Water as a Solvent and Ligand

Aqueous-Phase Organometallic Catalysis, Second Edition 

Copyright 2004 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim 

Classified as a ligand for metal ions, water has decent crystal field splitting properties, standing between oxygen-bound anions and nitrogen donors such as pyri-dines in the spectrochemical series  

The water molecule is a good cr-donor ligand, while jr-backbonding is negligible. For this reason, higher-valent transition metals form the more stable metal complexes, but the nature of the metal by itself is important, too (Table 1). 

For first-row metals (+ 2), an extra destabilization due to electrons in eg orbitals accounts for a ligand labilization. This effect is commonly referred to as crystal field activation energy (CFAE) [1 a—c]. 

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