Solution Spectroscopic and Chemical Properties of the Complex Hydride [FeH(6)](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(+).

Comparative preparations of homoleptic hydridic anions of iron and ruthenium using solution-based organometal hydrogenation techniques.

A study of the preparations of the complex hydridic anions [MH(6)](4)(-) (M = Fe and Ru) reveals a number of distinctive features. Here a soluble homoleptic ruthenium hydride has been prepared for the first time. For example, both FeX(2) and [Ru(eta(4)-1,5-COD)X(2)], X = Cl and Br, react with PhMgBr solutions under hydrogen to produce the title compounds. The benzene liberated in these reactions is more readily hydrogenated in the case of a homogeneous room temperature ruthenium hydride preparation to both cyclohexane and cyclohexene. The (1)H NMR spectroscopic data show that the two complex anions have hydride absorptions in the low-frequency region, delta -20.3 and -14.7, respectively. Further, (1)H spin-lattice relaxation times (T(1)) for M-H are longer in the case of Ru vs Fe.

Observation of Fe-H/D modes by nuclear resonant vibrational spectroscopy.

Metal-hydrogen bonding is important in chemistry and catalysis, but H atoms are often difficult to observe, especially in metalloproteins. In this work we show that Fe-H interactions can be probed by nuclear resonance vibrational spectroscopy at the 14.4 keV 57Fe nuclear resonance. An important advantage of this method, compared to Raman and IR spectroscopy, is the selectivity for modes that involve 57Fe motion. We present data on the FeS4 site in rubredoxin and the [FeH(D)6]2- ion. Prospects for studying more complex systems are discussed.