Inorg. Chem. 1997, 36, 3461-3465
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Solution Spectroscopic and Chemical Properties of the Complex Hydride [FeH6]4Donald E. Linn, Jr.,*,† and Sidney G. Gibbins‡ Departments of Chemistry, Indiana University-Purdue University at Fort Wayne, Fort Wayne, Indiana 46805-1499, and University of Victoria, Victoria V8W 2Y2, Canada ReceiVed October 31, 1996X Solution spectroscopic and chemical behavior was examined in the case of the homoleptic hydridic anion of iron [FeH6]4-. Examination of the UV-visible spectrum in THF revealed a LMCT band which occurs at 41 × 103 cm-1 ( ) 1200 L mol-1 cm-1). A manifold between 470 and 500 nm was consistent with overlapping spinforbidden transitions: 1A1g f 3T2g and 1A1g f 3T1g. The doubly spin-forbidden transition (1A1g f 5T2g) was not observed. Spin-allowed ligand field transitions, 1A1g f 1T2g and 1A1g f 1T1g, occurred at 28.2 ( ) 356 L mol-1 cm-1) and 24.2 × 103 cm-1 ( ) 414 L mol-1 cm-1), respectively. The latter data yielded the parameters ∆H) 25 × 103 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 Fe2+ as well beneath cyanide (35 × 103 cm-1 ) yet well above bipyridine (18 × 103 cm-1 ). Titration of solutions of [FeH6][MgX(THF)2]4 (1.2 × 10-3 M), I (X ) Cl, Br), with [MgCl2] ((1.8-45) × 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 {[FeH6][MgX(THF)2]3}-, II, where K1 ≈ (3 ( 1) × 10-3 (UV-visible). The 1H NMR (1.2 × 10-3 M, 25 °C) in THF-d8 displayed two hydride components at δ -20.3 and -20.4 ppm (5.6:1). Coalescence of the two hydride absorptions occurred near 40 °C and 200 MHz. Reaction of I with 6LiOH (8 equiv) was found by 6Li{1H} NMR to result in the replacement of the [MgX]+ unit in I with 6Li+.
Introduction Transition metal hydride compounds in which hydrogen is the only ligand, i.e. the homoleptic hydrides, are interesting from several standpoints. First, the stoichiometric density of hydrogen at the metal center is extremely high for such species, as, for example, the 9:1 ratio which is seen in Ginsberg’s nonahydride, K2[ReH9].1 Further, they represent species which are unambiguous examples of strictly transition metal hydride bonding. A number of these compounds have accumulated which can be described as either principally covalent or intermetallic complex hydrides.2 The soluble hexahydroferrate(II) [FeH6][MgX(THF)2]4, I, has been prepared by S.G.G. and its structure characterized by X-ray and neutron diffraction.3 These studies established the the existence of octahedral [FeH6]4- in which the short Fe-H bond (1.609 Å) indicated a covalent bond. Four [MgX(THF)2]+ units were positioned in a tetrahedral geometry on alternate faces of the [FeH6]4- octahedron. A long Mg-H bond (2.095 Å) indicated an ionic interaction. Magnetic susceptibility and Mo¨ssbauer measurements on I established that I was low-spin d6 [Fe(II)].4 Further, the present study takes the opportunity to study the solution spectroscopic and chemical properties of an example of this less ordinary class of compounds. * To whom correspondence should be addressed. † Indiana University-Purdue University at Fort Wayne. ‡ University of Victoria. X Abstract published in AdVance ACS Abstracts, July 1, 1997. (1) (a) Ginsberg, A. P.; Miller, J. M.; Koubek. J. Am. Chem. Soc. 1961, 83, 4909. (b) Abrahams, S. C.; Ginsberg, A. P.; Knox, K. Inorg. Chem. 1964, 3, 558. (2) (a) Bro¨nger, W. Angew. Chem., Int. Ed. Engl. 1991, 30, 759. (b) Yvon, K. Hydrides: Solid State Transition Metal Hydride Complexes. In Encyclopedia of Inorganic Chemistry; King, R. B., Ed.; Wiley: New York, 1994; p 1401. (3) (a) Gibbins, S. G. Inorg. Chem. 1977, 16, 2571. (b) Bau, R.; Chiang, M. Y.; Ho, D. M.; Gibbins, S. G.; Emge, T. J.; Koetzle, T. F. Inorg. Chem. 1984, 23, 2833. (c) The actual formulation of I is [FeH6][MgX(THF)2]4, where X ) Cl0.12Br0.88. This does not affect the interpretation of the results that follow because halides are principally “spectator ions”, and throughout this paper I will be referred to as simply [FeH6][MgCl(THF)2]4.
S0020-1669(96)01300-6 CCC: $14.00
The position of the hydride ligand in the spectrochemical series has remained an unresolved issue. An abbreviated order can be represented by the following sequence:5
I- < Br- < S2- < N3- < F- < OH- < O2- < H2O < SCN- < py < NH3 < en < SO32- < NO2- ) bipy ) o-phen < CH3- < Ph- < CN- < constrained phosphites ) CO Wilkinson placed the hydride ligand at about the same level as that of ammine.6 Bancroft used indirect Mo¨ssbauer data to position the ligand at a level higher than or about the same as that of cyanide.7 Further, Chatt concluded that hydride and methide occupied approximately the same position.8 Unlike earlier estimates, observation of the electronic absorption spectrum of I in THF yields information on the purely hydridic ligand field absorptions of [FeH6]4-. From these data, it is possible to deduce the effects of the ligand field stabilization energy parameter, ∆H-, on the octahedral environment of the ferrous ion. The position of the hydride ligand in the spectrochemical series is deduced thereby unambiguously. Proton NMR is a sensitive probe of the environment of the hydride ligand in solution and provides a picture of the solution structure. One earlier report indicated a single line, consistent with a single octahedral complex.3b Our recent data indicate that two species are present, which are related by chemical exchange. A goal of this study is to establish the relationship between a strong-field ligand, like cyanide, which is capable of π-backbonding, and the hydride ligand, which is capable of only (4) Moyer, R. O., Jr.; Lindsay, R.; Suib, S.; Zerger, R. P.; Tanaka, J.; Gibbins, S. G. Inorg. Chem. 1985, 24, 3890. (5) Lever, A. B. P. Inorganic Electronic Spectroscopy; Elsevier: Amsterdam, 1984; Chapter 9. (6) Thomas, K.; Osborn, J. A.; Wilkinson, G. J. Chem. Soc. A 1968, 1801. (7) Bancroft, G. M.; Mays, M. J.; Prater, B. E. J. Chem. Soc., Chem. Commun. 1969, 39. (8) Chatt, J.; Hayter, R. G. J. Chem. Soc. A 1961, 772.
© 1997 American Chemical Society
3462 Inorganic Chemistry, Vol. 36, No. 16, 1997
Figure 1. UV-visible spectrum of [FeH6][MgCl(THF)2]4 in THF. Conditions: concentration ) 9.80 × 10-4 M; temperature ) 22 °C; optical path length ) 10 mm. The solid line is the observed spectrum, and the points ()) are calculated to give the resolved spectrum. Parameters used to fit to the spectrum: λ1 ) 250 (15) nm; 1 ) 1400 (240) L mol-1 cm-1; δ1 ) 14.5 (2.7) × 103 cm-1; λ2 ) 355 (1) nm; 2 ) 360 (20) L mol-1 cm-1; δ2 ) 4.3 (0.2) × 103 cm-1; λ3 ) 414 (1) nm; 3 ) 250 (10) L mol-1 cm-1; δ3 ) 5.3 (0.2) × 103 cm-1 (the δ values represent bandwidths at half-height, and the numbers in parentheses are standard deviations.
σ-bonding. Further, the solution properties of the transition metal homoleptic hydrides provide relevant information on related solid state hydrides. Experimental Section The complex hydride [MgX(THF)2]4(FeH6) was prepared using methods previously described in the literature and handled under inert gas (drybox) or under vacuum ( Ru > Fe. This reflects the increased back-bonding for the heavier transition metals. Infrared data show the same trend for [MH6]4- and also the sensitivity of νMH to the cubic cell constant a in MII2MH6 (M ) Ru, Os).36 For [FeH6]4-, this sensitivity to a is also borne out with a 10% increase in a resulting in a 10% decrease in νFeH.37 Clearly, spatial relationships in the complex hydrides, i.e. the type of cation in the lattice, and indeed in solution, can have a dramatic influence on the energetics and corresponding chemical properties of the transition metal-to-hydrogen bond. Ion-pair reactions characterize these materials in THF. The fact that the [MgCl(THF)2]+ unit is dissociable points to the predominantly ionic bonding between this cation and the hydridic anion. It remains to be seen how sensitive such reactions are to the nature of the transition metal hydride complex. Acknowledgment. This paper is based on work supported by the U.S. Army Research Office under Grant DAAH04-940312. We also thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. A study leave grant (for S.G.G.) from the University of Victoria and the hospitality of M. L. H. Green (Oxford University) are gratefully acknowledged. Further, we wish to acknowledge the technical skills of undergraduates Aaron Trout and Ann Wilkinson. Perry Pellechia, Purdue University, West Lafayette, IN, graciously provided the 500 MHz NMR spectra. Supporting Information Available: 35Cl and 6Li{1H} NMR spectra (Figures S1 and S2) and a table of NOEs (Table S1) (3 pages). Ordering information is given on any current masthead page. IC961300D (34) The 6Li{1H} NOE technique was applied previously for determining the structures of organolithium hydrides in solution: Avent, A. G.; Eaborn, C.; El-Kheli, M. N. A.; Molla, E. M.; Smith, D. J.; Sullivan, A. C. J. Am. Chem. Soc. 1986, 108, 3854. (35) One reviewer suggested that [MgCl(THF)n]+ (contained in I) and [MgCl2] add possibly to the unoccupied triangular faces of [FeH6]4to form {[FeH6][MgCl(THF)2]5}+, III. Still another analogous redistribution reaction that the reviewer suggests is the formation of {[FeH6][Mg3Cl3(THF)n]}+, III′, from [MgCl(THF)n]+ (contained in I) and [MgCl2]. These equilibria require at least three hydride components: I, II, and III (or III′). The 1H NMR and UV-vis titration data indicate that only two hydride components are present. Formation of III or an analog is more complicated and more demanding (sterically) than equilibria 1 and 2. III (or III′) cannot be ruled out entirely, however. (36) Kritikos, M.; Nore´us, D. J. Solid State Chem. 1991, 93, 256. (37) A cubic cell constant for I is calculated to be 7.2 Å as compared to 6.4 Å for Mg2FeH6.
