Homoleptic Trimethylsilylacetylide Complexes of Chromium(III), Iron(II), and Cobalt(III): Syntheses, Structures, and Ligand Field Parameters

A straightforward method for synthesizing soluble homoleptic trimethylsilylacetylide complexes of first-row transition
metal ions is presented. Reaction of anhydrous CrCl2 with an excess of LiCCSiMe3 in THF at −25 °C affords
orange Li3[Cr(CCSiMe3)6]·6THF (1), while analogous reactions employing M(CF3SO3)2 (M = Fe or Co) generate
pale yellow Li4[Fe(CCSiMe3)6]·4LiCCSiMe3·4Et2O (2) and colorless Li3[Co(CCSiMe3)6]·6THF (3). Slightly modified
reaction conditions lead to Li8[Cr2O4(CCSiMe3)6]·6LiCCSiMe3·4glyme (4), featuring a bis-μ-oxo-bridged binuclear
complex, and Li3[Co(CCSiMe3)5(CCH)]·LiCF3SO3·8THF (5). The crystal structures of 1−3 show the trimethylsilylacetylide complexes to display an octahedral coordination geometry, with M−C distances of 2.077(3), 1.917(7)−1.935(7), and 1.908(3) Å for M = CrIII, FeII, and CoIII, respectively, and nearly linear M−C⋮C angles. The UV−visible absorption spectrum of [Cr(CCSiMe3)6]3- in hexanes exhibits one spin-allowed d−d transition (4T2g ← 4A1g)
and three lower-energy spin-forbidden d−d transitions. The spectra of [Fe(CCSiMe3)6]4- and [Co(CCSiMe3)6]3- in
acetonitrile display high-intensity charge-transfer bands, which obscure all d−d transitions except for the lowest-energy spin-allowed band (1T1g ← 1A1g) of the latter complex. Time-dependent density functional theory (TD-DFT)
calculations were employed as an aide in assigning the observed transitions. Taken together, the results are most
consistent with the ligand field parameters Δo = 20 200 cm-1 and B = 530 cm-1 for [Cr(CCSiMe3)6]3-, Δo =
32 450 cm-1 and B = 460 cm-1 for [Fe(CCSiMe3)6],- and Δo = 32 500 cm-1 and B = 516 cm-1 for
[Co(CCSiMe3)6]3-. Ground-state DFT calculations support the conclusion that trimethylsilylacetylide acts as a π-donor
ligand.