Volume 27
Inorganic Chemistry
Number 19 September 21, 1988
0 Copyright 1988 by the American Chemical Society
Communications Electronic Excitation MO Assignment of Rh2(02CR),(L,,), Using MCD Experiment and Theory Sir: The electric-dipole allowed absorption band that occurs in Rh2(02CR)*L2compounds14 at 550-600 nm (17-18 X lo3 cm-', emax = 240 M-' L-l) with x,y polarization, has been assigned (by different workers) in two ways, viz., r*(Rh2) a*(Rh2)'g2 and r*(Rh2) ~ * ( R h - 0 )(Figure ~ 1). For Rh2(02CCH3)4(0H2)2 (complex 1.2H20), this band (hereafter called band A) was first assigned by Dubicki and Martin (DM)5 to r*(Rh2) a*(Rh2), or 5e, 4a2,, on the basis of its x,y linear polarization, temperature independence of its intensity, and the relatively high sensitivity of this absorption band energy to axial ligand variation. Norman and Kolari (NK)2 then reported SCF-Xa-SW ground-state MO calculations for Rh2(02CH)4(OH2)2 that could be used to better understand the spectrum of 1-2H20;i.e., they provided for consideration several MOs near the HOMO and LUMO. Next, Martin et al. (MWRF)' published further evidence 'E) supporting the previously proposed DM assignment ('Al by showing that the x,y-static electric dipole and MO nature predicted theoretically2for band A were consistent with the results obtained from their single-crystal polarized absorption spectroscopy study. In addition, MWRF' also observed a 297-cm-' vibronic progression on band A ( ~ 5 5 2nm) of 1.2H20. This progression was attributed to v(Rh2) of their 'E(eia;,) electronic excited state, since 320 cm-' was earlier believed to be u(Rh2) of the ground state for 1.2H20. The direction of this 23-cm-' shift is consistent a*( -Rh2) or 5e, with the assigning of band A to r*( -Rh2) 4a2, (D4,,). Shortly thereafter Norman et al. (NRC)' also published excited-state SCF-Xa-SW computational results corresponding to the MWRF excited lE assignment of configuration cia:,, for band A. The degeneracy of this excited state and the similar experimental and calculated state energy differences were supportive of the MWRF assignment for A. Subsequently, the polarized crystal spectra of 1.2H20 were remeasured and extended by Miskowski et al. (MSSSG),3 since it was felt that (a) a 320-cm-' vibration of the ground state was too high for v(Rh2) but about right for the symmetric Rh-O(carboxy1ato) a*(-Rh2) type stretching mode, and that (b) for a r*(-Rh2) assignment (MWRF' and NK, NRC2s6)to be correct for the 18.1 X lo3 cm-' (552 nm) band A, the 23-cm-' difference between
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(1) Martin, D. S.;Webb, T. R.; Robbins, G. A.; Fanwick, R. E. Inorg. Chem. 1979, 18,475. (2) Norman, J. G.; Kolari, H. J. J . Am. Chem. SOC.1978, 100, 791. (3) Miskowski, V. M.; Schaefer, W. P.; Sadeghi, B.; Santarsiero, B. D.; Gray, H. B. Inorg. Chem. 1984, 23, 1154. (4) Johnson, S. A,; Hunt, H . R.; Neuman, H. M. Inorg. Chem. 1963, 2,
960. (5) Dubicki, L.; Martin, R. L. Inorg. Chem. 1970, 9, 673. (6) Ketteringham, A. P.; Oldham, C. J . Chem. SOC.,Dalfon Trans. 1973, 1067. (7) Norman, J. G.; Renzoni, G. E.; Case, D. A. J . Am. Chem. SOC.1979, 101, 5256.
0020-1 6691881 1327-3265$01.50/0
dxy'
Rh-RhS'
dxz,y;
Rh-Rhx'
Figure 1. Partial MO diagram for Rh2(02CH)4(0H2)22in hartrees (1 hartree = 27.21 eV), with A and B being the assignments of MWRFI-NK2 and MSSSG,3 respectively. The present MCD results directly confirm assignment A.
u(Rh2) values for the ground state and this excited state is too small. In this same study the resonance-enhanced Raman solution spectrum of complex Rh2(02CCH3)4(NCCH3)2 was also reported. For a band at ca. 345 cm-', the measured polarization ( p = 0.12) was closer to that of u(Rh-0) (alg, pxy = 0.125) than to the computed p of v(Rh2) (alg,pz = 0.333),8and they assigned v(Rh2) to the only other low-energy band observed (170 cm-'). On this basis together with points a and b above, as well as the computational results of NK? MSSSG3 reassigned band A to r*(-Rh2) u*( -Rh-O), or 5e, 4bh (D4*).Next there appeared a new electronic ground state vibrational study of complex Rh2(02CCH3)4(PPh3)2by Flint et al. (FHC).9 Their Raman, resonance Raman (RR), and Raman polarizations of isotopically ( l 6 0 , l80CH3, , CD3) substituted solids were all consistent with assigning bands at 289 and 338 cm-' to, respectively, v(Rh2) and u(Rh-0) for their complex, Rh2(02CCH3)4(PPh3)2, but no bands were found in the 150-170-cm-' region where the u(Rh2) band was previously suggested to be.3 Most recently Miskowski et al. (MDCMSW)'O chose various Rh2(carboxylato)4(L,)2 complexes that they take to have accessible a(-Rh-L) a*(-Rh2) electronic transitions (to nondegenerate excited states) in order to measure RR band depolarization ratios. MDCMSW concluded, as did C H F and MWRF,' that v(Rh2)was in the 300-cm-' region. With a view to providing some new and, we hoped, decisive experiment to distinguish between the two possible assignments, each having strong merit, of band A, we undertook the study of it by MCD experiment and theory and report the results now.
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(8) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th 4.; Wiley: New York, 1986; p 72. (9) Clark, R. J. H.; Hempleman, A. J.; Flint, C. D. J . Am. Chem. SOC. 1986, 108, 518. (10) Miskowski, V. M.; Dallinger, R. F.; Christoph, G. G.; Morris, D. E.; Spies, G. H.; Woodruff, W. H. Inorg. Chem. 1987, 26, 2127.
0 1988 American Chemical Society
Communications
3266 Inorganic Chemistry, Vol. 27, No. 19, 1988
Figure 2. EAS (a)and MCD (8) spectra and corresponding deconvoluted A ( y ) and B terms ( 6 ) for (A) Rh,(~-rnandelato)~(EtOH)~ and (B)
Rh2(02CCH,),(NCCH,)2. MCD spectra were recorded and calibrated by using the monochromator of a JASCO ORD/CD instrument and digital CD electronics'' of our design. The light beam was collimated, plane polarized, and alternately left and right circularly polarized by a Photoelastic Modulator (PEM) at 50 294 Hz. The resulting beam passed through a cuvette containing the sample solution subjected to a longitudinal magnetic field and was detected by using a Hamamatsu R-376 PMT (-20 "C). The output signal of the PMT was phase detected and amplified by a lock-in amplifier, the signal then being collected and stored with a microcomputer. In addition to collecting the spectral data the microcomputer also controlled the monochromator, PEM amplitude, and PMT high-voltage power supply in a feedback loop. The electronic absorption spectra were obtained with a Varian 2300 spectrophotometer. By calibration the two wavelength axes were matched throughout. Electronic absorption spectra (EAS) and MCD spectra for the compound Rh2(~-mandelato)4(EtOH)2 (complex 2.2EtOH) in ethanol and Rh2(02CCH3)4(NCCH3)2(compound 1.2NCMe) in acetonitrile were obtained at room temperature. (Isomer D is also R, 1, or +.) These and deconvoluted MCD A and B terms for the compounds 2.2EtOH and 2.2NCMe are shown in Figure 2. The dipole strength, bo(0.104 and 0.126 D2 for 2-2EtOH and 1*2N_CMe),for band A and the corresponding solution MCD Al and Boparameters (in the order 2.2EtOH then 1*2NCMe, and -9.3 X AI = 0.073 and 0.100 D2 and Bo= -5.0 X D2 cm) were obtained by careful deconvolution with Gaussian bands to obtain A , B, and D parameters of _earlier d_efinition.12 The These parametersI2 were converted to AI,Bo,and same EAS and MCD band centers, toEAS= cOMCD,and bandwidth parameters, FWHHEAs = 2(ln 2)1/2AMCD, were used.13 Optical band A maximizes at 16.9 X lo3 and 18.0 X lo3 cm-I, respectively, for compounds 2-2EtOH and 1.2NCMe. This band shift (( 17-18) X lo3 crn-') is attributed to axial ligand substitution of 0-coordinated EtOH for N-coordinated NCCH3. The sensitivity of band A is not the result of variation of the bridging carboxyl groups but is due to La, substitution, since very similar shifts are observed for the EtOH and NCCH3 adducts when the D-mandelato bridge" is replaced by the acetato bridge! The molar absorptivity corresponding to band A for compound 2-2EtOH and 1.2NCMe are 212 and 225 M-' L-I, respectively. Corresponding to band A in the EAS is a net negative band in the MCD spectra of both compounds 2.2EtOH and 1.2NCMe, but the minima are at significantly lower energies than EAS maxima (Figure 1). Deconvolution of the MCD spectra requir_es a positive A term (and Al = +) and a negative B term (and 23,
= -). A l / b oratios for compounds 2.2EtOH and 1-2NCMe are 0.70 and 0.80, respectively. The presence of the A term for band A is consistent with a 'Al 'E assignment of, for our molecules, any transition type yU(,) or y,,(,) e,(,,), where y can be a', a2, bl, or b2. Such transitions would have x,y polarization, and this is, indeed, the dominant polarization found for band A from single-crystal spectroscopy of the similar complex l-2H20.133Knowing the 'E excited state symmetry, or 'E, in D4*,_and the experimentally determined sign and magnitude of al/Bo,one now must decide 4azu,and MSSSG,3 between the assignments of MWRF,' 5e, 5e, 4b2,,. For these 'A!, 'E, transitions, use of the most recent formalism13 yields in reduced matrix element form our MCD parameter
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where = ( lEll@lllE), llrrfill = ( lAllrrfill'E), I l d k l l = ( 'Ell klI'A), and K~ = -kBLz and 1) is a 6 j symbol. Expanding the above summation, followed by explicit irreducible representation substitution (ground state, excited state and operators), evaluating the resulting 6 j coefficients, and evaluating the MCD factors in complex basis yield 1 -dl = llpf'll 1 1 4 lldkllD2 (2) 3(d%B) Our EAS electric dipole strength in this formalism is
J
lpf'jll
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(1 1) Trexler, J. W.; Schreiner, A. F., manuscript in preparation. (12) Stephens, P. J.; Suetaak Schatz, P. N. J . Chem. Phys. 1966,44,4592. (13) Piepho, S. B.; Schatz, P. N. Group Theory In Spectroscopy; WileyInterscience: New York, 1983.
After the summation was expanded, followed by explicit substitution of appropriate irreducible representations and evaluation of the 2 j and 3 j phases, Bobecomes 1/3 1 1 d j 1 1 l l d k l l , so that the final ratio is (4) which is to be compared to [Al(exp)/a0(exp)]. With further evaluation of the reduced matrix element, substituting the appropriate M O types proposed, we obtain (5)
for MWRF,' and for MSSSG3 the sign of ( 5 ) is negative. Un-
Inorg. Chem. 1988, 27, 3267-3269
3267
reducing the matrix elements yields, for M W R F
and for MSSSG the opposite sign. It is noteworthy that for both the M W R F and MSSSG assignments, evaluation of &t/i)oinvolves-computing identical multicenter integrals, but the signs o f A , / D o ratias are opposite. MO phases were carefully matched with all other computational features. (e,) was evaluated in the all-center owrator A 0 matrix mode".1' for Rh2(OzCH),. Bond distances and angles.I5 A 0 and f = 1.2 for I s single-f 2s and 2p functions for C and 0.l6 HI' were from the literature. Rh Sp (double-{) and 5s (triple-0 functions are our renormalized outermast terms of more complex functions,Is and the 4d (triple-() function is from the literature.I8 Basis orbital energies from the literature were used (2s. Zp C. 0 from ref 19; Is H is -13.6 eV; for Rh 4d. 5s. Sp the VOlPS of Co 3d, 4s, 4p19,20were used with 10000 cm-I subtracted from each." The 100 functions were used to compute the 70 X 70 S a n d L, matrices,"." and subsequently the M O C matrix?" all on an IBM 3081 (Triangle University Computing Center). We = +0.985 for the MWRF assignment, Se, 4azu, obtain sign and is in good which matches the experimental -%,/bo agreement with the experimental magnitudes for 1.2NCMe and 22EtOH and consistent with the r'(-Rh2) n*(-Rh2) excitation. The circular dichroism spectra of two Rhz(O2CR),L2 compounds (RCOz = L-mandelate and L = EtOH; R C 0 2 = D methoxyphenylacetate and L = THF) have been reported and ~ was shown to be consistent with the MSSSG a ~ i g n m e n t ?which at that time generally accepted. However, these data are also consistent with the MWRF assignment?' The all-center operator matrix treatment of the CD vicinal effect of RhZ(o-mandelato), will be published elsewhere."
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(14) Evans. R. S.; Schnincr, A. F.; Hauser, P. J.; Cavs. T. C. lmwg. Chem. 1975. 14. 163. (15) Cotton. F. A.; DcBoer. B. G.; LaPrade. M. D.; Pipal. J. R.; Ucko, D. A. Aero Crystollogr..Sccr. B: Stmcr. Crymllogr. Cry$r. Chem. 1971. 827, 1664. (16) Clementi. E.; Raimandi. D. L. 1. Chcm. Phys. 1963.38.2686. (17) Palke. W. E.; L i w m p . W. N . J. Am. Chem. Soc. 1966. 88. 2384. (18) Basch, H.; Gray. H. 8. Thcor. Chim. A m 1966.4.367. (19) Basch, H.; Virte. A.; Gray, H. B. Them. Chim. Acto 1965. 3 458. (20) Ballhausen. C. J.; Gray. H.B. Molecular &bird Theory; W. A. Benjamin: New York. 1965. (21) Barch. H.; Gray. H. B. Inorg. Chem. 1967.6.365. (22) Evans. R. S.; Schreincr. A. F.; Hauser. P. J. Inorg. Chem. 1974. 13. 71x5
(23) Agaskar, P. A.; Cotton, F. A,; Falvcllo. L. R.; Ham, S. J , Am. Chem. SOC.1986, 108, 1214. (24) Falvcllo. L. R.; Cotton. F. A,. unpublished rc~ults.
DeDartment of Chemistrv North Carolina State University Raleigh, North Carolina 27695
W.Trexler. Jr. Anton F. Sehreiner'
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0.16um .
Figure 1. Transmission electron micrograph of the solid product formed upon reacting [EtZn(SBu')], with H,S at a flow rate o f 5 cm'/min in CH,C12 solution at 22 OC.
window material should have low thermal expansion, high melting and decomposition temperatures, chemical inertness to hydrolysis and oxidation. and high fracture toughness. One of the most attractive materials for many IR optical applications is ZnS? but this material. as presently fabricated, does not possess the required mechanical properties. One approach to improving the mechanical properties of a material is to form a self-similar composite$* in this case, ZnS whiskers in a polycrystalline ZnS matrix. To fabricate IRtransmitting ZnS/ZnS composites, it would be necessary to use micron-sized whiskers of ZnS that have length to width ratios (aspect ratios) greater than IO. No present methodology exists for the convenient, large-scale preparation of such whiskers, although larger single crystals and whiskers of ZnS have been made by a variety of high-temperature (>900 "C), generally vaporphase, We have sought alternative methods for prcducing ZnS whiskers that would give greater wntrol over the product morphology. Low-temperature organometallic routes are attractive for this purpose, and we report herein a novel route to
Jack
Department of Chemistry and Laboratory for Molecular Structure and Bonding Texas A&M University College Station, Texas 77843
F. Albert Cotton
Received February 9. 1988
An Organometallic Route to Micron-Sized Whiskers of Zinc Sulfide Sir; A number of technologies require the development of highperformance optical materials that also meet stringent specifications of optical transparency and thermal, chemical, and mechanical properties.l,' For example, an infrared-transmitting 0020-1669/88/1327-3267$01.50/0
( I ) Musikant. S. Opricol Molcrir?ls; Dckkcr: New Yark. 1985. (2) Slack. G. A. 'Advanced Materials for Optical Windows"; General Elstrie Technical Information Sms; GE Co.: k h e n s l l d y . NY. 1979. (3) "Kodak IRTRAN Infrared Optical Materials"; Publication No. U-12: Eastman Kcdak Co.: 1971. (4) Fitzcr, E. 'Fiber Reinforced CeramicP and G l a w " ; Prcedingr of the lntcrnational Sympmium on Factors in Densification and Sintering of Oxide and Non-oxide Ccramicr, Hakanc. Japan; Gakujulsu Bunkcn Fukyu-kai: Tokyo. 1978; pp 618-673. (5) Fitrcr. E.; Schlichling. J. High Temp. Sci. 1980. 13. 149. (6) Yasuda. E.; Schlicting. J. 2. Wcrksrofprch. 1978, 9, 310. (7) Shichiri. T.; Aikami, T.; Kaninski, J. J . Crysf. Growth 1978, 43, 320. ( 8 ) Tempe% P. A.: Ballinlync, W. G. J . Crysf. Growrh 1974. 21. 219. (9) Steinberg. 1. T.; et al. J . Cryst. Growrh 1972. 13/14, 8 5 . (IO) Nitschc. R.; Bolstcrli. H.U.; Lichtcnstcigcr, M. Phys. Chem. Solids 1961, 21, 199.
( I I ) Fujita. S.; Minoto. H.;Takcbe. H.;Noguchi. T.J. Crysf. Gmwrh 1979,
47. 326. (12) Benoti, M.; Farkas-Jahnkc. M.; Lcndvay, E.; Ncmc1h.T. J. Mater, Sei. 1969, 4, 699. (13) Ujiie, S.; Kotera. Y.J. Crysr. Growrh 1971. IO, 320. (14) Lillcy, P.; Jones. P. L.; Lilting. C. N . W. J. Molcr. Sei. 1970. 5. 891. (IS) Vohl. P.; Buchan, W. R.;Genthc. J. E. J . Electrtroehem.Stroe. 1969, 118, 1842. (16) Yim. W. M.;Stafko. E. J. J . Eleerrwhem.Soe. 1972. 121. 381.
0 1988 American Chemical Society
