A coherent overview of the physical and chemical properties of the family of transitionmetal hydrido complexes is addressed from ab initio through an exhaustive treatment from the solid state at different levels to the molecular state, taking a promising class of compounds for potential applications, the nonahydrido complexes as an illustrative case study. A new chemical vision is presented, pertaining to structure-property relationships such as the different chemical behaviors of the tricapped trigonal prismatic {TH9} (T = Tc, Re) complex anions at the two distinct lattice sites. From energy differences different stability and binding of the Tc versus Re based compounds are underlined whereby the latter is found more tightly bonded. Such solid state results are also supported by calculations at the molecular level with larger infra-red frequencies within the ReH9 complex anion versus TcH9 corresponding to both bending and stretching T-H modes in agreement with experiment. Electronic density of states show both compounds to be insulating with large band gaps and narrow valence bands which are differentiated for the two sites of the transition element (T1 and T2): T1H9 sub-motifs at the corners and T2H9 ones forming a honeycomb arrangement as it is illustrated from the electron localization function (ELF) plots and chemical bonding. ELF maps are also significant of largely ionic 2K+(TH9)2− compound within which covalent-like (TH9)2− show electron localization on hydrogen and free electron behavior within the tricapped trigonal prism.< Réduire