The DFT optimized structure (Fig. ) is in particular notable for the unusually short (ca. A similar effect was previously observed for the [CH(SiMe 3 ) 2 ] 3 ligands of the Lu(III) or Ti(III) analogues. This interaction is essentially described as the donation of electrons from the filled molecular orbital corresponding to the C-H bond to a metal d-orbital of appropriate symmetry, and has been related to a metal-carbon bond acquiring a π (or alkylidene) character, often called α-H agostic interaction in metal alkyls. In order to investigate the possible presence of the π character within the Yb-C bonds, we conducted a Natural Bond Orbital (NBO) analysis, using the program NBO 7.0 54 (see SI Chapter 5.2