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 [FeH6](4-). Examination of th e W-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) --> T-3(2g) and (1)A(1g) --> T-3(1g). The doubly spin-forbi dden transition ((1)A(1g) --> ST2g) was not observed. Spin-allowed lig and field transitions, (1)A(1g) --> T-1(2g) and (1)A(1g)- T-1(1g), occ urred 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 yield ed the parameters QK = 25 x 10(3) cm(-1) and B = 310 cm(-1), assuming C/B = 4. Thus, the position of hydride was established in the spectroc hemical series of low-spin Fe2+ as well beneath cyanide (35 x 10(3) cm (-1)) yet well above bipyridine (18 x 10(3) cm(-1)). Titration of solu tions of [FeH6]MgX(THF)(2)](4) (1.2 x 10(-3) M), I(X = Cl, Br), with [ MgCl2] ((1.8-45) x 10(-3) M) did not perturb the ligand field absorpti ons but caused a hypsochromic shift in the LMCT band consistent with t he formation of the less anionic polyhydride complex, I, from [MgX(THF )(n)](+) and ([FeH6][MgX(THF)(2)](3))(-) II, where K-1 approximate to (3 +/- 1) x 10(-3) (UV-visible). The H-1 NMR (1.2 x 10(-3) M, 25 degre es C) in THF-d(s) 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 (LiOH)-Li-6 (8 equi v) was found by Li-6{H-1} NMRto result in the replacement of the [MgX] (+) unit in 1 with Li-6(+).