Homoleptic allyl complexes of chromium with trimethylsilylated ligands. Formation and molecular structure of {[1-(SiMe3)C3H4]2Cr}2, [1,3-(SiMe3)2C3H3]2Cr, and [1,1′,3-(SiMe3)3C3H2]2Cr

Reaction of two equivalents of K[1-(SiMe3)C3H4], K[1,3-(SiMe3)2C3H3], or K[1,1′,3-(SiMe3)3C3H2] with CrCl2 in THF at −78 °C produces the red complexes {[1-(SiMe3)C3H4]2Cr}2 (1), [1,3-(SiMe3)2C3H3]2Cr (2) and [1,1′,3-(SiMe3)3C3H2]2Cr (3), respectively. They are thermally stable compounds, remarkably so for the monomeric 2 and 3, which possess formal 12-electron counts. Single crystal X-ray structures confirm the dimeric nature of {[1-(SiMe3)C3H4]2Cr}2, which is constructed around a Cr2 core with CrCr′=1.9784(7) Å. In the monomeric complexes, the allyl ligands are bound in a trihapto manner to the metals, with CrC distances of 2.193(2)–2.257(2) Å in 2 and 2.223(5)–2.319(5) Å in 3. The allyl ligands adopt staggered conformations, with a 10.4° angle between the C3 planes in (2) and parallel ligands in (3). The trimethylsilyl groups in both complexes are in a syn, anti conformation. The monomeric complexes are high-spin, with four unpaired electrons. The steric shielding provided to the metal by the trimethylsilyl groups is probably responsible for the lack of reactivity of 2 with bulky donors such as PPh3, although it forms a monoadduct with the more sterically compact PMe3. Density functional theory calculations were performed on (C3H5)2Cr, and comparisons made with the structures of the trimethylsilylated derivatives.