Water as a bridging ligand

Table 2 Water as a Bridging Ligand Between Metal Ions...

D. L. Toppen and R. G. Linck (Inorg. Chem., 1971,10, 2635) have recently discussed the role of co-ordinated water as a bridging ligand in redox systems and have compared the relative rates of reduction of various complex ions with Cr" and V". For outer-sphere reactions, it appears that fc(Cr")VAc(V") a 0.02. 

The role of co-ordinated water as a bridging ligand in redox systems has been discussed, the data for the reaction... 

Clusters substituted with water or water fragments are known. The 58-electron anion [Re4(/ -H)3(/ 3-H)(CO)i2] (Fig. 8) reacts reversibly with a water molecule, which is probably coordinated as a bridging ligand. This complex is unstable above 270 K and gives a derivative with a triply bridging OFF, presumably via a tetrahy-dridic intermediate substituted with a water molecule coordinated to only one metal center. ... 

The exchange of water for ammonia in the inner coordination shell of Co is a rapid process and is not included in the mechanisms.) The intermediate compound actually represents two species, one with the electron to be transferred associated with Cr and the other with the electron associated with Co. The role of the solvent molecules in outer-sphere electron transfers is not known, although it has been suggested that solvent may play a role as a bridging ligand. 

This approach emphasizes the role of water as a source of 0X0 ligands. It is important to note that crystalline tetranuclear 4B and undecanuclear 11 aggregates form only if the amounts and concentrations of water vary within certain narrow limits. Perhaps the number of potentially bridging oxo and hydroxo groups that can form in solution determine the nuclearity of the aggregates. 

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