In the present work, a convenient and straightforward approach to the preparation of borylated amidines based on the closo‐dodecaborate anion [B12H11NCCH3NHR]–, R=H, Alk, Ar was developed. This method has two stages. A nitrile derivative of the general form [B12H11NCCH3]⁻ was obtained, using a modified technique, in the first stage. On the second stage the resulting molecular system interacted with primary amines to form the target amidine products. This approach is characterised by a simple chemical apparatus, mild conditions and high yields of the final products. The mechanism of the addition of amine to the nitrile derivative of the closo-dodecaborate anion was studied, using quantum‐chemical methods. The interaction between NH3 and [B12H11NCCH3]⁻ ammonia was chosen as an example. It was found that the structure of the transition state determines the stereo‐selectivity of the process. A study of the biological properties of borylated amidine sodium salts indicated that the substances had low toxicity and could accumulate in cancer cells in significant amounts.