Complexes of the type M AA

The two optical forms designated A and A are shown in Fig. 7.4. Since the bidentate ligand is symmetrical, geometrical isomerism cannot arise. 

In (a) the A configuration is shown and in (b) the mirror-image, A configuration, is represented. The designations A and A were previously termed d and /, or D and L forms (not necessarily respectively). 

Stereochemical rearrangement of the two isomers can occur by intermolecular and intramolecular mechanisms. These take the following form  

This mechanism is supported by identical dissociation and racemization rate constants. This further implies either that the bis species M(AA)2 is racemic as formed, or that it may racemize (by a cis-trans change, or by a dissociative or intramolecular path) more rapidly than it re-forms iris in the dynamic equilibrium (7.23). Identical activation parameters for the dissociation (to the bis species) and racemization in aqueous acid (Table 7.5) and other solvents of Nifphen) and Ni(bpy)3 indicate that these ions racemize by an intermolecular mechanism. This is the only such example for an M(phen) + or M(bpy) + species (see Table 7.5) although recently it has been observed that Fe(bps)3 (bps is the disulfonated phenanthroline ligand shown in 13, Chap. 1) but not Fe(phen)3+ also racemizes predominantly by a dissociative mechanism in water. For the other tr/s-phenanthroline complexes (and for Fe(bps)3 in MeOH rich, MeOH/HjO mixtures ) an intramolecular mechanism pertains since the racemization rate constant is larger than that for complete dissociation of one ligand, Table 7.5. 

There are basically two types of intramolecular rearrangements which are not easy to distiguish experimentally. In one type, intramolecular twisting occurs without any metal-donor atom bond rupture. In the other, rupture of one metal-ligand bond of the chelate occurs to give a five-coordinated intermediate. 

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A number of complexes of the type M(AA)2X2 are neutral and soluble in organic solvents. They are stereochemically non-rigid on an nmr time scale, that is they display nmr line broadening or collapse in accessible temperature ranges. Concommitant solvolysis problems can usually be avoided. The behavior shown is typified by the complexes Ti(P-diketone)2X2. The cis-configuration is invariably the stable one in solution, e.g. 6... 

The stereoisomerism in some asymmetric tetrahedral complexes of the type M(AA,)2 has been characterized by nmr. - Tetrahedral complexes of the type M(AA,)2 are chiral, but their lability requires nmr methods for measurement of the rates of interconversion of the enantiomers. The pioneering work of Holm and his collaborators has been consolidated by Minkin and his Russian group and Zn, Cd and, more recently, Ni chelates of the type M(AA[)2 have been extensively studied. 

A complex of the type [M(AA)2X2] is optically active giving a suitable example, comment on the structure of such complex. 

Some very interesting structural features exist for neutral jS-keto-enolate complexes (18) of the type M(AA)nLm. Adducts with nitrogenous bases or water have yielded a 5-coordinate irregular Zn(AA)2 H20 complex and trans 6-coordinate cobalt(II) and nickel (II) complexes M(AA)2 2B, with B = H2O or pyridine. The vanadyl complex, VO(AA)2, contains a square pyramid of oxygens about the metal, while U02(AA)2-H2O shows a pentagonal bipyramidal structure. The anhydrous material is dimeric. 

Unlike four-coordinated systems, six-coordinated complexes afford many examples of optical isomerism. These are very common among compounds or ions of the type [M(AA)3]. For example, the optical isomers of trioxalatochromate(lll) are complexes XX and XXL... 

Miscellaneous.—twisting rather than a bond-rupture mechanism has been established by n.m.r. line-broadening studies for the racemization of Ti complexes of the type [Ti(AA)(BB)2] [BB = acac or 3-acetylpentane-2,4-dionate ion AA = MeaC(0-)(CH2) C(0-)Me2, n=0 or 1]. ... 

Raubenheimer HG, Kruger GJ, Lombard AA, Linford L, Vdjoen JC. Sulfar-containing metal complexes. 12. Reactions of a-thio carbanions with carbene complexes of the type [M(CO)5 0(alkyl)Ar ] and with the carbyne [(f -MeC5H4)-Mn(CO)2(CPh)][BCl4]. Organometallics. 1985 4 275-284. 

In more recent years attention has turned from studying the equilibria of binary metal-amino acid complexes to that of ternary complex formation in aqueous media, particularly to complexes of the type (aa)—M"—L, where L is some other ligand or a different amino acid to (aa), and M" is a Idnetically labile metal ion. Ternary complexes involving kinetically inert metal ions, e.g. Co" and Pt", are more well known since they can be separated from mixtures and studied in isolation. Such is not the case with the labile systems. Because of the facile nature of their equilibria they must be studied in situ (claims regarding the separation of labile spedcs by chromatographic procedures... 

In this type of complex the helicity is determined by the relative position of the lines (AA) and (BB). In the case shown in Figure 2.53, the absolute configuration of the complex will therefore be OC-6-(A). This rule that we have just defined can also be applied to complexes of the type (symmetry C2) and M(L 2) b (symmetry C ). 

Mechanisms leading to geometrical isomerization in complexes of this type resemble those already discussed with M(AA,)3 and M(Li)(L2), Secs. 7.6.2 and 7.6.3. As well as twist mechanisms, dissociation of either the unidentate ligand or one-ended dissociation of the... 

The marked dependence of 7( P—M— P) on stereochemistry for complexes of the platinum group metals has been used in NMR spectroscopy for several years 109,115). For methyl or tert-h xty tertiary phosphines, when the two phosphines are mutually trans then a triplet pattern results, but if the two phosphines are mutually cis, then a doublet pattern results as a consequence of the spectrum being of the AA X X type and the dependence of V( P—M— P) on stereochemistry. Exactly the same behavior is found in NMR spectroscopy with the advantage that the technique is far more versatile and is not normally troubled by resolution problems. Thus for cr-[RhCl3(CO)(PBu"2Ph)2] triplet patterns have been observed for six of the eight different carbon atoms in the tertiary phosphine ligand 164). When —M—is... 

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