Using the density functional method with the generalized gradient approximation for the exchange and correlation, we investigated the geometrical and electronic properties of free-standing complexes of Co-n clusters combined with hydrogen-saturated boron-nitrogen (BN) rings [(BN)(3)H-6](m). The Co atoms tend to form a subcluster capped by BN rings that preserve the Co subcluster against the environment and with which they weakly interact. Thus, the Co subcluster is capable of sustaining a noticeable magnetic moment. These facts are relevant for designing grains with localized magnetic moments. We also optimized those TMn[(BN)(3)H-6](n) complexes with n = 1-3 and TM = Fe, Ni, Ru, Rh, and Pd, starting with the ground-state geometry obtained previously for TM = Co, in order to analyze the dependence of the electronic properties with the number of d electrons in the transition-metal atoms.