Unidentate metal complexes

Only one tautomeric form will give the chelate directly. The other might form the chelate via the unidentate metal complex. A biphasic reaction might be anticipated. 

Placing a proton on the X group presumably weakens the Cr-X bond. The enhanced lability is due to a reduced enthalpy of activation, compared with that associated with the step. Normally, the removal of unidentate ammonia or amine ligands from metal complexes is not accelerated by acid, since the nitrogen is coordinately saturated. This situation changes when we consider multidentates (Sec. 4.4.2). 

Multiple metal-metal bonds have since been found in Re and many other transition metal complexes [41]. The structure of the unsupported Re2 unit (2) is shown in Chart 1. Bonds to the ligands in the equator around each metal may be staggered or eclipsed depending on electronic structure, and solvent or unidentate ligands may bind weakly at the axial positions. In some cases, polydentate ligands may bridge the metal-metal-bonded unit (3). 

The equilibrium constant for the formation of a metal complex is known as its stability constant. (Some authors, however, present the datum as its reciprocal, the instability constant of the complex, by analogy with the dissociation of a weak acid.) There are two kinds of stability constants stepwise (Ad, Ad, Ad,..., Kn) and overall (/ ). We will assume that there are six aqua ligands to be replaced by some other unidentate ligand Xx, in an aqueous solution of Mm+ ... 

Table 9 Stereochemical Parameters for Some Octahedral Transition Metal Complexes, [M(unidentate...

As unidentate ligands, carboxylates are expected to (i) lose the equivalence of the two carbon- oxygen bonds found in the anion and (ii) have one metal-oxygen distance considerably shorter than the next shortest M—O contact. Lithium acetate dihydrate exemplifies this14 with C—O distances of 133 and 122 pm and Li—O distances of 227 and 257 pm. Most examples of unidentate carboxylate complexes have this classical configuration of M(0—C) and C=0 respectively so certainly the presence of features (i) and (ii) unambiguously determine this mode of coordination. 

Dicarboxylic acids metal complexes, 443 bridging systems, 444 chelate coordination, 443 naturally occurring, 962 reactivity, 446 synthesis, 446 tetradentate bridging, 444 unidentate, 443... 

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 metal complexes of QDO and PDO have been the subject of some studies since the middle of the 20th century [67]. In these cases, the hetero cycle N-oxidc has acted either as a unidentate N-oxidc oxygen ligand or as a chelating or bridging bidentate ligand when heterocycle substituents were involved in the coordination processes. A wide variety of metals have been used in the coordination reactions, among Cr(III), Mn(ll), Fe(III), Co(II), Ni(Il), Cu(II), and Zn(ll). 

The most facile syntheses of diolefin complexes are those in which the diolefin reacts with a coordinatively unsaturated metal complex without ligand displacement. Thus a unidentate norbornadiene complex of iron, [Fe(f -Cp)(CO)2( 7 -NBD)][BF4], can be prepared by treating [Fe( j -Cp)(CO)2]2 with xs Ph3C[BFJ, followed by quenching with cycloheptatriene prior to addition of norbornadiene . Tetracoordinate complexes of Ir(I) undergo diolefin addition ... 

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