Chelation stepwise coordination

The step-by-step process of chelation described above is believed to be the usual way all polydentate ligands go about coordinating to metal ions in general - knitting themselves onto the metal ion stitch by stitch (or, more correctly, undergoing stepwise coordination). It also provides a way whereby compounds can achieve only partial coordination of a potential donor set, when there are insufficient coordination sites for the full suite of potential donor groups available on a ligand. 

Complexes formed by tetradentate siderophores involve stepwise complex formation and therefore, have somewhat different equilibria from their hexadentate analogs. Initial chelation will occur with a tetracoordinate FeL complex forming. A subsequent equilibrium then occurs, where the FeL complexes will react in a 2 1 stoichiometry with free ligands in solution to form a single Fe2L3 complex (coordinated water and charges not shown for clarity). 

By the stepwise thermal decomposition of the tris chelate, mono adducts of the type [NiX2(N—N)] (X = halides, NCS) were obtained.852-854 These complexes are polynuclear six-coordinate with bridging anions. In the thiocyanato derivative the nickel atoms are ferromagnetically coupled. 

In 1959, the coordinated mercaptide ion in the gold(III) complex (4) was found to undergo rapid alkylation with methyl iodide and ethyl bromide (e.g. equation 3).9 The reaction has since been used to great effect particularly in nickel(II) (3-mercaptoamine complexes.10,11 It has been demonstrated by kinetic studies that alkylation occurs without dissociation of the sulfur atom from nickel. The binuclear nickel complex (5) underwent stepwise alkylation with methyl iodide, benzyl bromide and substituted benzyl chlorides in second order reactions (equation 4). Bridging sulfur atoms were unreactive, as would be expected. Relative rate data were consistent with SN2 attack of sulfur at the saturated carbon atoms of the alkyl halide. The mononuclear complex (6) yielded octahedral complexes on alkylation (equation 5), but the reaction was complicated by the independent reversible formation of the trinuclear complex (7). Further reactions of this type have been used to form new chelate rings (see Section 7.4.3.1). 

Our previous results on hypericin indicate that excited-state H-atom transfer occurs even when one of the carbonyls is prohibited from accepting a hydrogen. The presence of such a transfer is apparent under very acidic conditions in AOT reverse micelles and cannot be excluded upon chelation of Tb3+ [76]. There is thus no evidence for a concerted H-atom-transfer mechanism in hypericin. In the present study, contrary to our initial expectations, we are not even able to demonstrate that hypomycin B executes an excited-state H-atom transfer hence our investigation sheds no light on the general question of how many H atoms are transferred in the perylene quinones and whether the transfer is concerted or stepwise. On the other hand, if further investigation reveals that H-atom transfer does not occur in hypomycin B, the result would have considerable implications for an understanding of the reaction coordinate for the H-atom transfer. 

Reaction of aluminum halides with acetonitrile solutions of terpy leads to the formation of [Al(terpy)Cl3] or [Al(terpy)2]Xj (X = Cl, Br, or I) (48). Spectroscopic and conductivity measurements suggest that these complexes are octahedral. Moore and co-workers have investigated the reaction of terpy with [A1(DMS0)6] (DMSO, dimethylsulfoxide) by NMR methods stepwise release of DMSO indicated sequential coordination of the pyridine rings, and it is noteworthy that the closure of the final ring is significantly slower than the other two, as predicted by our concepts of increasing strain in the chelated complex (78). 

Precipitation equilibria are described in part in Chapter 7. An important type of precipitation equilibrium considered here involves a coordination reaction of a metal ion with univalent anions to form an uncharged chelate that is insoluble. In general, it is necessary to consider stepwise equilibria ... 

These two reactions are in accordance with the stepwise deprotonation of citric acid (c.f. Fig. la). The oxidation state of Re generated by electroreduction of the perrhenate-citrate complex was found to be Re(V), which could easily be reduced further, forming Re(IV). The enhanced reduction of the perrhenate-citrate complex was ascribed to expansion of the Re coordination sphere from 4 to 6, through formation of chelated structures by a concerted process, in which the incoming ligand transferred protons to coordinated oxo-groups. 

The ethylene methyl acrylate system is amenable to low-temperature NMR mechanistic studies with all of the critical intermediates visible spectroscopically." " Acrylate insertion occurs predominantly in a 2,1-fashion, yielding a strained four-membered chelate ring in which the carbonyl oxygen atom is coordinated to the palladium atom. This insertion is followed by a series of /3-hydride eliminations and readditions expanding the ring stepwise to the six-membered chelate complex this is the catalyst resting state shown in Scheme 6. 

The stepwise alternating insertions of CO and an alkene into Pd-C bonds comprise important steps in living catalysts for the alternating copolymerization [65]. The olefin insertion into acyl complexes provides cationic alkyl species in which a carbonyl oxygen is coordinated to the palladium center as shown in Eq. 7.4 The chelating alkyl complexes, whose presence has been confirmed by several research groups, would give extra stabilization to prevent occurrence of /I-elimination. 

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