Tris bidentate complexes

There are many examples of tris-bidentate complexes ML3. An early example is provided by the germanium complex [GeCmaltjs], whose formation, along with [Ge(OH)2(malt)2], was documented in 1966 149). Better-characterized examples are provided by the series of tin(IV) compounds [SnLaJX with L — l,2-dimethyl-3-hydroxy-4-pyri-... 

Fig. 1. The absolute configurations of two optical isomers of a tris-bidentate complex with symmetrical ligands...

The tris-bidentate complexes, [Os(phen)2(das)]2+, [Os(bpy)2(das)]2+, [Os(phen)2(dpae)]2+, [Os(phen)(das)2]2+, and [Os(bpy)(das)2]2+, are prepared by refluxing cis-[Os(N-N)2Cl2] or [Os(N-N)C14] with the appropriate diarsine in ethylene glycol. These complexes exhibit reversible Os(III/II) oxidations and a reversible reduction that is centred at the phen or bpy ligand. In the case of [Os(bpy)2(das)]2+, a second ligand-centered reduction is observed in acetonitrile. The electronic absorption and emission spectra of these complexes have also been reported (97). [Os(bpy)(das)2]2+ has been used as an effective sensitizer in a photoelec-trochemical half-cell (312). 

Figure 89 shows that at b = 1.2 an additional feature has appeared on the potential energy surface, which at b = 1.3 has formed a deep minimum (Figure 90). The two minima correspond to the two optical isomers of the Z)4 square antiprism (Figure 93). It may be noted that in contrast to the three-bladed propeller , which is the dominant stereochemistry for tris(bidentate) complexes (Section 2.3.3), this four-bladed propeller is only expected in tetrakis(bidentate) complexes where the bidentate ligands have exceptionally large normalized bites. At b = 1.26, the angle of twist 0 = 22.5° and the two square faces are staggered with respect to each other (Figure 93). In an analogous way to the behaviour of tris(bidentate) complexes, a decrease in b leads to a decrease in 9.

In the numerous octahedral bis- and tris-bidentate complexes of this type, the configurational chirality can be associated simply with the array of chelate-spanned edges, and is designated A or A, as in (19a) and (19b), as discussed previously. The nature of the donor atoms is immaterial in this regard, although their consideration is necessary in the analysis of diastereomers with unsymmetrical bidentate ligands and in the assessment of any quantitative measures of chirality, such as the rotational strengths of electronic transitions.121,149... 

Octahedral Six Coordinate In addition to the tris-bidentate complexes and the unidentate complexes discussed above, there are many other examples of chiral metal complexes that are based on octahedral geometry. For example, there are a number of reports of edge-sharing binuclear structures containing four bidentate chelates. The individual metal centers in these binuclear species can exist as either A or A enantiomers, and, therefore, the overall structure can be chiral AA and AA, or the achiral (meso) AA pair as illustrated in Figure 5.12a and b. Studies of complexes of this type were important in the proof of Werner s coordination theory. In these pioneering studies, Werner was able to prove by experiment the existence of the three isomers (AA, AA, and AA) of [Co2(p-NH2)( l-NC>2)(en)2]4 +. 23 For Cr(III) complexes, Werner showed that the / >.vo-AA- Cr2(p-OH)2(en)414 1 may be prepared by... 

The compounds that we will study are tris-bidentate complexes with trans-cyclohexane-1,2-diamine (see Section 17.6). We start from [Co(NH3)6]3+ (see Section 17.1). Using the usual tools in HyperChem, edit that complex so that it has six identical Co-N distances of 1.955 A and valence angles involving Co of 90° or 180°. Select the cobalt and the six nitrogen atoms with the select tool and save this structure make sure that the same name also appears as the MOMEC input structure (Setup/Files/Input). Open the module that allows you to compute the structure with the metal center and the six donor atoms fixed Execute/Rigid Geo-metry/Fixed Coordinates. Read File will mark the coordinates to be fixed, i.e., the x,y,z coordinates of the atoms that you have selected in HyperChem. The window shown in Fig. 17.20.1 will appear. 

There are few well-characterized examples of this type. Neutral tris(bidentate) complexes of the type... 

Fig. 5. The trigonal twist angle for a tris-bidentate complex.

Very few other studies have been carried out on the hydrolytic reactivity of tris-bidentate complexes. In some instances it is noted that the complexes are prone to decomposition in basic solution. This applies to strained complexes such as those with seven-membered rings 89) or complexes with many axially disposed substituents 162). 

If [Colenla] is taken as a reference complex for the tris-bidentate complexes, one observes that the unstrained complexes with five-membered rings have rather similar reduction potentials. The complexes of [Colpnlal , in acetone at least, become progressively easier to reduce with increasing ob content. The reduction potential of [Co(t-menlal is 450 mV more positive than [Co(en)3], indicating the destabilizing effect that axially oriented substituents have on the Co(III) complex. 

Tris(bidentate) complexes constitute a general family for which a useful, unambiguous convention has been developed based on the orientation of skew lines which define a helix. 

The twist boat form is chiral and there are two enantiomeric conformations. The chirality can be defined in the same way as a cobalt-ethylenediamine ring and designated as S and X. The third form, a boat form cannot be accommodated to form a tris(bidentate) complex and, in fact, such a structure has not yet been reported. Niketii and Woldbye showed that there exist 16 possible conformers of the [M(tn),] system for each of the absolute configurations A and A, in which they adopt the three stable configurations chair andS- and X-skew boat forms (32). 

The circular dichroism spectra in the region of the first absorption band of the tris-bidentate complex ions having six-membered chelate rings sure known to be particularly sensitive to experimental conditions. For example, the CD spectrum of A-lel -[Co(R,R-ptn) ]C1, in an aqueous solution shows two peaks A = -O.589, 522 nm A = - 0.104, 462.5 nm, whereas that of A-lel,-[Co(R,R-ptn) ](CIO,) in an aqueous solution gives a negative peak (A , = -O.587T at 518 nm (41). The solid state CD differ from the solution CD and the solution CD are sensitive to the temperature of measurement and are affected by the presence of oxo anions (42, 43, 44). Table V lists the lowest frequency CD spectra of tris-diamine cobalt(III) complexes in the CT region. 

The solution circular dichroism spectra also reveals R(AO, giving the sum, R(T.) = R(E) + R(A ). A comparison of the single crystal and the solution CD spectra indicates that R(E) has minor rotatory strength of -0.16 Dp and R(A2) +0.32 DP (72). This result supports the observed relation between the relative magnitudes of R(E) and R(A2) and the arrangement of non-ligating atoms in tris-bidentate complexes (Table VII). The polar capping of A -(+). g -... 

Stereochemical Correlations in the Circular Dichroism of d—d and Charge-Transfer Transitions Applications to Tris(bidentate) Complexes... 

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