itially proposed formulation of Me Mo IV Cl2 as a coordination polymer. We observed that all the Mo(IV) dichloride complexes crystallized with the two chloride ligands trans to each other. Note that while both Me Mo IV Cl2 and tBu Mo IV Cl2 can be crystallized directly from the reaction mixture as distorted octahedral complexes with the two chloride ligands in apical positions with nearly identical bond metrics, Ph Mo IV Cl2 precipitates from the reaction mixture as a dark black amorphous powder. We only achieved growing suitable crystals for XRD analysis by slow diffusion of n pentane into a saturated pyridine solution. The solid state structure yet revealed the presence of an additional equatorial pyridine ligand, giving rise to a seven-coordinate complex, further emphasizing the high Lewis-acidity of this family of Mo(IV) complexes. XRD analysis of the bis-triflate complexes (R = Me, t Bu) showed that both complexes display distorted octahedral geometries where the two triflate ligands are positioned cis to each other, in contrast to the trans coordination geometry observed for their dichloride counterparts. Despite this change in coordination geometry, the Mo IV -O( R diket) average bond lengths are essentially identical when moving from the dichloride (dMo-O = 2.01 Å) to the bistriflate (dMo-O = 1.97 Å) complexes, and in the same range than in previously reported solid state structures of diketonate Mo complexes such as ). Worthy of note is the fact that, in the case of Me Mo IV Cl2, the chelate formed by the diketonate ligand and the Mo center significantly deviates from planarity (∡ = 21.4 °), breaking up the complexes' overall D4h symmetry (Fig. ). We further identified that the chelate is nearly planar displays a similar bending of the diketonate ligand (∡ = 22.1 °). We propose that this deviation results from a non-zero orbital overlap between the HOMO of the anionic β-diketonate ligand and an empty orbital of appropriate symmetry of the metal fragment (an example with the dz² orbital is highlighted in Fig. ). Population of these empty orbitals upon reduction of the Mo center will disfavor this interaction, resulting in a more planar coordination mode of the βdiketonate ligand. The larger angle observed in the case of tBu Mo IV Cl2 (∡ = 25.4 °) is in line with the stronger electron-donating properties of the t Bu groups of its supporting ligand, increasing its 𝜋-donation capabilities. DFT calculations further support this hypothesis, reproducing well the trend, yet at a smaller magnitude (e.g for tBu Mo IV Cl2: ∡ = 12.0 ° vs. ∡ = 25.4 ° from XRD data, see Section S4 & S7). Both the Mo(IV) dichloride and bistriflate complexes are paramagnetic compounds, as highlighted by magnetic susceptibility measurements in both the solid and liquid state, suggesting a high spin d 2 electronic configuration (Section S2). Yet, this paramagnetic character did not prevent recording of solution NMR spectra. All complexes display clean signals in solution 1 H and 19 F (for bis-triflate complexes) NMR spectra (Section S5). At the difference with the single crystal structures presented above, both cisand trans-isomers are observed in solution for all the complexes synthetized, independently of the present X -ligand (X -= Cl -or TfO -) (Fig. ). As an exception, in MeCN, the most polar solvent studied here, tBu Mo IV (OTf)2 was found to be uniquely present as its cis isomer. Owing to its increased solubility, the following detailed solution studies were only performed for the t Bu-substituted diketonate complexes but the close similitudes between the 1 H and F NMR spectra of all three complex families let us infer that the same conclusions on the analysis of the tBu diket complexes properties can be applied to their Me diket and Ph diket counterparts. The ratio between the cisand trans-isomers determined by 1 H NMR and summarized for tBu Mo IV Cl2 and tBu Mo IV (OTf)2 in Table appeared to be both dependent of the anionic X -ligand and the solvent used. A significant influence of the polarity of the media on the cis:trans ratio was observed, with more polar solvents favoring the cis isomer. The behavior at different temperatures was studied using variable temperature NMR (Fig. ). The Cl -and TfO -complexes displayed significantly different behavior: for tBu Mo IV Cl2 the cis:trans ratio is nearly independent on temperature, while for tBu Mo IV (OTf)2 the cis isomer is more favored at low temperature, in agreement with the cis-configuration observed in the single crystal structure. In case of the triflate complexes, structural information could be extracted from F NMR spectroscopy. Even in a coordinating solvent such as MeCN, the triflate ligands retain a characteristic F chemical shift (e.g. 𝛿 ≈ -40 ppm for tBu Mo IV (OTf)2), very distinct from free, non-coordinated -OTf anions (𝛿 ≈ -80 ppm). This result suggests that the triflate ligands remain bound to Mo, in contrast to previous literature examples, and highlights the relatively strong Lewis acidity of the Mo center in this series. We investigated the catalytic activity for allylic substitution reactions of the in-situ generated mixtures in the presence of R Mo IV Cl2/AgOTf (R = Me, Ph, t Bu) in a 1:2 ratio, using the coupling of cinnamyl alcohol (1) with electron-rich aromatics as carbon nucleophiles (e.g. phenol (2), anisole or p-cresol) as a prototypical reaction. All three complexes proved efficient to promote the coupling (Section S3). As no significant difference in catalytic activity could be detected with Mo(IV) complexes supported by the other 𝛽-diketonate ligands investigated here, we decided to restrict the more detailed reactivity and mechanistic studies to the most soluble complexes of the series, supported by tBu diket ligands.