Coordination capability

Studies on metal-pyrazole complexes in solution are few. The enthalpy and entropy of association of Co(II), Ni(II), Cu(II) and Zn(II) with pyrazole in aqueous solution have been determined by direct calorimetry (81MI40406). The nature of the nitrogen atom, pyridinic or pyrrolic, involved in the coordination with the metal cannot be determined from the available thermodynamic data. However, other experiments in solution (Section 4.04.1.3.3(i)) prove conclusively that only the N-2 atom has coordinating capabilities. 

Monodentate thioether ligands are relatively poor ligands towards Ni11. However, several Ni11 complexes with polydentate thioether ligands are known, where the coordination capability is enhanced by chelation. In particular, complexes with macrocyclic thioether ligands, which are discussed in Sections 6.3.4.10.l(iii) and 6.3.4.10.3, are very stable. 

An estimation of the relative coordinating capabilities of the different arenes has been obtained by measuring the equilibrium constants for arene replacement by acetone-dis (Scheme 2.20). The values of the equiUbrium constants show the lack of acetone-ds replacement in the case of hexamethylbenzene, and the higher coordination capability of aniUne in comparison with trimethylbenzene or toluene [22]. 

Fig. 2.15 Gerold Schwarzenbach (1904-1978) did his Ph.D. in 1928 at the ETH Zurich with Professor William D. Treadwell in analytical chemistry and, after a year with Sir Robert Robinson at Manchester and London, became Oberassistent at the Chemische Institut of the University of Zurich. In the 1930s he started his work on the coordination capabilities of a new class of polydentate ligands (Komplexone) which soon received world-wide recognition. In 1942, he was promoted as Associate Professor and in 1947 as Professor of Analytical Chemistry at the University of Zurich but returned in 1955 to the ETH, where he was the Director of the Laboratory of Inorganic Chemistry until his retirement in 1973 (photo from Helv. Chim. Acta 75, 21-61 (1992) reproduced with permission of Dr. Kisakurek, Editor of Helvetica Chimica Acta)...

Equation (65) is an example of a process that may be readily promoted by a cluster but not necessarily with a mononuclear complex. Scheme 8 shows how the multiple-coordination capabilities of a trinuclear cluster could reduce the N—C bond and eventually cleave it. To date, examples of the NCO ligand bridging more than two metals in either a mono- or polyhapto fashion are unknown. The reverse process represents a unique method for forming carbon-nitrogen bonds. Although the above chemis-... 

It appears that the difference in coordinating capability of the anions in the above structures is associated with differences in polarizability of the F ligands in the MF. In general, the CsCl type structure occurs only when at least one of the ions is highly polarizable. In those hexafluorometalates which can be unambiguously described as salts A+MF the small, less polarizable (harder) cations such as Li " and Na generally... 

It is widely accepted that the carbonyl reactivity toward nucleophiles increases in the order aldehyde>ketone>ester>amide [6]. This reactivity order is simply based on the extent to which each carbonyl carbon is sterically and electronically activated. However, reactivities might change when these carbonyl substrates are subjected to a Lewis acid. It is generally assumed that the coordination capability of the carbonyl oxygen to Lewis acids is the means by which Lewis acids activate carbonyl substrates. Thus, in some re.spects, the reaction rate parallels the Lewis basicity of the carbonyls. Furthermore, the reactivity of a carbonyl substrate depends on the reaction type as well as the Lewis acid employed. Special care must be taken in assessing the relationship between the relative reaction rate, the relative Lewis basicity, and the inherent carbonyl reactivity of each substrate. It is instructive to take a look at the following example (Schemes 2-2 and 2-3 Fig. 2-1). 

Fig. 6. Free energy landscape in which during a transition from A to B both the variable q and q change systematically left hand side). For this case, the variable q is a good order parameter and suffices to distinguish the equilibrium fluctuations in the stable states A and B. By following g as a function of time t we can detect transitions between the stables states right hand. side). However, this variable does not capture all essential aspects of the transition and is therefore not a good reaction coordinate. In the definition of such of a good reaction coordinate capable of describing the complete course of the reaction the variable q can not be neglected...

Thomas has reported elegant studies of transmetalations in the course of allylation reactions with aldehydes. As illustrated in Scheme 5.2.47, chiral, non-racemic 207 undergoes transmetalation with SnCU to provide stabilization of the trichlorostannyl species 208 by vicinal coordination with the benzyl ether. Activation of aldehyde and addition proceeds via the closed, chair-like transition state 209. The features of the four-membered metallocycle 209 account for the exclusive formation of the Z-homoallylic alcohol and the observed 1,5-xyn-diastereoselection in 210. Similar results are obtained with ethers that have coordinating capabilities (PMB and MOM). 

Finally, several other metal-substituted zinc metalloprotein derivatives have been prepared, including those of VO, Fe(II), Co(III), Pt(II), and HgCl2. Although these systems add little directly to our understanding of the relationship between structure and function of the enzymes, nonetheless they represent new bioinorganic compounds and are of interest in themselves, or can add information on the coordinating capabilities, and reactivity in general, of the residues present in the active cavity. 

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