In the present work, the synthesis and characterization of new metal-rich borides of the Ti3Co5B2- and of the perovskite-type structure is reported. The samples were prepared by arc melting the elements. The products were characterized by powder and single crystal X-ray diffraction as well as energy and wavelength dispersive X-ray spectroscopy. Suitable samples were investigated by SQUID magnetometry. The experimental works were completed by quantum chemical calculations based on density functional theory. With A3Ru5B2 (A = Nb, Ta) and A3–xRu5+xB2 (A = Zr, Hf) four new ternary representatives of the Ti3Co5B2-type structure have been synthesized. Zr2.86(5)Ru5.14(5)B2 and Hf2.83(2)Ru5.17(2)B2 are the first ternary phases of the Ti3Co5B2 structure type, which contain mixed occupied Wyckoff sites. Quantum chemical calculations show that these site mixings stabilize the compounds by increasing the valence electron count. Independent of the experimental works a technique was tested to predict the stability of ternary phases of the Ti3Co5B2-type structure by ab-initio calculations of the lattice vibrations. Theory and experiment are in good agreement. As a result, 24 potential new phases of the Ti3Co5B2-type structure could be predicted as dynamically stable. Quaternary substitutional variants of the Nb3Ru5B2 compound were synthesized and characterized, in which niobium was partially replaced by 3d metals. The quaternary phases crystallize in three structural variants, namely (Nb2–xScx)NbRu5B2, Nb3–xMxRu5B2 (M = Ti, V), and Nb2+xM1–xRu5B2 (M = Cr, Mn, Fe, Co, Ni). These variants can be distinguished by different site mixings on the tetragonal and pentagonal prismatic coordinated sites. Theoretical investigations show that competing electronic and size effects are responsible for these mixings. In Ti3–xRu5–yIryB2+x, titanium was successively replaced by boron on the tetragonal prismatic coordinated site. Ti2Ru2.8(2)Ir2.2(2)B3 is the first boride in which boron occupies a tetragonal prismatic environment. In addition, these phases are the first quaternary representatives of their structure type, which show a mixing of 4d and 5d elements (Ru/Ir) on the cobalt sites, with ruthenium preferring the 2c site, while iridium is preferably found on the 8j site. In the quinary Sc2FeRu5–xIrxB2 series, the first borides were synthesized, in which such a 4d/5d mixture exists and whose magnetic properties were studied. The incremental increase of the valence electron count by increasing the iridium amount led to drastic changes in the magnetic properties. Iridium-rich phases show dominant ferromagnetic interactions, while ruthenium-rich phases have dominating antiferromagnetic interactions. The phases with x = 2 and 3 represent the first hard magnetic borides of transition metals. In the second part of this thesis, ternary and quaternary phases of the Ti2Rh6B-type structure were investigated. The Ti2Rh6B-type structure is a 2x2x2 superstructure of the normal perovskite-type structure. Based on Zr2Ir6B and Sc2Ir6B, with the latter discovered in this work, quaternary substitutional variants were synthesized. Zr2Ir6–xPdxB (x = 1, 2), Zr2Ir4Ni2B, Sc2Ir6–xPdxB (x = 1, 2, 3, 4, 5), and Sc2Ir6–xNixB (x = 1, 2, 3, 4, 5) crystallize in the Ti2Rh6B structure type. The occurrence of this structure type is strongly dependent on the valence electron count. All phases have a valence electron count between 63 and 68. Quantum-chemical calculations on Zr2Ir6B and the successful application of a rigid band model show that a large pseudogap in the density of states coincides with the range of stable phases and is responsible for the upper limit of 68 valence electrons.