The preparation of new Co-II-mu-OH-Co-II dimers with the binucleating ligands 3,5-bis{bis[(N'-R-ureaylato)-N-ethyl]-aminomethyl}-1H-pyrazolate ([H(4)P(R)buam](5-), R = Bu-t, Pr-i) is described. The molecular structure of the isopropyl derivative reveals that each Coll center has a trigonal-bipyramidial coordination geometry, with a Co center dot center dot center dot Co separation of 3.5857(5) angstrom. Structural and spectroscopic studies show that there are four hydrogen-bond (H-bond) donors near the Co-II-mu-OH-Co-II moiety; however, they are too far away to be form intramolecular H-bonds with the bridging hydroxo ligand. Treating [Co(2)(II)H(4)P(R)buam(mu-OH)](2-) with acetonitrile led to the formation of bridging acetamidato complexes, [Co(2)(II)H(4)P(R)buam(mu-1,3-OC(NH)CH3)](2-); in addition, these Co-II-mu-OH-Co-II dimers hydrolyze ethyl acetate to form Co-II complexes with bridging acetato ligands. The Co-II-1,3-mu-X'-Co-II complexes (X' = OAc-, [OC(NH)CH3](-)) were prepared independently by reacting [Co(2)(II)H(3)P(R)buam](2-) with acetamide or [CO(2)(II)H(4)P(R)buam](-) with acetate. X-ray diffraction studies show that the orientation of the acetate ligand within the H-bonding cavity depends on the size of the R substituent appended from the urea groups. The tetradentate ligand 3-{bis[(N'-tert-butylureaylato)N-ethyl]aminomethyl}-5-tert-butyl-1H-pyra zolato ([H(2)P(tBu)uam](3-)) was also developed and its Co-II-OH complex prepared. In the crystalline state, [Co(II)H(2)P(tBu)uam(OH)](2-) contains two intramolecular H-bonds between the urea groups of [H(2)P(tBu)ua m](3-) and the terminal hydroxo ligand, [(Pr4N)-Pr-n](2)[Co(II)H(2)P(tBu)uam(OH)] does not hydrate acetonitrile or hydrolyze ethyl acetate. In contrast, K-2[CO(II)H(2)P(tBu)uam(OH)] does react with ethyl acetate to produce KOAc; this enhanced reactivity is attributed to the presence of the K+ ions, which can possibly interact with the Co-II-OH unit and ester substrate to assist in hydrolysis. However, K-2[Co(II)H(2)P(tBu)uam(OH)] was still unable to hydrate acetonitrile.