Octahedral complexes polydentate ligands

Complexes with coordination number 6 tend to be octahedral those with coordination number 4 are either tetrahedral or square planar. Polydentate ligands can form chelates. 

There are more examples of a second type in which the chirality of the metal center is the result of the coordination of polydentate ligands. The easiest case is that of octahedral complexes with at least two achiral bidentate ligands coordinated to the metal ion. The prototype complex with chirality exclusively at the metal site is the octahedral tris-diimine ruthenium complex [Ru(diimine)3 with diimine = bipyridine or phenanthroline. As shown in Fig. 2 such a complex can exist in two enantiomeric forms named A and A [6,7]. The bidentate ligands are achiral and the stereoisomery results from the hehcal chirality of the coordination and the propeller shape of the complex. The absolute configuration is related to the handness of the hehx formed by the hgands when rotated... 

In contrast to the facile reduction of aqueous V(III) (—0.26 V versus NHE) [23, 24], coordination of anionic polydentate ligands decreases the reduction potential dramatically. The reduction of the seven-coordinate capped-octahedral [23] [V(EDTA)(H20)] complex = —1.440 V versus Cp2Fe/H20) has been studied extensively [25,26]. The redox reaction shows moderately slow electron-transfer kinetics, but is independent of pH in the range from 5.0 to 9.0, with no follow-up reactions, a feature that reflects the substitutional inertness of both oxidation states. In the presence of nitrate ion, reduction of [V(EDTA) (H20)] results in electrocatalytic regeneration of this V(III) complex. The mechanism was found to consist of two second-order pathways - a major pathway due to oxidation of V(II) by nitrate, and a minor pathway which is second order in nitrate. This mechanism is different from the comproportionation observed during... 

Finally, polydentate ligands can affect the geometry of a complex merely as a result of their own steric requirements. For example, we find some tetradentate ligands such as tris(2-dimethylaminoethyl)amine, [Me6tren = ((CHj NCHjCH N], form only five-coordinate complexes (Fig. 12.10), apparently because the polydentate ligand cannot span a four-coordinate tetrahedral or square planar complex and cannot conform ( fold ) to fit a portion of an octahedral coordination sphere. 

The importance of connectivity in polydentate ligands has been demonstrated by Dahlenberg.193 The classic QP ligand of Venanzi, P(o-PPh2C6H4)3, containing four phosphorus donor atoms, forms a trigonal bipyramidal complex with iron(II), [FeX(QP)]X (X = Cl, Br, I).194 When flexibility is allowed between the phosphorus donor atoms in P(CH2CH2CH2PMe3)3 a distorted octahedral... 

If we consider the geometry of the [M(en)3]"+ complex ion, we have further possibilities to consider. Whereas an octahedral complex with six identical ligands can only exist in one form, one with three didentate chelating ligands is chiral and can exist as two (non-superimposable) enantiomers (Fig. 2-7). The incorporation of polydentate ligands into a co-ordination compound may well lead to a rather considerable increase in the complexity of the system, with regard both to the stereochemical properties and any related chemical reactivity. 

The ammonia and amine complexes are the most numerous chromium derivatives and the most extensively studied. They include the pure ammine [CrAm6]3+, the mixed ammine-aqua types, that is, [CrAm4 (H20) ]3+ (n = 0-4, 6), the mixed ammine-acido types, that is, [CrAm6 X ](3" )+ (n = 1-4, 6), and mixed ammine-aqua-acido types, for example, [CrAm6 m(H20) Xm](3 m7+ (here Am represents the monodentate ligand NH3 or half of a polydentate amine such as ethylenediamine, and X an acido ligand such as halide, nitrite, or sulfate ion). These complexes provide examples of virtually all kinds of isomerism possible in octahedral complexes. 

It is common practice to consider the traditional Werner octahedral complex ions [MlLNle]" [M = Co(III), Rh(III), Ir(III), Cr(III), Ru(III), Pt(IV) LN = donor atom of unidentate or polydentate ammine or amine] as well as square-planar [M(LN)4p [M = Pt(II), Pd(II)] as kinetically inert compounds. Bound ammonia is generally less labile than bound water, and it has been suggested that this observation can be related to the presence of an extra and exposed electron pair in water. This may make it more sensitive to electrophilic groups in the solvation sheath, which could assist its dissociation from the metal ion (274). If we take the stance of assigning lability as a property of the ligand in such complexes, then ammonia and amines in general can be... 

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