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 Crl(2) with an excess of LiCCSiMe3 in THF at -25 degrees C affords orange Li-3[Cr(CCSiMe3)(6)]center dot 6THF (1), while analogous reactions employing M(CF3SO3)(2) (M = Fe or Cc) generate pale yellow Li-4[Fe(CCSiMe3)(6)]center dot 4LiCCSiMe(3)center dot 4Et(2)O (2) and colorless Li-3[Co(CCSiMe3)(6)]center dot 6THF (3). Slightly modified reaction conditions lead to Li-8[Cr2O4(CCSiMe3)(6)]center dot 6LiCCSiMe(3)-4glyme (4), featuring a bis-mu-oxo-bridged binuclear complex, and Li-3[Co(CCSiMe3)(5)(CCH)]center dot LiCF3SO3 center dot 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) angstrom for M = Cr-III, Fe-II, and Co-III, 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 (T-4(2g) -> (4)A(1g)) and three lower-energy spin-forbidden d-d transitions. The spectra of [Fe(CCSiMe3)(6)](4-) and [Co(CCSiMe3)(6)](3-) acetonitrile display high-intensity charge-transfer bands, which obscure all d-d transitions except for the lowest-energy spin-allowed band (T-1(1g) <- (1)A(1g)) 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 Delta(0) = 20 200 cm(-1) and B = 530 cm(-1) for [Cr(CCSiMe3)(6)](3-), Delta(0) = 32 450 cm(-1) and B = 460 cm(-1) for [Fe(CCSiMe3)(6)],(4-) and Delta(0) = 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 pi-donor ligand.