and Ru17 correlate well with the one of reference Ru5, as illustrated by the superimposed structures (Figure ). The lengths of the Ru-C NHC and the Ru=C alkylidene in the three complexes do not differ by more than three estimated standard deviations, as well as the geometry index (τ 5 ), which characterizes the distortion of the square-pyramidal geometry [Ru5: 0.35; Ru16: 0.38; Ru17: 0.40]. However, Ru-O bond distances significantly increase in the order Ru17, Ru5, Ru16, respectively, which perfectly correlates with the NHCs electronic donation abilities (overall and σ-donation rising in the order BI*FPh < BIPh < BI*Ph), and which reflects the structural trans effect from BI*FPh to BIPh and to BI*Ph. Interestingly, such observation differs from phosphine derived second generation olefin metathesis catalysts (e.g. Ru1) where the increased πacidity of NHC ligand accounts for a stronger trans effect as the resulting metal-phosphine πbackdonation weakens. Obviously isopropoxy ligand is a purer σ-donor hence stronger σdonating NHC will have a stronger trans effect. Such a trend can also be observed in other examples in the literature although, to the best of our knowledge, no such assumption has been made so far. The relative steric constraints of the parent unsubstituted BIPh and the two new double-decker BI*Ph and BIF*Ph ligands were quantified by establishing their topographical steric maps in complexes Ru5, Ru16, and Ru17 respectively, using the SambVca2 web application (Figure ), and were compared to the steric hindrance brought by the SIMes ligand in the general-purpose catalyst Ru2. Interestingly, it appeared that the substitution of the benzimidazolyl backbone led to a slight decrease of the %V bur from 32 spectively, which hinders the N-phenyl rotation as hypothesised. This through-space effect of the backbone substitution is the opposite of the buttressing effect observed in amino-decorated NHC catalysts, in which the bulky amino substituents on the backbone forced the N-aryl groups to be twisted. As expected, all three benzimidazolylidene ligands provide a lower steric pressure than the standard SIMes ligand [%V bur (SIMes) = 33.7%], due to the presence of small N-phenyl groups. In order to rationalize the effect of the insertion of a phenyl or pentafluorophenyl ring in the backbone of the benzimidazolyl group of Ru16 and Ru17 respectively, we carried out a DFT study at the B3PW91-D3 level of theory. The optimised structure of Ru16 and Ru17 (Ru16-opt and Ru17-opt, respectively), are shown in Figure . Selected bond distances, angles, and torsion angles are listed in Table and compared with those experimentally observed. The computed bond distances and angles accurately reproduce the experimental values with variations of less than 0.04 Å and 1.0°. Taking into account the free rotation of both the N-phenyl and the benzimidazolyl-phenyl or pentafluorophenyl rings, the disparities observed in the Cl1-Ru-Cl2 angles and in some of the torsion angles between the experimental and computed structures (Table ) are likely to be attributed to packing interactions within the crystal lattices.