Cluster models of SiO2-200 are proposed and compared with spectroscopic (IR and NMR) experimental data. Five models describing the variety of surface silanols (isolated, vicinal and germinal) at the SiO2-200 surface have then been derived and used to study the grafting reaction of homoleptic silylamide lanthanum(III) complexes. Three different grafting modes have been obtained (mono-, bi- and tri-grafted) in line with the experimental knowledge. In terms of energetic stability as well as spectroscopic properties of coordinated OPPh3 (as a probe), all modes could coexist at the surface. The analysis of the δ(31P) chemical shifts for the coordinated OPPh3 indicates some possible important differences in Lewis acidity of the lanthanide centre, which may impact the catalysis.