A new catalytic procedure for atom-transfer radical addition (ATRA) and cyclization (ATRC) reactions is described. The combination of the ruthenium(III) complex [RuCl2Cp*(PPh3)] with magnesium allows these reactions to be performed under mild conditions with high efficiency. It is suggested that magnesium acts as a reducing agent that generates and regenerates the catalytically active ruthenium(II) species. With magnesium as the co-catalyst, various dinuclear ruthenium ethylene and β-diketiminato-ruthenium complexes were tested in ATRA and ATRC reactions. It was found that these complexes in combination with Mg are potent catalysts. Furthermore, the catalytic activity of [CpOsBr2(PPh3)] in conjunction with Mg as the co-catalyst has been evaluated. The Os complex allowed to perform the reactions with similar efficiency as the analogous Ru complex [CpRuCl2(PPh3)]. A novel method for the synthesis of substituted cyclopropanes is described. Olefins are reacted with 1,1'-dichlorides in a Ru-catalyzed atom transfer radical addition process and the resulting 1,3-dichlorides are directly converted into cyclopropanes by reductive coupling with magnesium. This one-pot reaction is applicable to a variety of substrates and can be performed in an inter – or intramolecular fashion. A sequential intermolecular ATRA reaction is reported. By using an excess of the olefin, the ATRA mono adduct can be directly coupled with a second equivalent of olefin to give 1,5-dichlorides. These "double Kharasch" products can be employed in a following dechlorination step to give substituted cyclopentanes. The possibility of performing ATRA reactions in an enantioselective fashion was explored. Ruthenium complexes with different P-chiral phoshines or phosphoramidites were tested for selectivity in combination with magnesium.