The crystal structures of the LaMnSi 2 , NdMnSi 2 and PrMnCu 0 . 5 Ge 1 . 5 compounds have been refined by single crystal X-ray diffraction. All three compounds crystallize in the orthorhombic LaMnSi 2 -type structure (Cmcm), previously denoted as the 'TbFeSi 2 -type'. The variation of the atomic coordinates and interatomic distances, observed between the RMnSi 2 and RMnCu 0 . 5 Ge 1 . 5 series, well account for enhanced Cu-Ge bonds compared to the corresponding Si-Si ones. Simultaneously, a weakening of the Mn-Ge bonds is observed. The modifications of these chemical bondings are correlated to drastic changes of the Mn sublattice magnetic properties. The variations of the manganese local environment are related to the more or less pronounced magnetic anisotropy observed in these series of materials.