The delicate regulatory network of nitric oxide (NO) includes the control of its precursor arginine (arg) and its effecting second messenger cyclic guanosine 3 ,5 -monophosphate (cGMP). There are several enzymes involving in the regulation of arg and cGMP. They are arginase (ARG) and phosphodiesterase (type-V PDE), and probably indirectly urease (URA), respectively. These enzymes are metal-dependent hydrolases that utilize water as a co-substrate to hydrolytically destroy the primary substrate arg (by ARG) or cGMP (by PDE), or urea (by urease), respectively. The functions of these enzymes require closely coupled di-metallic centers at their active sites, a di-Mn(II) active site for ARG, a di-Mg(II) or a di-Zn(II) active site for PDE, and a closely coupled di-Ni(II) active site for URA.The objective of this study is to synthesize and characterize aqua-bridged di-metallic compounds that can be used as the chemical, structural and spectroscopic models for the active sites of ARG, PDE, URA hydrolases, and therefore to facilitate the understanding of their structural and functional properties. The prior requirements for these chemical models are (a) they have to be di-metallic, (b) they have to contain a coordinated water molecule as the functional co-substrate, and (c) they have to be assembled by the close mimics of amino acid's side chains.In this study, we have successfully synthesized a class of novel aqua-bridged di-metallic compounds with strict biomimetic ligands. The some structural features of these model compounds are summarised in Figure-1. The two metal ions, either Mn(II) or Mg(II) or Ni(II) or Co(II) are assembled together by a water and two bi-dentate carboxylic bridges, and are sterically enclosed by two terminal imidazoles and one mono-dentate carboxylic group. Imidazole and acetate were used to mimic the side chains of histidine, aspartic acid and glutamic acid, respectively. A detailed examination of the high resolution X-ray structures of these compounds reveals a very intriguing net work of intra- and intermoleular hydrogen bond (H-bond). The terminal monodentate carboxylic groups serve as auxiliary H-bond acceptors for the activation of aqua bridge. Both metal ions are found in high spin states and are antiferromagnetically coupled at low temperature.