Bridging ligand remote attack

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 ... 

A related example of inner-sphere reaction is shown in reaction (5), where an additional mechanistic subtlety appears.10 As shown by the products, inner-sphere electron transfer also occurs but now by remote attack in which the sites of bridging ligand binding to the reductant and oxidant are at different atoms on the bridging ligand. 

The preparation of europium metasilicate hydrate from solutions of europium bromide and sodium metasilicate has been described.The kinetics and mechanism of the reduction of thiocyanato- and isothiocyanato-penta-amminecobalt(iii) ions by europium(ii) in acid solution have been discussed in terms of europium(ii) attack on the ambidentate bridging ligand at the end remote from the cobalt centre. The differences in the activation enthalpies for the reduction of the complexes were attributed to (a) differences in enthalpy of formation of the precursor complex Eu - X - Co (NH3)5, and (b) ease of stretching of the Co—S or Co—N bond in the precursor complex. The low-temperature Mossbauer spectrum of EUH2 suggested the covalent transfer of electron density. to the metal 6s orbital to be more marked in EuH2 than EuO. ... 

Some basic issues are raised by studies which have been done with NCS as the bridging ligand. When (NHjjjCoNCS is reduced either by Cr(H20)6 or Co(CN)5 , - attack is quantitatively at the remote position. However, when (NH3)5CoSCN + is reduced by both... 

The systems listed in Table 6.7 are examples of what is called remote attack, in which the Cr(II) is attached at a ligand atom considerably removed from the Co(III). In such systems, the rate constant is sensitive to the nature of the remote substituent through its effect on K. The rate constants also correlate generally with the ease of reduction of the bridging ligand, which seems to reflect an influence on k. ... 

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