Metal linkage isomers

Linkage isomerism This is a special type of structural isomerism in which the differences arise from a particular ligand which may coordinate to a metal ion in more than one way. In Table 1-3 we indicated that a ligand such as thiocyanate could bond to a metal through either the nitrogen or the sulfur atom, and the complex ions [Co(NH3)5(ACS)]2+ and [Co(NH3)5(5CN)]2+ are related as linkage isomers. 

Another type of isomerism displayed by coordination complexes Is based on the bonding of the ligand. Linkage isomers occur when a ligand can bond to a metal using either of two donor atoms. Figure 20-10 shows the two... 

Linkage isomers occur when a ligand is capable of bonding to a metal through either of two donors. In... 

We have already used the HSAB principle as it applies to linkage isomers in metal complexes. This application to bonding site preference can also be used to show the behavior of other systems. For example, the reactions of organic compounds also obey the principles when reacting with nucleophiles such as SCN- or N02 ... 

Many studies have been carried out on this reaction. Some of the unusual features of this reaction will be described in Chapter 20. Note that in writing the formulas for linkage isomers, it is customary to write the ligand with the atom that functions as the electron pair donor closest to the metal ion. Special consideration will be given in Chapter 20 to the behavior of cyanide complexes because CN-is also an ambidentate ligand. 

There are basically two types of mechanisms which can explain rearrangement of linkage isomers. In the intermolecular mechanism, the ligand giving rise to linkage isomerism (L-L, with two donor centers) dissociates from and recombines with the metal M ... 

If the bridging ligand contains only one atom (e.g.. Cl ). both metal atoms of the complex must be bound to it. However. If the bridging ligand contains more than one atom (e.g.. SCN ). the two metal atoms may or may not be bound to the same bridging-ligand atom (see Problem 13.30). Hie two conditions are called adjacent and remote attack, respectively. A remote attack may lead to both linkage isomers ... 

One area of silver chemistry that has received much recent attention is the synthesis of multinuclear aggregates.315 Anionic metal dithiolato complexes such as K M(S2C202)2 were found to form adducts with silver(I) phosphines. In these complexes, the inert cations (K+) that accompanied the metallothio anions could be readily substituted by coordinatively unsaturated silver phosphines Ag(PR3)2. For dithiooxalto complexes this led to the isolation of a remarkable range of linkage isomers (34)-(37) 316,317... 

Electronic effects become apparent in the M—NCS/M—SCN linkage switches observed in a series of Pd11 (iso)thiocyanate complexes. Ligands positioned trans to the pseudohalide and that are suited to accept electron density from the metal into empty orbitals (backbonding) stabilize the Pd—NCS linkage isomer. However, this rationale is contradicted by the trend in Co—(NCS) bonding in a series of cobalt complexes (see ref. 204b for a review). 

The synthesis of both 0- and N-bonded linkage isomers of the [Rh(NH3)5(urea)]3+ complexes has been described505 and their reactions studied in detail. The formation of [Rh(NH3)6]3+ from [Rh(NH3)5(NH2CONH2)]3+ represents the first observation of non-enzymatic enhancement of the decomposition of urea at low pH. Comparison of the rate constant (k = 2.0x 10-5 s 1) with that observed for the slow decomposition of urea at low pH (equation 33 k = 6xlO 10s 1 at 25 °C) indicates that coordination to the metal centre increases the reactivity by some 3 x 104-fold. 

M—ONO) and nitro (M—N02) isomers of some acidoammine-cobalt(III) complexes. Many years later Adell2 made extensive studies of the kinetics of isomerization in these systems. However, it was not until 1962 that linkage isomers of other metal complexes of this type were reported. ... 

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