Two new crystalline modifications of cobalt tris-(β-aminoethylate) with composition [Co(OCH2CH2NH2)3]⋅5,44H2O (II) and [Co(OCH2CH2NH2)3]⋅3H2O (III) have been separated from basic solutions at different pH. On the basis of physico-chemical investigations, it has been established, that in compounds II monoethanolamine coordinated to cobalt atom in the facial mode similar to compound I described in the periodic literature [4]. It has been found that this is a pseudo-polymorph modification of compound I, the only difference being the different number of water molecules, meanwhile compounds is the meridional isomer. As a result of the synthesis condition modifications the compound [Co(HOCH2 CH2NH2)2(OCH2CH2NH2)](NO3)2 (IV) has been isolated, a specie that contains two protonated and one deprotonated monoethanolamine molecules. Biological activity study of the synthesized compounds revealed that the algal biomass, obtained in nutritive media with coordination compounds as additional agents, is differently affected and depends on structural characteristics of cobalt complexes.