Indeed, the NBO analysis revealed a high degree of deviation of the natural hybrid orbitals (NHO) on α-C atoms from the Yb-C axis (θ NHO-C-Yb = 17.8 • for one [CH(SiMe 3 ) 2 ] 3 ligand and 17.9 • for the other two alkyl ligands). The value of such deviation can be used as a quantitative descriptor of the degree of the π character within the metal-carbon bonds (for a pure σ bond, no deviation would be expected). In fact, the degree of the π character within the Yb-C bonds is higher than the one observed for Ti(III), Ti(IV) and Zr(IV) neutral alkyl complexes, while being somewhat smaller than for the related cationic complexes. Furthermore, the NBO analysis revealed three low-lying natural orbitals, which have been attributed to the low-lying 5d orbitals, with their occupancies being slightly higher than 5 % (see Tab. S11). The spatial distribution of these orbitals (Fig. ) includes, in The obtained significant stabilization energies for these 5d orbitals (see Tab. S9) imply that there are other secondary interactions within the structure of Yb[CH(SiMe 3 ) 2 ] 3 that involve these 5d orbitals. One example of such an interaction is the three-centre -two-electron Yb-Me-Si interaction, previously observed for the Lu[CH(SiMe 3 ) 2 ] 3 analogue. Indeed, the NBO analysis reveals the constructive overlap between these 5d orbitals and the bonding orbitals of the Si-C γ bonds (Fig. ), thus showing that such interactions are also predicted 22 for the Yb[CH(SiMe 3 ) 2 ] 3 complex.