(M)ixed pyrazole-acetonitrile complexes, both neutral fac-[ReBr(CO)3(NCMe)(pz*H)] (pz*H = pzH, pyrazole; dmpzH, 3,5-dimethylpyrazole; or indzH, indazole) and cationic fac-[Re(CO)(3)(NCMe)(pz*H)(2)]A (A = BF4, ClO4, or OTf), are described. Their role as the only starting products to obtain final pyrazolylamidino complexes fac-[ReBr(CO)(3)(NH-C(Me)pz*-eta N-2,N)] and fac-[Re(CO)(3)(pz*H)(NH-C(Me)pz*-eta N-2,N)]A, respectively, is examined. Other products involved in the processes, such as fac-[ReBr(CO)(3)(pz*H)(2)], fac-[Re(CO)(3)(NCMe)(NH-C(Me)pz*-eta N-2,N)]A, and fac-[Re(CO)(3)(pz*H)(2)(OTf)] are also described. Warming CD3CN solutions of fac-[Re(CO)(3)(NCMe)(pz*H)(2)]A at 40 degrees C gives cleanly the pyrazolylamidino complexes [Re(CO)(3)(pz*H)(NH-C(Me)pz*-eta N-2,N)]A as the only products, pointing to an intramolecular process. This is confirmed by carrying out reactions in the presence of one equivalent of a pyrazole different from that coordinated, which affords complexes where the pyrazolylamidino ligand contains only the pyrazole previously coordinated. When the reactions lead to an equilibrium mixture of the final and starting products, the reverse reaction gives the same equilibrium mixture, which indicates that the coupling reaction of pyrazoles and nitriles to obtain pyrazolylamidino ligands is a reversible intramolecular process. A systematic study of the possible factors which may affect the reaction gives the following results: (a) the yields of the direct reactions are higher for lower temperatures; (b) the tendency of the pyrazoles to give pyrazolylamidino complexes follows the sequence indzH > pzH > dmpzH; and (c) the reaction rates do not depend on the nature of the anion even when a large excess is added. The presence of a small amount of aqueous solution of NaOH catalyzes the reaction. Thus, addition of 0.5-1% of NaOH (aq) to solutions of fac-[ReBr(CO)(3)(NCMe)(pz*H)] (in CD3CN) or fac-[Re(CO)(3)(NCMe)(pz*H)(2)]A (in CD3CN, CD3NO2 or (CD3)(2)CO) allowed the syntheses of the corresponding pyrazolylamidino complexes [ReBr(CO)(3)(NH-C(Me)pz*-eta N-2,N)] or [Re(CO)(3)(pz*H)(NH-C(Me)pz*-eta N-2,N)]A with better yields, more rapidly, and in milder conditions.