Bidentate Entering Groups

Annibale, L. Cattalini, L. Maresca, G. Michelon, and G. Natile, Inorg. Chim. Acta, 1974, 

Trozzi, Inorg. Chim. Acta, 1974,11, 231. 

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The kinetics of reaction of Re(NCO)(CO)5 with phen, bipy, or diphos indicate rate-determining loss of carbon monoxide. Rate constants for the series ReX(CO)6 are in the order NCO > Cl > Br > I. The same rate trend, Cl > Br > I, is observed for reaction of ReX(CO)6 with triphenyl-phosphine this trend is, as expected, the same as that for analogous reactions of MnX(CO)s. Surprisingly, an earlier study of ReX(CO)5 with bidentate ligands indicated the opposite rate trend, viz. I > Br > Cl whether this difference is due to a difference in mechanism for bidentate entering groups or merely to large experimental uncertainties has not yet been established. 

Table 3 Activation parameters for the reaction o/[Pt(bipy)Cl2] with uni- and bidentate entering groups (L) in methanol...

Enterobactin (MW 669) is derived from certain enteric bacteria such as Escherichia coli and is classified as a siderochrome by Neilands (1) in which the iron is bound by three bidentate catechol groups instead of the hydroxamic acid ligands described above. There are three charged oxygens which bind the iron either in a facial or meridional configuration. 

A proper consideration of the kinetic situation requires an analysis of the theoretical rate law for such a system. The following analysis is somewhat simplified in that a symmetrical bidentate chelate is assumed and the ring closing is not reversible. A complete version has been given by Letter and Jordan. These reactions are usually done in dilute acid (pH 5-7), so that protonation of the entering group has been included in Scheme 3.9, but charges and coordinated solvent have been omitted. 

The first characteristic reaction of arynes to be discovered was the addition of polar species, especially nucleophiles, to the triple bond. Since arynes are bidentate intermediates, such additions could lead to two different products in the event of unsymmetrical substitution in the aryne ring. If, as is often the case for such polar additions, the aryne was generated by elimination of HX from a monosubstituted aromatic compound 126, then the product 127 with the entering group in the position previously occupied by the substituent is called the product of normal or ipso-substitution while the rearranged product 128 is referred to as that of cine-substitution. Should the aryne be generated from the isomeric precursor 129 then the designation of which is the normal... 

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