Octahedral metal complex studies

Experimentally based intuitive arguments have been presented to arrive at a regional rule for optical activity of d-d transitions of conformational isomers of octahedral metal complexes. Conformational preferences for chelate rings formed by 1,3-pn in its octahedral mono, bis, and tris metal complexes have been studied by calculation of the conformational energies. In all cases, the chair conformation was found to be the most stable. The lowest energy pathway for converting from one chair configuration into another has a barrier to activation of about 7 kcal mol Conformational types of metal-edta complexes have been studied. ... 

A process that has been studied widely in relation to phenomena such as chiral symmetry breaking, spontaneous resolution and chiral amplification is the reaction (Fig. 18) of [Co(H20)2 (OH)2Co(en)2 2](S04)2 (denoted 1) with NH4Br to give the chiral complex cA-[CoBr(NH3)(en)2]Br2 (denoted 2). This reaction is historically important, as 2 was one of the first octahedral metal complexes to be resolved into A and A stereoisomers, some years after Werner predicted that octahedral ions M(en)2XY should exist as enantiomeric pairs. 

The substitution reactions of octahedral metal complexes (Fig. 5-38) have been the subject of intensive investigation over the past forty years, with complexes of the non-labile ions such as chromium(m) and cobalt(m) playing a vital role in these studies. 

Accordingly, the search for unambiguous data leads to studies of kinetically inert octahedral metal complexes or pyridine, bpy and phen, which are not expected to form seven co-ordinate products. There have been numerous studies of [M(bpy)3]"+ and [M(phen)3]"+ complexes, but it is probably fair to state that no definitive evidence for activation towards attack of hydroxide at the ligand has been presented. There is no doubt, however, that sufficient kinetic anomalies exist for further study to be worthwhile, and the importance of [M(bpy)3]"+ and [M(phen)3]"+ complexes in photocatalytic systems should justify this. A possible chemical consequence of the reaction of a nucleophile with a co-ordinated heterocycle is seen in the reaction of (2,2 - bipyrimidinejruthenium(11) and hydroxide ... 

The steric course of substitution reactions of octahedral metal complexes other than cobalt (III) has been studied to a limited extent. Garner... 

Most studies of the mechanism of substitution in octahedral metal complexes have been concerned with Werner-type complexes organometallic complexes have entered the research field more recently. Among the former, the popular candidates for study have been Cr(III) d ) and low-spin Co(III) (fi ) species. These complexes are kineti-cally inert and their rates of reaction are relatively slow and readily followed by conventional techniques. Both Rh(III) and Ir(III) (both low-spin d ) also undergo very slow substitution reactions. There is no universal mechanism... 

A-Ru(DIP),l. Octahedral metal complexes of the 4,7-diphenyl-phenanthroline (DIP) family are versatile probes for Z-form structure. Using both spectroscopic studies of [A-Ru(DIP)3p binding (59, 60) and photochemical cleavage studies with [A-Co(DIP)3] (see below), one observes that the complexes can scan large pieces of primarily B-form DNA and bind exclusively to Z-form sites or sites of other unusual geometries. The battery of spectroscopic techniques described in the first part of this section was used to determine the relative affinity for [Ru(DIP)3] enantiomers to different forms of DNA and to derive spectroscopic probes for Z-like DNA structures. 

Other studies complemented the previous results and have also shown that a homochiral ion-pairing is favoured for Ru(DEAS-bpy)3 + complex (4.9) DEAS-bpy = 4,4 -bis(diethylamino-styryl)-[2,2 ]-bipyridine " and for the compounds [Ru(bpy)2(L-L)] [PFgli L-L = cmbpy = 4-carboxy-4 -methyl-2,2 -bipyridine (4.10) L-L = dcbpy = 4, 4 -dicarboxy-2,2 bipyridine (4.11). Interestingly, in the previous two examples the octahedral metal complexes have D3 symmetry as A-TRISPHAT (4.7) while in the last example complex 4.10 possesses a C symmetry while 4.11 displays a C2-symmetry (Figure 4.6). These results suggest that a homochiral association between A-TRISPHAT and the octahedral ruthenium complexes is not dependent on a prerequisite D3-symmetry of the metal complex. 

DNA binding with octahedral metal complexes has been the focus of many studies of different groups however. Barton and coworkers were the first to report on homochiral association (A, A) between A-[Ru(phen)3] with B-DNA (A-helix) on the basis of equihbrium dialysis Although the enantiospecific interaction of the A-... 

Most studies of the mechanism of substitution in octahedral metal complexes have been concerned with Werner-type complexes organometallic complexes have entered the... 

Interaction of fluorophores with natural biopolymers such as proteins, polysaccharides, or DNA is also an enantioselective process and can be used to discriminate between two enantiomers of a specific compound. A remarkable example is the differential interaction of A and A isomers of luminescent octahedric metal complexes with DNA (metallointercalators), which generate a fluorescent signal and was shown to be dependent on the DNA heUcity, as was reversed for right-handed B-DNA and left-handed Z-DNA [38], However, in most studies, the main interest has usually been to probe the biopolymer conformation and not to generate an enantioselective fluorescence response of the probe. 

Focusing on reactions using the Fluid Matrix Technique, we have studied the interaction of chromium vapor with 2 at 200 K (13). The resulting film was found to contain metal complexes encapsulated within the polymer in which the isocyanide group adopts a well-defined octahedral arrangement around the chromium center, i.e. a species of type Cr(CN-[P])g. Since characterization of this metal complex within the polymer is not trivial we shall develop the analysis in a little detail. 

Studies of the base-hydrolysis mechanism for hydrolysis of technetium complexes have further been expanded to an octahedral tris(acetylacetonato)techne-tium(III) [30], Although a large number of studies dealing with base hydrolysis of octahedral metal(III) complexes have been published [31], the mechanism of the tris(acetylacetonato)metal complex is still unclear. The second-order base hydrolysis of the cationic complex tris(acetylacetonato)silicon(IV) takes place by nucleophilic attack of hydroxide ion at carbonyl groups, followed by acetylacetone liberation, and finally silicon dioxide production [32], The kinetic runs were followed spectrophotometrically by the disappearance of the absorbance at 505 nm for Tc(acac)3. The rate law has the following equation ... 

This account is concerned with the rate and mechanism of the important group of reactions involving metal complex formation. Since the bulk of the studies have been performed in aqueous solution, the reaction will generally refer, specifically, to the replacement of water in the coordination sphere of the metal ion, usually octahedral, by another ligand. The participation of outer sphere complexes (ion pair formation) as intermediates in the formation of inner sphere complexes has been considered for some time (122). Thermodynamic, and kinetic studies of the slowly reacting cobalt(III) and chromium(III) complexes (45, 122) indicate active participation of outer sphere complexes. However, the role of outer sphere complexes in the reactions of labile metal complexes and their general importance in complex formation (33, 34, 41, 111) had to await modern techniques for the study of very rapid reactions. Little evidence has appeared so far for direct participation of the... 

As already mentioned, UV-Vis spectroscopy is an effective tool to study metal complexes. For different actinide(IV) compounds in ILs (Np(IV), Pu(IV), U(IV) as [C4CiIm]2[AnCy complexes in [C4CiIm][Tf2N]) a similarity with solid complexes having an octahedral An(IV) environment was estimated [14,15]. In other ILs, for example, uranyl ions dissolved in [C4QIm] [NfO], [U02] may be present as a bare cation [16]. 

There is growing evidence that substitution reactions in Co(lll) complexes may not be typical of octahedral transition metal complexes. Early studies of substitution reactions for Cr(III) complexes revealed a rather strong dependence of reaction rate... 

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