CORRESPONDENCE 1691
Vol. 7, No. 8)August 1968 inlet system can be very deceptive if the temperature range is below the ion-chamber temperature. ( 3 ) Reactions which occur in the mass spectrometer may be more pronounced than expected from other experimental evidence. For example, Mn(C0)5C1 can be purified by vacuum sublimation in a clean apparatus a t 60’ where gross amounts are transferred onto a cold glass surface. Yet this material will decompose in a few minutes after introduction into a previously used molecular-leak reservoir heated to only 50’. INSTITUTE FOR ATOMICRESEARCH AND HARRY J. SVEC GREGOR THE DEPARTMENT OF CHEMISTRY A. JUNK IOWASTATE UNIVERSITY AMES,IOWA50010 RECEIVED FEBRUARY 2, 1968
Transition Metal Nuclear Quadrupole Resonance. 11. Interpretation of the Rhenium Nuclear Quadrupole Resonance Spectra in Dirhenium Decacarbonyl Sir : Since nuclear quadrupole resonance spectroscopy provides a sensitive measure of the charge and electron distribution in molecules, Rels5 and Re18’ nqr may be useful in the study of rhenium-rhenium1s2and rheniumligand bonds. Although quadrupole coupling has been observed in the microwave spectrum of rhenium compound^,^ to the author’s knowledge the only pure Re185and Re187 nqr have been detected4 in dirhenium decacarbonyl. Because of the unusual nature of the results, no explanation has heretofore been proposed. This communication reports the low-temperature rhenium quadrupole coupling constant and offers a credible explanation for the anomalies. All resonances (cf. Table I) were obtained on a highpower superregenerative spectrometer5 and all frequency measurements were made utilizing spectrumanalyzing techniques.6 The rhenium field gradient, qEZ,and the asymmetry parameter, q, defined in terms of its irreducible tensor 7 =
(Pzz
-
al/J/azz
components7 can be calculated using solutions8 of the secular equation for a nucleus with a spin of 5//z and the value for the electric quadrupole moment of Re1s7,9 The room-temperature values are qzz = -6.9 X esu/cm3 and 7 = 0.63 0.03 while the liquid nitrogen
*
(1) F. A. Cotton and C . €3. Harris, Inovp. Chem., 4,330 (1965). (2) F. A. Cotton, ibid., 4,334 (1965). ( 3 ) J. F. Lotspeich, J . Chem. Phys., 81,633, 643 (1959). (4) S. L.Segel and R. G. Barnes, Phys. Rev., 107, 638 (1957). (5) C. B. Harris, unpublished work; G. E. Peterson, private communica-
TABLE I RHENIUM NUCLEAR QUADRUPOLE RESONANCE FREQUENCIES Nucleus
Re186 Re187 Rels5 Rel8’ Re185 RelS7 Re185 Re187
Temp, OC
Signal t o noise ratio
26.5 26.5 26.5 26.5 - 196 - 196 - 196 - 196
11:l 24: 1 44:l 92: 1 -1O:l -20: 1 -30:l -60: 1
Freq (+0.05),
M Hz
28.94 27.35 39.93 37.70 35.67 33.75 39.80 37.51
temperature values are qzr = - 7.1 X esu/cm3 and These results show that the “average”10field gradient tensor in the y direction is twice as large as the “average” field gradient tensor in the x direction a t room temperature. This is totally unexpected in view of the symmetryll of Rez(C0)lo. The D a d “ideal” molecular symmetry requires c4v site symmetry a t the Re nucleus. Under a fourfold rotation axis qzz should be the same as qVy,;.e., q = 0. The deviations from ideal CAvsite symmetry in iMnz( C O ) ~ and O ~ ~T c ~ ( C 0 ) are l ~ ~very ~ small. In fact, all the measured bond angles and bond distances in both molecules are within three standard deviations of ideal D l d molecular symmetry. Unfortunately, precise structural parameters for Rez(CO)lo are not available from a two-dimension crystal structure.ll However, it is difficult to imagine the ReZ(CO)lo structure deviating enough from its Tcz(C0)lo and Mn2(CO)l~isomorphs to produce such a large asymmetry parameter. Since the molecules are neutral, there can be no point charge lattice sum contributions to the field gradient in any direction. Induced dipole-induced dipole interactions cannot contribute any appreciable field gradient a t the Re nucleus. At a distance of -2 A the induced-dipole interactions are about one-tenth the coulombic contributions and thus would account14 for less than 1% of the total field gradient in neutral molecules. There does appear to be a credible explanation for the large asymmetry parameter if one accepts structural isomorphism and assumes that the anisotropic temperature factors, B r j ,of the form exp[-(Bllh2 B22k2 B d 2 2Bl2hk 2B13hl 2B23kl) 3 are similar for Mnz(CO)lo, Tcz(CO)lo, and Re~(C0)lo.’~ It is interesting to note that in both Mn2(CO)loand Tcz(C0)lo an analysis of the anisotropic temperature 17 = 0.88 f 0.03.
+
+
+
+
+
(10) The phrase “average” field gradient connotes the field gradient from contributions of electron distributions averaged over the dynamical properlies of the solid, e.g., large thermal or molecular vibrations in the lattice. The term molecular field gradient denoles the idealized field gradient of a nucleus a t its zero-point vibrational energy. (11) L. F. Dahl, I. Ishishi, and R. E. Rundle, J. Chem. Phys., 26, 1750
tion. ( 6 ) G. E. Peterson and P. Ivt. Bridenbaugh, Rev. Sci. Iwst7., 37, I081
(1957). (12) L. F. Dahl and R. E. Rundle, Acta Cryst., 16, 419 (1963). ( 1 3 ) M. F. Bailey and L. F. Dahl, Inovg. Chem., 4,1140 (1965). (14) T. P.Das and E. L. Hahn, Solid Slate Phys. Sufifil., 1 , 165 (1958). (15) Mnz(CO)lo and TcI(C0)in but not Rez(CO)ln crystal structures were
(1966). (7) T. P. Das and E. L. Hahn, Solid Stale Phys. Suppl, 1, 4 (1958). (8) M.1%.Cohen, Phys. Rev., 96, 1278 (1954). (9) H.Schuler and H. Korsching, Z. Physik, 106, 168 (1937)
refined anisotropically. The anisotropic temperature factors of both Mnz(C0)in and Tcz(C0)ro indicate the same relative amplitude of anisotropic thermal motion in the same directions. It seems safe t o assume that Rei(C0)in should be similar to isomorphs in this respect.
1692 CORRESPONDENCE
Inorgunic Chemistry
factors shows significant differences in thermal motion of the carbonyls cis to each other. In Mnz(CO)lo, for instance, the root-mean-square component of the thermal displacement lowers the X n ChVsite symmetry to CZv;e.g., the CO’s in the x direction have 30% less thermal displacement than the CO’s in the y direction. Assuming a similar behavior in Rez(CO)lo, the over-all effect, therefore, of the large anisotropic vibration is to leave pzz relatively unchanged in both direction (along the Re-Re bond) and magnitude but to make ;qyu/qz21 much larger than 1. In other words, the “average” electron density around the Re is distributed more by thermal motion along the y direction than the x direction; thus, a large asymmetry parameter may be observed. The increase in the low temperature 7 may be due to a decrease in the x and y motion but a greater decrease in the x direction. Low-temperature Rez(CO)lo anisotropic thermal parameters are needed to verify this point. The room-temperature 7 can be estimated theoretically as follows. Consider the vibration of a carbonyl attached to a rhenium as depicted in Figure 1. Under the harmonic oscillator approximation the rms component of vibration along the M-CO bond can be calculated16 from the anisotropic temperature tensors. In the case of Mn2(CO)lothere is a 30% difference between CO’s cis to each other. Relating the rms amplitude to the exponentional function, u, in a Gaussiun distribution, i.e., rms amplitude = ( 1 / 2 r ) ) ’ ” the expectation value of r P 3 centered a t the metal nucleus can be calculated, i.e. (r/T)1/2
S-+:
r-3e-4Y-Ro)2
dr
(1)
where Ro is the distance from the metal to the carbon. Using rms values for CO’s in the x and y directions, respectively, as 0.26 and 0.18 8, the expectation values of r V 3 are calculated with Ro = 1.83A as 0.157 and 0.161 A-3. Because the 5d radial wave functions of Re are so diffuse” (cf. Figure 2 ) , the effect of a carbonyl vibration is transmitted the Re nucleus as a dynamical variation in the ( Y - ~ ) of the d electrons. One must consider the two electrons forming the Re-C u bond as well as the four electrons participating in the Re-C T bond. These are depicted in Figure 2 . Note that the 7~ electrons are localized in a region of space about 45’ off the Re-C bond. Because of the angular dependence in the field gradient tensor, the effects of all T electrons are about equal to those of the u electrons. Thus eq 2 can be written, where N is the number of effective (qzz
7 =
--
-
qyy)dynamicaIN(1
Pzz
- “I”)
(2)
Figure 1.-Coordinate system for (r-3) and its relation to the CO vibrational amplitude.
,
--- Re
5d radical wavefunction
\
f i ‘.d T t
Ii I
OL‘
I
1.45
I
2.10
I
3.20
I
4.25
I
5.32
I
6.30
Bohr Units
Figure 2.-Radial 5d wave function for Re and its relation to the u and T electrons in an Re-CO bond.
0.753 X l o x 4and 0.772 X 1014 esu/cm3, respectively, using the calculated ( Y - ~ ) ~ , ~ N. is equal to 4 and a (1 - ym) of 28 is usedIg to calculate an 7 equal to 0.31. This compares favorably to the observed r] of 0.63. It should be emphasized that the above is only a qualitative argument meant to show that dynamical vibration anisotropy can in a credible fashion account for a sizable r]. Acknowledgment.-The author acknowledges the assistance in this work of M. D. Fayer and the U. S. Atomic Energy Commission under the auspices of the Inorganic Materials Research Division, Lawrence Radiation Laboratory, for financial support. (18) R. M. Sternheimer, Phys. Rev., 84, 244 (1951); 86, 316 (1952); 96, 736 (1954); 130, 1423 (1963). (19) J. F. Lotspeich, J . Chem. Phys., 31, 643 (1959).
DEPARTMEKT OF CHEMISTRY OF CHARLES B. HARRIS THE UNIVERSITY OF CALIFORNIA AND THE INORGAXIC M.4TERIALS RESEARCH DIVISIONOF THE LAWRESCE RADIATION LABORATORY BFRKELEY, CALIFORKIA 94720 RECEIVED FEBRUARY 9, 1968
MGssbauer Experiments on Iron(11) Phthalocyanine
electrons projected on the Re-C bond and (1 -- rm) is the Sternheimer factorl8 for inner-core polarizaSiY : tions. The dynamical qxx and puz,are calculated as We wish to report the results of Miissbauer spectro(16) R . W. James, “The Optical Principles of the llifiraction of X-rays,” G. Bell and Sons Ltd., London, 1962. scopic experiments on iron(I1) phthalocyanines. Our (17) F. Herman and S. Skillman, “Atomic Structure Calculations,” Prenindependent experiments are very similar to those of tice-Hall Inc., Englewood Cli5s, N. J., 1963.
