The interactions of the beryllium(II) ion with the cyclopentadienyltris(diethylphosphito-P)cobaltate monoanion, L-, have been investigated, in aqueous solution, by synthetic methods, potentiometry, ESMS, and H-1, P-31, and Be-9 NMR spectroscopy. L- has been found able to displace either two or three water molecules in the beryllium(II) coordination sphere, to form mononuclear, dinuclear, and trinuclear derivatives, in which the metal ion is pseudotetrahedrally coordinated. The species [BeL(H2O)](+) and [Be2L2(mu -OH)]+ have been identified in solution while complexes of formula BeL2 and [Be3L4](ClO4)(2) have been isolated as solid materials. The species [BeL(OPPh2)](+), closely related to [BeL(H2O)](+), has been characterized in acetone solution and isolated as tetraphenylborate salt. The structure of the unusual trimeric complex [Be3L4](2+) has been elucidated by an unprecedented 2D Be-9-P-31 NMR correlation spectrum showing the presence of a single central beryllium nucleus and two equivalent terminal beryllium nuclei. The three beryllium centers are held together by four cobaltate ligands, which display two different bonding modes: two ligands are terminally linked with all the three oxygen donors to one terminal beryllium, and the other two bridge two metal centers, sharing the oxygen donors between central and terminal beryllium atoms.