Ligands anation

Anations of cis- and trans-[Co(en)2(N02)OH2r with NO2 and CN8" are stereoretentive. Use of this fact has been made, to prepare ds- and fra s-[Co(en)2Co(N02)(C204)] containing the monodentate oxalato ligand. Anation of cis-[Co(NH3)4(OH2)2f by NOJ in HNO2-NO2 buffers has been studied kinetically. A two-step reaction was observed, with a second-order rate law applying to each step. No evidence was obtained for any nitrito intermediates in the reaction. 

The distinction between D and mechanisms in polar solvents which can coordinate to the metal is very difficult. Therefore, the replacements of water in [Fe(CN)5(H20)] with various anions as entering ligands (anation processes), as well as with uncharged entering ligands, have been extensively investigated. 

IS further extended by the replacement of X by other anionic or neutral ligands. The inertness of the compounds makes such substitution reactions slow (taking hours or days to attain equilibrium) and, being therefore amenable to examination by conventional analytical techniques, they have provided a continuing focus for kinetic studies. The forward (aquation) and backward (anation) reactions of the pentaammines ... 

Halides are ubiquitous co-ligands for cobalt(III), and are met throughout this review. Anation of (solvent)cobalt(III) complexes by halide has been examined from time to time. An example is substitution of coordinated acetonitrile in [Co(L)(MeCN)2]3+ (L = tetraaza-macrocycle) by Cl-and Br-.1096 A mechanism involving interchange from within tight ion pairs was proposed. Halo-bridged polymeric complexes are well known with both classical and organometallic complexes. 

The most common reaction exhibited by coordination compounds is ligand substitution. Part of this chapter has been devoted to describing these reactions and the factors that affect their rates. In the solid state, the most common reaction of a coordination compound occurs when the compound is heated and a volatile ligand is driven off. When this occurs, another electron pair donor attaches at the vacant site. The donor may be an anion from outside the coordination sphere or it may be some other ligand that changes bonding mode. When the reaction involves an anion entering the coordination sphere of the metal, the reaction is known as anation. One type of anation reaction that has been extensively studied is illustrated by the equation... 

A unique type of anation occurs when the loss of water is followed by coordination of a ligand that is already part of the coordination sphere of another metal. A reaction of this type is... 

For the low-spin t2g aqua ions [Ru(H20)6]2+, [Rh(H20)6]3+, and [Ir(H20)e]3+ a d-activation mode would a priori be predicted. The approach of a seventh water molecule towards a face or edge of the coordination octahedron is electrostatically disfavored by the filled t2g orbitals which are spread out between the ligands. Rate constants for anation reactions of Cl-, Br-, and I- on [Ru(H20)e]2+ are very similar, indicating identical steps to reach the transition state, namely the dissociation of a water molecule (130). An extension of this study to a large variety of ligands demonstrated clearly that the rate determining... 

Kinetic studies of aquation of dinuclear [ traras-PtCl(NH3)2 2 (p-NH2(CH2)6NH2)]2+ established rate constants for the loss of the first and second chloride ligands (7.9 x 10-5 and 10.6 x 10-4s-1), and for the reverse anations (1.2 and 1.5M-1s-1). Reactivities here are very similar to those in analogous mononuclear systems [Pt(amine)3Cl]+ (204). A kinetic and equilibrium study of axial ligand substitution reactions... 

Likewise the two anation rate constants for the reverse reaction are almost identical. Clearly the two terminal platinum atoms are very well insulated from each other by the long alkyl chains of the bridging ligands (207). 

On the general question of the anation reaction of the aquo complex by various anions, there is a little more information. In particular, of course, there are a few cases where one may identify rates of the same sort that are discussed in the paper on nonaqueous systems-i.e., rates of interchange between outer and inner sphere ligands. 

A. Photosubstitution reactions can be aquation, anation or ligand exchange. (i) Photoaquation reactions of the type... 

The isomers cis- and trans-[Cr(CN)2(NH3)4]+ as well as m-[Cr(CN)(NH3)4DMS0](G04)2 have also been synthesized by the anation reactions shown in Scheme 29.325 While anation in DMSO is accompanied by stereochemical change, the reaction in H20 is stereoretentive. The dicyano complexes undergo H+-assisted thermal aquation, involving the successive loss of the CN- ligands. The trans complex is about ten-fold more reactive in the first step than the cis, an observation attributed to the trans labilizing effect of CN-. 

Because of the inertness of Co(III) and Cr(III) complexes, their substitution reactions were the first among those of octahedral complexes to be extensively studied. Most evidence supports the fd mechanism for substitution in Co(fll) complexes. First, there is little dependence of reaction rates on the nature of the incoming ligand, if bond making were of significant importance, the opposite would be expected. Data are presented in Table 13.4 for the anation reaction of penta-ammineaquacobaltdll) ... 

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