Direct synthesis of magnesium amidinates of the general formula [RNC(R1)NR]MgR2 has been performed from appropriate carbodimide and Grignard reagents (R = iPr, Cy, pTol; R1 = Me, nBu; R2 = nBu, Cl, I). Magnesium bisamidinates of the composition [RNC(R1)NR]2Mg(solvent)2 are accessible from [RNC(R1)NR]MgR2 and via the Schlenk-like equilibrium in coordinating solvents. The only isolated amidinatomagnesium halide, preserved in the dinuclear form of {[pTolNC(Me)N-pTol]Mg(THF)}2-μ-(THF)-μ-(Cl)2, has been obtained from the reaction of pTol-N[double bond, length as m-dash]C[double bond, length as m-dash]N-pTol with MeMgCl(THF)n in hexane. The reaction of pTol-N[double bond, length as m-dash]C[double bond, length as m-dash]N-pTol with two equivalents of MeMgI gives an unprecedented dinuclear cyclic adduct {μ-[pTolNC(Me)N-pTol][MgI(OEt2)]2-μ-Me}. The structures of these complexes have been investigated by NMR spectroscopy, sc-XRD and theoretical methods. Selected complexes have been tested as initiators of ring-opening polymerization reactions with various substrates, the copolymerization of oxiranes and CO2 as well as the esterification of lactides.