Complex formation between ATP (adenosine 5'-triphosphate) and tn2COIII(aq) (tn = trimethylenediamine) and resulting hydrolysis of the ATP to ADP (adenosine 5'-diphosphate), AMP (adenosine 5'-monophosphate), PPi (pyrophosphate), and Pi (orthophosphate) have been examined by means of 31P nmr. With ATP approximately 0.1 M and tn2CoIII-(aq) up to 0.3 M, complex formation was promoted by equilibrating solutions for a period at pH 4, after which hydrolysis was allowed to proceed at each of several pHs in the range 5 to 9 prior to quenching by addition of strong base. With ATP 0.01 M and tn2CoIII(aq) up to 0.08 M, the above procedure was followed in some cases; in other experiments the pH of each ATP/tn2CoIII(aq) solution was adjusted immediately to a value in the range 5 to 9 with the remainder of the procedure as before. In most cases the hydrolysis was at 25 degrees C, but temperature dependence was also examined. The integrals for the beta-phosphorus resonance have been used to analyze for ATP in the quenched solutions; independent measurements of ATP by an enzyme/spectrophotometric method (Bergmeyer) gave similar results. Cobalt to ATP molar ratios up to 1 produce tn2CoIII-ATP as the predominant ATP complex; this 1:1 complex shows no detectable acceleration in hydrolysis compared to free ATP. Cobalt to ATP molar ratios of greater than 1 lead to complexes of type (tn2CoIII)2ATP and (tn2CoIII)3ATP, which exhibit greatly enhanced reactivity towards ATP hydrolysis. At a 2:1 molar ratio (0.1 or 0.01 M ATP), the enhancement in rate is approximately 10(5) at pH 7 where the rate is a maximum (comparison for 25 degrees C); at higher molar ratios the rate enhancements are even greater. The results support the view that effective metal ion catalysis of ATP hydrolysis requires formation of reactive species involving more than one metal ion per ATP.