A critical issue in Cognitive Radio Networks is the interference generated by secondary users when they simultaneously access and transmit over a spectrum portion reserved to licensed users. In this paper, we propose a method that allows to decide whether a pair of unlicensed users can communicate over a licensed spectrum without causing harmful interference at the primary receiver. The proposed solution is fully distributed and is based on opportunistic information acquired via simple CTS/RTS messages exchanged in the system. We show that the solution can be applied when both primary and secondary users are equipped with multiple-antennas. In this way on one side we achieve the coexistence, when possible, of the two kind of users and on the other side, for both of them, we are able to exploit the MIMO paradigm and increase the transmission performance. A result of the proposed scheme is that the access of the secondary nodes, when feasible, is done with transmission powers that are suitably dimensioned in order to not deteriorate the primary users Signal-to-Interference-plus-Noise-Ratio (SINR). To show this we provide a mapping of the areas in the transmission space that are most likely to produce high levels of interference at the primary receiver. From these maps the effects of the interference management and the advantages of the spatial diversity can be appreciated.