The tripodal ligand N[CH2CH2NHC(O)NHC(CH3)(3)](3) ([H(6)1]) was used to synthesize a series of monomeric complexes with terminal hydroxo ligands. The complexes [Co(II/III)H(3)1(OH)](2-/1-), [Fe(II/III)H(3)1(OH)](2-/1-), and [Zn(II)H(3)1(OH)](2-) have been isolated and characterized. The source of the hydroxo ligand in these complexes is water, which was confirmed with an isotopic labeling study for [Co(III)H(3)1(OH)](1-). The synthesis of [M(II)H(3)1(OH)](2-) complexes was accomplished by two routes. Method A used 3 equiv of base prior to metalation and water binding, affording yields of less than or equal to 40% for [Co(II)H(3)1(OH)](2-). When 4 equiv of base was used (method B), yields ranged from 50% to 70% for all of the M(II)H(3)1(OH)](2-) complexes. This improvement is attributed to the presence of an intramolecular basic site within the cavity, which. scavenges protons produced during formation of the M-II-OH complexes. The molecular structures of [Zn(II)H(3)1(OH)](2-), [Fe(II)H(3)1(OH)](2-), [Co(II)H(3)1(OH)](2-), and [Co(III)H(3)1(OH)](1-) were examined by X-ray diffraction methods. The complexes have trigonal bipyramidal coordination geometry with the hydroxo oxygen trans to the apical nitrogen. The three M-II-OH complexes crystallized with nearly identical lattice parameters, and each contains two independent anions in the asymmetric unit. The complexes have intramolecular H-bonds from the urea cavity of [H(3)1](3-) to the coordinated hydroxo oxygen. All the complexes have long M-O(H) bond lengths (> 2.00 Angstrom) compared to those of the few previously characterized synthetic examples. The longer bond distances in [M(II)H(3)1(OH)](2-) reflect the intramolecular H-bonds in the complexes. The five-coordinate [Zn(II)H(3)1(OH)](2-) has an average Zn-O(H) distance of 2.024(2) Angstrom, which is similar to that found for the zinc site in carbonic anhydrase H (2.05(2) Angstrom). The enzyme active site also has an extensive network of intramolecular H-bonds to the hydroxo oxygen. [Co(II)H(3)1(OH)]2- and [Fe(II)H(3)1(OH)]2- have one-electron redox processes at -0.74 and -1.40 V vs SCE. Both complexes can be chemically oxidized to yield their corresponding M-III-OH complexes. [Co(III)H(3)1(OH)](1-), with an S = 1 ground state, is a rare example of a paramagnetic Co-III complex.