al. 1997). Center I contains a distorted rubredoxin-type [Fe(SCys)4] coordination sphere. [Fe(SCys)4] sites in proteins are known to catalyze exclusively electron transfer, which is, therefore, the putative function for center I. Center II contains a unique [Fe(NHis)4(SCys)] site that is rapidly oxidized by 0, and is, therefore, the likely site of superoxide reduction (Lombard et al. 2000). A blue nonheme iron protein, neelaredoxin (Nlr) from Desulfovibrio gigas (Silva et al. 1999), contains an iron center closely resembling that of Rbo center II (Table 10.1). The blue color is due to the oxidized (i.e., Fe(III)) form [Fe(NHis)4(SCys)] site, which, in both Nlr and Rbo, has a prominent absorption feature at -650 nm. Reduction of center II to its Fe(II) form fully bleaches its visible absorption. These absorption features have been used to probe the reactivity of Rbo with superoxidie. 

Anions can promote hydrolysis of complex cations by producing ion pairs of enhanced reactivity (see 2.178). Usually however, ligands accelerate the removal of a coordinated ligand by entering the metal coordination sphere with it and thereby labilizing it towards hydrolysis. We have already seen the effect of coordinated OH on the enhanced labilities of Fe(III) and Cr(III). Dissociative mechanisms and considerable acceleration are promoted by CFlj, CN , SOj and other groups on inert Cr(III), Co(III) and Pt(IV) complexes. Nitrate ions, for example, reduce the half-life for replacement of water in Cr(H20)g by dmso from —380 h to 10 s ... 

In Eq. (5.25) H2O represents a water molecule initially present outside the coordination sphere of the metal ion, which, as a result of the exchange, has entered the first coordination sphere. It follows that the degree of kinetic reactivity of aquometal ions with complexing agents parallels their kinetic lability toward water exchange. Moreover, since the water exchange rate constants of most metal ions are known, predictions on the rate of complex formation of aquometal ions can be made. 

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