σ- vs π‑Bonding in Manganese(II) Allyl Complexes

Reaction of two equivalents of K­[1,3-(SiMe₃)₂C₃H₃] (= K­[A′]) with MnCl₂ in THF produces the allyl complex A′₂Mn­(thf)₂; if the reaction is conducted in ether, the solvent-free heterometallic manganate species K₂MnA′₄ is isolated instead. With the related allyl K­[1,1′,3-(SiMe₃)₃C₃H₂] (= K­[A″]), reaction with MnCl₂ in THF/TMEDA produces the corresponding adduct A″₂Mn­(tmeda). In the solid state, both A′₂Mn­(thf)₂ and A″₂Mn­(tmeda) are monomeric complexes with σ-bonded allyl ligands (Mn–C = 2.174(2) and 2.189(2) Å, respectively). In contrast, K₂MnA′₄ is a two-dimensional coordination polymer, in which two of the allyl ligands on the Mn cation are σ-bonded (Mn–C = 2.197(6), 2.232(7) Å) and the third is π-bonded (Mn–C = 2.342(7)–2.477(7) Å). Both σ-allyls are π-coordinated to potassium cations, promoting the polymer in two directions; the π-allyl ligand is terminal. Density functional theory (DFT) calculations indicate that isolated high-spin (C₃R₂H₃)₂Mn (R = H, SiMe₃) complexes would possess π-bound ligands. A mixed hapticity (π-allyl)­(σ-allyl)­MnE structure would result with the addition of either a neutral ligand (e.g., THF, MeCN) or one that is charged (Cl, H). Both allyl ligands in a bis­(allyl)manganese complex are expected to adopt a σ-bonded mode if two THF ligands are added, as is experimentally observed in A′₂Mn­(thf)₂. The geometry of allyl–Mn­(II) bonding is readily modified; DFT results predict that [(C₃H₅)­Mn]⁺ and (C₃H₅)­MnCl should be σ-bonded, but the allyl in (C₃H₅)­MnH is found to exhibit a symmetrical π-bonded arrangement. Some of this behavior is reminiscent of that found in bis­(allyl)magnesium chemistry