Solution Spectroscopic and Chemical Properties of the Complex Hydride [FeH₆]⁴⁻

Solution spectroscopic and chemical behavior was examined in the case of the homoleptic hydridic anion of iron [FeH(6)](4)(-). Examination of the UV-visible spectrum in THF revealed a LMCT band which occurs at 41 x 10(3) cm(-)(1) (epsilon = 1200 L mol(-)(1) cm(-)(1)). A manifold between 470 and 500 nm was consistent with overlapping spin-forbidden transitions: (1)A(1g) --> (3)T(2g) and (1)A(1g) --> (3)T(1g). The doubly spin-forbidden transition ((1)A(1g) --> (5)T(2g)) was not observed. Spin-allowed ligand field transitions, (1)A(1g) --> (1)T(2g) and (1)A(1g) --> (1)T(1g), occurred at 28.2 (epsilon = 356 L mol(-)(1) cm(-)(1)) and 24.2 x 10(3) cm(-)(1) (epsilon = 414 L mol(-)(1) cm(-)(1)), respectively. The latter data yielded the parameters Delta(H)()- = 25 x 10(3) cm(-)(1) and B = 310 cm(-)(1), assuming C/B = 4. Thus, the position of hydride was established in the spectrochemical series of low-spin Fe(2+) as well beneath cyanide (35 x 10(3) cm(-)(1) ) yet well above bipyridine (18 x 10(3) cm(-)(1) ). Titration of solutions of [FeH(6])[MgX(THF)(2)](4) (1.2 x 10(-)(3) M), I (X = Cl, Br), with [MgCl(2)] ((1.8-45) x 10(-)(3) M) did not perturb the ligand field absorptions but caused a hypsochromic shift in the LMCT band consistent with the formation of the less anionic polyhydride complex, I, from [MgX(THF)(n)()](+) and {[FeH(6)][MgX(THF)(2)](3)}(-), II, where K(1)()() approximately (3 +/- 1) x 10(-)(3) (UV-visible). The (1)H NMR (1.2 x 10(-)(3) M, 25 degrees C) in THF-d(8) displayed two hydride components at delta -20.3 and -20.4 ppm (5.6:1). Coalescence of the two hydride absorptions occurred near 40 degrees C and 200 MHz. Reaction of I with (6)LiOH (8 equiv) was found by (6)Li{(1)H} NMR to result in the replacement of the [MgX](+) unit in I with (6)Li(+).