Synthesis of complex compounds with programmed properties with the use of "building blocks" has proven to be very effective, especially in obtaining of molecular magnets, MOFs or optical materials. In the works [1-3] it is shown that structure of such "building block" influence the magnetic properties of complexes with dimeric anions [Cr2(m-OH)2(nta)2]2-and [Cr2(m-OH)(m-Ac)(nta)2]2-. In the present work a possible three-steps mechanism of reaction between the anion [Cr2(m-OH)2(nta)2]2-(I) and isonicotinic acid resulting in formation of a new dimeric complex [Cr2(m-OH)(m-iNic)(nta)2]2-(II) is proposed, on the base of DFT PBE calculations using the quantum-chemical program PRIRODA 06 [4]. Step 1: In aqueous solution the electrolytic dissociation of isonicotinic acidtakes place: R-COOH + H2O=RCOO- + H3O+ Step 2: Interaction of hydronium cation with (I) leads to the Product 1 (Prod1 in Fig. below). The energy barrier of this step is quite small (the calculated value is 3.39 kcal/mol).Step 3: The obtained Prod1 interacts with isonicotinic anion resulting in final complex (II).The energy barrier of this reaction is high enough (the calculated value is 99.58 kcal/mol in Fig. above), which is also confirmed by the conditions of synthesis (T and P).