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This work is dedicated to the investigation on intermetallic compounds of beryllium and magnesium with late transition metals. By conducting fundamental research, the objective is to unveil novel intermetallic compounds possessing distinctive chemical bonding and interesting physical properties, with the aim to identify potential semiconductor materials for further thermoelectric applications. Following the recent discovery of the semiconducting properties of Be5Pt, it was initially hypothesised that replacing Be with Mg, while preserving the semiconducting properties, could enhance the widespread applicability of said material considering the lower toxicity of magnesium compared to that of beryllium. The study of the already well-investigated Mg–Pt system, revealed that a phase with composition Mg5Pt does not exist, instead two new phases, Mg3Pt2 and Mg29-xPt4+y (x = 0.47, y = 0.07), were discovered. Mg3Pt2 can be synthesised by direct reaction of its constituent elements or through spark plasma sintering (SPS) using MgH2 and PtCl2 as precursors. An in-depth analysis of the chemical bonding in Mg3Pt2 allowed to conclude that belonging to the same structural prototype (Eu3Ga2) does not necessarily indicate the same chemical bonding scenario. The isolation of single crystals for diffraction experiments combined with atomic-resolution transmission electron microscopy (TEM), enabled the determination and examination of the crystal structure of Mg29-xPt4+y, the existence of which had previously only been hinted on the basis of powder diffraction or metallography analysis. The investigation of the chemical bonding in Mg29-xPt4+y revealed a unique characteristic, that distinguishes it from other complex intermetallic compounds (CMAs). Notably, a spatial separation of regions with different bonding features was observed, explaining a distinctive mixed Mg/Pt site occupancy near the origin of the unit cell. Beryllium has garnered considerable interest due to its versatile behaviour when combined with other elements. These combinations can give rise to materials exhibiting distinctive physical properties and intriguing chemical bonding characteristics. However, the high toxicity associated with beryllium and its compounds as well as difficulties in characterisation, e.g. very low X-ray scattering power, has limited systematic investigations of Be–based intermetallic compounds. This comprehensive study focuses on the binary Be–Ru system. The redetermination of the Be3Ru crystal structure, showed that it crystallises with TiCu3–type structure. The crystal structure can be derived by ‘colouring’ the hexagonal closest packing of spheres characteristic for large groups of intermetallic compounds. Be3Ru exhibits diamagnetic properties, and its metallic electrical resistivity is in good agreement both with electronic structure calculations and experimental measurements. Be2Ru crystallises with Fe2P–type structure, instead of the previously reported MgZn2–type one. Detailed investigations using single crystal X-ray diffraction experiments together with atomic-resolution electron microscopy have revealed the presence of minor orthorhombic inclusions dispersed within the hexagonal Fe2P–type matrix crystal structure. Despite these structural variations, both atomic arrangements primarily consist of similar structural layers and exhibit comparable chemical bonding characteristics. It has been also discovered that Be3Ru2 crystallises with U3Si2–type structure, in contrast to the previously reported (Mn0.5Fe0.5)2O3–type structure. Be7Ru4 and Be12Ru7 represent two new phases in the Be–Ru system. They possess a very close atomic composition (63.6 at. % Be and 63.2 at. % Be, respectively) and are situated between Be2Ru and Be3Ru2 in the Be–Ru phase diagram. Together with Be2Ru, these two new phases form a series of two-dimensional intergrowth structures, incorporating building blocks of Be2Ru and Be3Ru2 (Fe2P– and U3Si2– type structure). The first one is comprised of hexagonal channels of Ru atoms accompanied by embedded columns of [Be@Be6] trigonal prisms, while the second structure consists of columns composed of tetragonal [Be@Ru8] and trigonal [Be@Ru6] prisms. The structural organisation observed in Be7Ru4 and Be12Ru7 has not been documented previously, indicating that these two phases represent novel structural prototypes. A careful investigation of the crystal structure of Be17Ru3, revealed that the center of a cage [X@Be12] around at the origin of the unit cell, is not completely empty, but rather partly occupied by either Be or Ru. Furthermore, it was observed that this cage can be filled by rare earth and actinide elements giving rise to a novel family of ternary compounds with composition RBe68Ru12 (R = U, Th, Ce, Pr, Gd, Ho). Finally, two new Be–based Laves phases C15–Be2Fe1-xRux (x = 0.52) and C14–Be2Fe1-xOsx (x= 0.57) were discovered through alloying Ru and Os to C14–Be2Fe Laves phase. This study confirmed that the stability of C15 or C14 AB2 Laves phases cannot be predicted by simple reasoning such as atomic size ratio between the A and B atoms, difference in electronegativity or valence electron concentration (VEC), particularly when all three elements, Fe, Ru and Os, belong to the same group of the periodic table. Despite their different chemical behaviour, the investigation of chemical bonding using quantum chemical techniques in the Be– and Mg–based intermetallic compounds with late transition metals, unveiled shared characteristics whereby their crystal structures are stabilised by the formation of polar multiatomic bonds. The observed charge transfer not only serves a decisive role in stabilising the atomic configurations, but also contributes to the emergence of distinct structuring of the calculated electronic density of states of states, DOS, i.e. appearance of more or less prominent dips in the vicinity of the Fermi level, implying their proximity to a semiconducting state, in particular as far as Be–based intermetallic compounds are concerned.

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