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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.

A pair of POCOP-supported mono-and dicarbonyl complexes of Co have been prepared and crystallographically characterized. The reactivity of ( tBu POCOP)Co(CO) with H 2 , acids, and carbon monoxide has been compared to that of the previously reported Rh and Ir counterparts. Co is found to share reactivity traits with both Rh and Ir.

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A microscale laboratory is described to prepare the compounds [RuH2L(P{C6H5)3}3] (L = P(C6H5)3, H2, CO, and N2 in a pedagogical introduction to inert atmosphere inorganic syntheses. The starting material, [RuCl2{P(C6H5)3}3], is prepared routinely in 100% yield and is air-stable. The hydrogenation of this compound is accomplished using borohydride to give [RuH2L{P(C6H5)3}3] (L = P(C6H5)3 and H2). Derivatives where L = CO and N2 are prepared from [RuH2(H2){P(C6H5)3}3] by direct reaction in solution with gaseous CO and N2. 31P{1H} NMR is used to appraise the chemical purity and to determine the structures of these compounds in solution.

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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(+).

Internal electron transfer (IET) within II and V yields the radical Ar02H', for which Cr(V) and Cr(IV) compete in le oxidations to the quinone "Ar(=0)2". The latter competition (as measured by the ratio k2/kt) is related to the extent of autocatalysis. Two molecules of the diol have been incorporated into intermediate V, for the diol unit that appears in the ligand sphere of the (substitution-inert) Cr(III) product, VI, cannot be that which undergoes oxidation to the Ar02H* radical. Since the Ar(OH)2-Cr(IV) step (ks) in this reaction exhibits no kinetic dependency on [diol], we infer that conversion to V is rapid and essentially complete under our reaction conditions. An analogous scheme may be applied to the oxidation of hydroquinone, but chelation is precluded with this 1,4-diol. In an alternate path, Ar02H' formed in the initial internal electron transfer remains bound to Cr(IV) while undergoing a second le oxidation to yield Cr(III) and quinone. Although conceptually simple, this cannot be a major contributor, for release of free Ar02H* appears to be essential for the observed catalysis.In the consideration of the reactions of Cr(V) with the several "ambifunctional" reductants (Table V) that utilize sequences related to (3)-( 6), it has been noted36 that the most dramatic autocatalysis and the appearance of clocklike behavior are associated with the greatest reversals in selectivities toward Cr(V) and Cr(IV) when these reductants are compared with the radicals that they generate, i.e., when the differences between the ratios k]/k3 and k2/kA are most marked. Moreover, there is considerably less variation in the latter ratio than in the former, and in previous instances, the severity of autocatalysis was determined principally by the selectivity of the primary reductant. This is not the case with reduction by hydroquinone, for which ki/k} is found to lie very close to the corresponding ratio for iodide, a strongly autocatalytic reductant. Were it not for the poor selectivity of the semiquinone radical, we should observe a clock reaction in this reaction as well.39Acknowledgment. We are grateful to Aria White for technical assistance.