Znorg. Chem. 1994, 33, 1725-1726
1725
.rr-Bonding of the Dihydrogen Ligand Probed by Mossbauer Spectroscopy
have d x electrons have failed, even in a low-temperature matrix.17 However the adduct of H2 with CH3+, (H2)CH3+, is stable in the gas phase and in strong acid solution at low temperature;I8 here the C-H2 bonding is described as 3-center, 2-electron with no Robert H. Morris' and Marcel Schlaf barrier to rotation of the Hz.l9 Department of Chemistry, University of Toronto, Mossbauer spectroscopy has been used by Bancroft et al.20 2' Toronto, Ontario, Canada M5S 1Al to estimate the relative importance of u-and *-effects in complexes of the type truns-[Fe(L)H(depe)~]BPh4.~A plot of isomer shift Received November 4, 1993 versus quadrupole splitting showed a linear correlation for the Introduction ligands L = CH3CN, PhCN, P(OMe)3, and (CH3)3CNC where a-bonding was thought to dominate. By contrast, strong *-acid There is much evidence for the importance of d?r-a* backligands N2 and CO deviate significantly from the line, signaling bonding in the stabilization of transition metal-dihydrogen an important *-effect. The use of additive partial isomer shifts bonding. The H2 ligand in the Kubas complex W(H2)(CO)3(Piand partial quadrupole splittings (pqs) allowed the definition of Pr& is oriented along the P-W-P axis in the solid state to a slightly different line on the basis of a wider range of purely maximize *-bonding.' This leads to a barrier to H2 rotation a-bonding ligands; this work revealed that P(OMe)3 and ArNC which can be detected in the inelastic neutron scattering (INS) ligands also have a *-bonding component.21 Bancroft's studies spectra for M ( H z ) ( C O ) ~ ( P C ~(M ~ ) Z= Cr, Mo, W), Mo(H2)suggested that N2 is a very poor a-donor and a moderate (CO)(dppeh, [Fe(H~)H(dppe)2l+,Fe(H2)(H)dPEtPh2)3, whereas CO is a good x-acceptor and moderately .~,~ [M(Hz)H(PP3)]+ (M = Fe, Ru), and R u ( H ~ ) ~ ( H ) ~ ( P C ~ ~ ) Z*-acceptor, good u-donor.22 This work was extended to a wider range of The energy of the d r electrons as probed by electrochemistry ligands LZ3and to a parallel series of complexes trans-[Fe(L)Hcorrelates qualitatively with the stability of dihydrogen complexes (dppe)~]+ z4 and tr~ns-FeH(X)(dppe)2.~~9~~ In the dppe series, with respect to loss of H2 (d energy too low) and to splitting of the ligands L = P(OMe)3, PhCN, CH3CN, and NH3 defined a the H2 (d energy too high).4 However there was not a correlation line for mainly a-bonding ligands while *-acid ligands deviated between electrochemical parameters and the barrier derived from from the line to a greater extent following the trend CNCH2Ph I N S Z Infrared studies of metal carbonyl complexes containing < N2 < CO.Z4 Use of the anionic ligands X = CN, N02, NCS, a dinitrogen or a dihydrogen ligand suggest that H2 and N Zhave C1, N3, Br, and I provided a line parallel to and close to the line similar bonding ~ r 0 p e r t i e s . jThe ~ separation of u-and *-bonding for the a-donor ligands just mentioned.24 Our dihydrogen contributions on the basis of reported CO stretching frequencies complexes [Fe(Hz)H(dppe)z] BF4 and [Fe(Hz)H(de~e)~] BPh425 is a difficult problem; Andrea et alS8on the basis of an analysis are the subject of the present study. of the spectra of W(Hz)(CO), concluded that the dihydrogen ligand is a relatively strong *-acceptor. N M R studies have Experimental Section provided evidence for a barrier to rotation in [Ru(CSMes)(H2)( d ~ p m ) ] + ~ arecent n d workconfirms that this isdue toa preferred The complexes were prepared as described previously.25 Samples (150 orientation of the H2 ligand determined by r-bonding.10 Themg) were sealed with epoxy resin into poly(methy1 methacrylate) cells in a argon-filled glovebag. The Mbsbauer spectra were collected at oretical studies suggest that both a-bonding (a(Hz)-d(M)) and room temperature for 2 days. Isomer shifts were referenced to stainless ?r-back-bonding (dr(M) a*(Hz)) are important in stable steel; some literature data24were converted from the sodium nitroprusside c o m p l e x e ~ . ~ ~There - ~ 5 is some indication that the a-interaction scale by subtracting 0.16 from the isomer shift. dominates. For example cationic dihydrogen complexes appear qualitatively to be more stable than neutral ones of the same Results and Discussion meta1.I6 However this might have to do with destabilization of the coordinatively unsaturated decomposition product rather than The isomer shifts and quadrupole splittings for the dihydrogen stabilization of the dihydrogen complex. Some recent attempts complexes are listed in Table 1 along with those reported for at preparing H2 complexes with strong Lewis acids that do not related complexes. Also included are IH N M R chemical shifts of the terminal hydride of the complexes in solution. The chemical Eckert, J.; Kubas, G . J.; Hall, J. H.; Hay, P. J.; Boyle, C. M. J . Am. shifts for the dihydrogen complexes have to be measured at low Chem. SOC.1990, 112, 2324-2332. temperature where intramolecular exchange of H atoms is frozen Eckert, J.; Kubas, G. J. J . Phys. Chem. 1993, 97, 2378-2384. OUt.25 dppe = PPh2CHzCH2PPh2, PP3 = P(CH2CHzPPh2)3, and depe = PEt2CH2CHzPEt2. Plotsof isomer shift versus quadrupole splitting (Figures 1 and Morris, R. H. Inorg. Chem. 1992, 31, 1471-1478. 2) reveal that dihydrogen has the largest deviation from the lines Morris, R. H.; Earl, K. A.; Luck, R. L.; Lazarowych, N. J.; Sella, A. defined by Bancroft 21 (Figure 1) and Mori and Takashima24 Inorg. Chem. 1987, 26, 2674-83. Zheng, Y. F.; Wang, W. H.; Lin, J. G.; She, Y. B.; Fu, K. J. J. Phys. (Figure 2) for a-donor ligands. The position of H2 on the plots Chem. 1992, 96, 9821-9827. suggests that it is a very weak a-donor ligand and a strong *-acid Howdle, S.M.; Healy, M. A.; Poliakoff, M. J. Am. Chem. SOC.1990, ligand. The magnitude of the deviation indicates that *-bonding 112, 4804-4813. Andrea, R. R.; Vuurman, M. A.; Stufkens, D. J.; Oskam, A. D. Red. is relatively more important for H2 than for CO or N2. A Trav. Chim. Pays Bas 1986, 105, 372-374. combination of a-withdrawal from the a(H2) orbital and *-backJia,G.;Lough,A. J.;Morris,R.H.Organometallics1992,ll. 161-171.
-
Klooster, W. T.; Koetzle, T. F.; Jia, G.; Morris, R. H.; Fong, T. P.; Albinati, A. Submitted for publication. Hay, P. J. J . Am. Chem. SOC.1987, 109, 705-710. Jean, Y.; Lledos,A.; Maouche, B.;Aiad, R. J. Chim. Phys., Phys. Chim. Biol. 1987, 84, 805-8. Maseras, F.;Duran, M.; L1edos.A.; Bertran, J.J.Am. Chem. SOC.1991, 113, 2819-2884. Van Der Sluys, L. S.; Eckert, J.; Eisenstein, 0.; Hall, J. H.; Huffman, J. C.; Jackson, S. A.; Koetzle, T. F.; Kubas, G. J.; Vergamini, P. J.; Caulton, K. G. J. Am. Chem. SOC.1990, 112, 4831-4841. Albinati, A.; Bakhmutov, V. I.; Caulton, K. G.; Clot, E.; Eckert, J.; Eisenstein, 0.;Gusev, D. G.; Grushin, V. V.; Hauger, B. E.; Klooster, W. T.; Koetzle, T. F.; McMullan, R. K.; O'Loughlin, T. J.; Pelissier, M.; Ricci, R. S.;Sigalas, M. P.; Vymenits, A. B. J . Am. Chem. SOC.1993, 115, 7300-7312. Heinekey, D. M.; Oldham, W. J. Chem. Rev. 1993, 93, 913-926.
0020-1669/94/1333-1725$04.50/0
(17) Moroz, A.; Sweany, R. L. Inorg. Chem. 1992,31, 5236-5242. (18) Hiraoka, K.; Kebarle, P. J. Am. Chem. SOC.1976,98,6119and references therein. (19) Raghavachari, K.; Whiteside, R. A.; Pople, J. A.; Schleyer, P. v. R. J. Am. Chem. SOC.1981, 103, 5649-5657. (20) Bancroft, G. M.; Mays, M. J.; Prater, B. E.; Stefanini, F. P. J . Chem. SOC.A 1970, 2146-2149. (21) Bancroft, G. M. Coord. Chem. Rev. 1973, 11, 247-262. (22) Bancroft, G. M.; Platt, R. H. Ado. Inorg. Chem.. Radiochem. 1972,15, 59-245. (23) Silver, J. Inorg. Chim. Acta 1991, 184, 235-242. (24) Mori, G.; Takashima, Y. Chem. Lett. 1979, 4 2 5 4 2 8 . (25) Bautista, M. T.; Cappellani, E. P.; Drouin, S. D.; Morris, R. H.; Schweitzer, C. T.; Sella, A,; Zubkowski, J. J. Am. Chem. Soc. 1991, 113, 48764887.
0 1994 American Chemical Society
1726 Inorganic Chemistry, Vol. 33, No. 8, 1994
Notes
Table 1. Mossbauer and IH N M R Data for the Complexes trans-[Fe(L or X-)H(diphos)2]+
-7
-
-17 -17
-
Mossbauer
IS, mm s-I
trans complex [Fe(HdH(depe)21 BPh4 [Fe(CO)H(depe)z] BPh4" [Fe(CNCMe3)H(depe)2]BPh.p [Fe(P(OMe)s)H(depe)~]BPLa [Fe(Nz)H(depe)21BPhP [Fe(NCMe)H(depe)z] BPh4" Fe(CI)H(depe)2
-0.03 -0.04 0.05 0.09 0.16 0.19 0.23"
[F~(H~)H(~PP~)~IBF~ [Fe(CO)H(dppe)zlBF4 [Fe(P(OMe)s)H(dppe)2] BF4 IF~(N~)H(~PP~)~IBF~ [Fe(NCPh)H(dppe)2lBF4 [Fe(NCMe)H(dppe)21BF4 [Fe(NH3)H(dppe)21BF4 Fe(CN)H(dppe)2 Fe(Cl)H(dppe)2 a
0.00 0.12' 0.25' 0.31' 0.33' 0.34b 0.38' 0.25' O.3gb
QS, mm s-1 0.00
NMR6(lH), PPm -14.6 -10.9 -13.9 -14.4 -18.2 -23.8 -31.9
-1.00 -1.13 -0.90 -0.33 -0.46 0.0" -0.56 -1.32' -1.5 1' -0.63' -1.01' -0.85' -0.78' -1.77' -0.75'
1H shin
c
-12.9 -7.8'
-27 1
-15.3'
I -32 i
-20.5'
i
-0.05
H2
m
-13
-11
-09 -07 -05 -03 qUadNpole splitting
-01
01
Figure 1. Plot of isomer shift (mm s-I) versus quadrupole splitting (mm for some complexes trans- [Fe(L or X-)H(depe)z] BPh4. Isomer shifts are referenced to stainless steel. The line for u-donating groups is similar to the one found in ref 22. s-1)
05
-
04
+ I
03
-
isomer shift 0 2
-
.\
iI
I
I
-
0
01
02
A dppe
\
,
8
03
04
05
06
isomer shift
-0 1 -15
~
depe
Figure 3. Plot of hydride N M R chemical shift (ppm) versus isomer shift (mm s-I) for the complexes. Refer to Table 1 for the identity of the points.
-13.1' -29.2
co _.
1
t \
-01
N2
O f
+I I
-
0.15
\
~
-22
Data from ref 20. Reference 24. Reference 29. Reference 30. 03
\
H2
MeCN
interligand repulsions around the small Fe center which is not compensated for by the formation of iron-hydride bondsS26Several other dihydrogen complexes have an observable dihydride tautomer.26 A plot (Figure 3) of the lH N M R shift of the terminal hydride versus the isomer shift of the Fe for the complexes [Fe(L or X-)H(diphos)z]+ displays parallel curves for diphos ligands depe and dppe. A correlation between these parameters might be expected because both depend to a certain extent on the s electron density at the Fe center. The isomer shifts and/or the IH chemical shifts for the dihydrogen complexes are more negative than would be expected from such a trend (Figure 3). Although the interpretation of 'H N M R chemical shifts of hydrides is very complex,27one possible explanation is that the strong r-acid ligand, dihydrogen, has removed 3d electron density from the iron to an unusually large extent (compared to CO and N2) which reduces the shielding of the Fe nucleus from the 4s electrons by the 3d electrons. An increase in s electron density at the Fe nucleus is known to decrease the isomer shift. This would have the indirect effect of diamagnetically shielding the hydride as well. A partial quadruple coupling constant, pqs(H2). of -0.22 for the dihydrogen ligand is calculated from the data for the dppe and depe complexes. For this calculation values of pqs(depe) = -0.62,21 pqs(dppe) = -0.50,2* and pqs(H) = -1.0421 were used. Such pqs values might be useful in predicting quadrupole complexes in related, unknown complexes and to distinguish between cis and trans stereochemistries.21, 23
Conclusions
sigma bonding I
01
,
increasing x-bonding
m
O f
-0 I
H2
! -2
