Soft metal‐organic grafting of periodic mesoporous silica (PMS) has recently been demonstrated to give gadolinium‐loaded nanostructured hybrid materials with interesting 1H relaxometric properties. Here, the sterically demanding alkoxide and aryloxide complexes, [Ln(OCH2CMe3)3]4 and Ln(OC6H3tBu2‐2,6)3 (Ln = La, Gd), respectively, were grafted onto PMS materials MCM‐41, SBA‐15, and SBA‐1. Support materials with distinct topology and pore diameters were utilized in order to direct the metal precursors towards the PMSs internal or external surfaces. All hybrid materials were examined by N2 physisorption, Diffuse Reflectance Infrared Fourier Transform (DRIFT) and solid‐state NMR spectroscopy, as well as elemental analysis. The characterization data demonstrated the successful silica attachment of the alk(aryl)oxide complexes via the surface silanol groups. Careful design of the hybrid materials based on findings in our previous studies resulted in GdIII loaded materials with particularly high 1H longitudinal relaxivities at high magnetic field strength (400 MHz). The highest relaxivity of r1 = 31 mM–1s–1 was found for material [Gd(OCH2CMe3)3]4@SBA‐1 showing no intrapore metal loading. This value is amongst the highest relaxivities reported for GdIII loaded mesoporous nanomaterials so far at such high field strength. The metal loading, localization of metal centers (directed by the topology of the PMS), and the particular properties of the metal‐organic precursor are factors found to influence r1.