The cobalt(II) urease model complex [Co-2(mu -OAc)(3)(urea)(tmen)(2)][OTf] (2) prepared from the cobalt model hydrolase [Co-2(mu -H2O)(mu -OAc)(2)(OAc)(2)(tmen)(2)] (1) undergoes facile reaction with acetohydroxamic acid (ARA) to give the monobridged hydroxamate, complex [Co-2(mu -OAc)(2)(mu -AA)(urea)(tmen)(2)][OTf] (3) while 1 gives the dibridged hydroxamate complex [Co-2(mu -OAc)(mu -AA)(2)(tmen)(2)][OTf] (4). The structures and Co-Co distances of the hydroxamate derivatives of 1 and 2 are very close to those of their nickel analogues and suggest that hydroxamic acids can also inhibit cobalt-based hydrolases as well as inhibiting urease 1 also reacts, with glutarodihydroxamic acid (gluH(2)A(2)) to eliminate hydroxylamine with formation of [CO2(mu -OAC)(2){mu -O(N) (OC)2(CH2)(3)}(tmen)(2)][OTf] (5), the structure of which is very close to that of its, nickel analogue. Both 1 and 3 show weak antiferromagnetic coupling. Oxidation of 1 with H2O2 gives three dicobalt(M) hydroxy complexes (7-9), the first of which [Co-2(mu -OAc)(2)(OAc)(2) mu -OH)(tmen)(2)][OTf] (7) contains a bridging hydroxyl and the second [CO2(mu -OAc)(2)(OAc)(mu -OH)(OH)(tmen)(2)][OTf] (8) containing both a bridging and terminal hydroxyl, while the third [Co-2(mu -OAc): (OAC)(2)(mu -OM)(2)(tmen)(2)][OTf] (9) contains, two bridging OH groups with mixed-valence Co(II)/(Co(III) intermediates.