7247
J. Phys. Chem. 1992,96,7247-7251
(IO) Miyasaka, H.; Mataga, N . Bull. Chcm. Soc. Jpn. 1990, 63, 131.
troscopic experiments are in progress to reveal the dynamics of the complex in the excited state.
(1 1) Miyasaka, H.; Morita, K.; Kamada, K.; Mataga, N . Bull. Chem. SOC. Jpn. 1990,63,3385. Chem. Phys. Lett. 1991, 178,504. (12) Topp, M. R. Chem. Phys. Lett. 1976, 39,423. (13) Obi. K.: Yamanuchi. H. Chem. Phvs. Letr. 1978. 51.448. (14) Hiratsuka, H.; kamazaki, T.; Machwa. Y.; Hikida, T,;Mori, Y.J . Phys. Chem. 1986,90,774. (1 5 ) Nagarajan, V.; Fesscnden, R. W. Chem. Phys. Lett. 1984.11 2,207. (16) Johnston, L. J.; Lougnot, D. J.; Wintgens, V.; Scaiano, J. C. J . Am. Chem. Soc. 1988.110.518. Redmond, R. W.; Scaiano. J. C. Chem. Phvs. Lett. 1990. 166. 20. (17) KBjii, Y.; Fujita, M.; Hiratsuh, H.; Obi, K.; Mori, Y.; Tanake I. J. Phys. Chem. 1987, 91, 2791. (18) Melhuish, W. H. J . Opt. Soc. Am. 1964, 54, 183. (19) Miyasaka, H.; Morita, K.; Mataga, N. Private communication. (20) Beckett A.: Porter, G. Trans. Faraday Soc. 1963.59. 2038. (21) Miyasaka.H.; Makga, N. Proceed& of "Dynamics and Mechanisms of Photoinduced Electron Trmfer and Related Phenomenan;Elsevier: New York, in press. (22) Nagakura, S.J . Am. Chem. Soc. 19!58,80, 520. (23) Mataga, N.; Kubota, T. Molecular Interactions and Electronic Spectra; Marcel Dekkcr: New York, 1970. (24) Bromberg, A.; Meisel, D. J. Phys. Chem. 1985, 89, 2507. (25) Minegishi, T.; Hiratsuka, H.; Tanizaki, Y.; Mori, Y. Bull. Chem. SOC. Jpn. 1984, 57, 162. (26) Mataga, N.; Kaifu, Y. Mol. Phys. 1963, 7, 137.
Acknowledgment. This work was supported in part by the Grant-in-Aid for Scientific Research (No.03453019) from the Ministry of Education, Science and Culture. R&W NO. BPH, 16592-08-8; TEA, 121-44-8.
References md Notes
.-
(\ -1I ) Cohen. H. -H. J. Am. --,-- - - - S. G.: - Chao.
Chem. SOC.1968.90. , . 165. Cohen.
S.G. Cohcn, J. 1..; Phys. Chem. 1968, 72, 3782.
(2) Guttenplan, J. B.; Cohen, S. G. J . Am. Chem. Soc. 1972, 94, 4040. (3) Cohen,-S. G.; Parola, A,; Parsons, G. H. Chem. Rev. 1973, 73, 141. (4) Arimitsu, S.;Masuhara, H. Chem. Phys. Lett. 1973,22,543. Arimitsu, S.;Masuhara, H.; Mataga, N.; Tsubomura, H. J. Phys. Chem. 1975, 79, 1255. (5) Inbar, S.; Linschitz, H.; Cohen, S. G.J . Am. Chem. Soc. 1980,102, 1419. (6) Peters, K. S.; Freilich, S. C.; Schaeffer, C. G.J. Am. Chem. Soc.1980, 102, 5701. Schaeffer, C. G.; Peters, K. S. Ibld. 1980, 102, 7566. (7) Bhattacharyya, K.; Das, P. K. J . Phys. Chrm. 1986, 90, 3987. (8) Harhino, M.; Shizuka. H. J. Phys. Chem. 1987,91,714. Harhino, M.; Kogure, M. Ibid. 1989, 93, 728. (9) Devadoss, C.; Fasenden, R. W. J . Phys. Chem 1990,94,4540.
Cobalt, Rhodium, and Iridium Dioxide Molecules and Waish-Type Rules R. J. Van Zee, Y.M. Hamrick,+ S. Li, and W.Weltner, Jr.* Department of Chemistry and Chemical Physics Center, University of Florida, Gainesville, Florida 32611-2046 (Received: March 27, 1992; In Final Form: May 18. 1992)
COO,, Rho,, and IrO,, readily formed by the reaction of laser-vaporized metal atoms with oxygen, have been shown to be linear with 22ground states. This was established by electron-spin-resonancespectra of the molecules isolated in solid argon lo3Rh , (I = '/2): 191J931r (I = 3/2), and 170(I = hyperfine splittings have been obtained and at -2 K. 59C0(I= 'I,) indicate that the electron spin is predominantly on the metal atom and has -50% so character. Attempts to observe the metal atom-02 complexes were not successful. A Walsh-type rule applied to transition-metal dioxides is supported by the geometries found here.
I. Introduction In the course of research on the clusters of the group VI11 metals, Co, Rh, and Ir,' electron-spin-resonance (ESR) signals were detected which were established by addition of O2(and 1702) to be due to the metal dioxide molecules. There are several reasons for being interested in these dioxides. There is the question of whether a dioxide complex M+(O,-) is formed by these metals and whether its ESR spectrum can be detected, as has been the case for the alkali2-s and coinage metals.61' The IR spectrum of such a Rh complex has been reported by Hanlan and Ozin.12 Is this complex a necessary precursor to the formation of a covalently-bonded dioxide mole cule? Is the latter linear or bent and of high or low spin? The geometry and electronic properties in the ground state are also of intertst relative to the suggestion that Walsh-like rules may apply to such transition-metal (TM) triatomics. A simple molecular orbital scheme can be set up for transition-metal dioxides and dihalides,13-17 analogous to Walsh's treatment of BAB molecules1Ebut emphasizing d rather than p orbitals on A.ie2i Instead of an abrupt change from linear to bent geometry upon addition of the 17th valence electron (e.g., linear C 0 2 to bent NO,), the TM triatomics are proposed to change from bent to lincar when the 19th valeme electron is added (e.&, bent TiF2to linear VF2).13-17Among the first-raw dioxides, the geometry change would then occur from CrO, to Mn02, and most experimental evidence supports this scheme.22-26 Some
dioxides are still controversial, such as Fe0227-m and perhaps C U O ~ . ~Here ~ * ~the * ESR evidence that COO2, Rho2,and Ir02 (with 21 valence electrons) are linear is in accord with this Walsh-type rule. Finally, these metal oxides are of interest bccause of their possible relevance to the well-known catalytic activity of these metals. It is worth noting that there is not a monotonic change in the electronic properties of these metal atoms in going down the periodic table. Rh is ammalous in having a d8s (4F) ground state, whereas Co and Ir have d7s2(4F) lowest states with the d's lying approximately 0.4 eV h i g h ~ r . ~ )Thus, . ~ ~ for Rh, a promotion energy is not needed to utilize its extended 5s electron, and its exceptional reactivity is noticeable in matrices Containing H2,02, or CH,. This difference in the atomic properties should also ultimately be reflected in the molecular orbitals of the dioxides and in their electronic and magnetic properties. This is indeed found to be the case.
II. ExperimentalSection The Heli-Tran and ESR apparatus have been described prev i o u ~ l y ?The ~ ~ metal ~ vapor was produced with a Nd:YAG laser operating at 1064 nm. Cobalt rod, 5 mm diameter X 12 mm long (99.998% purity, 100% s9c~, I = 7/2), rhodium powder pressed into a pellet ap roximately 1 mm X 10 mm diameter (99.95% purity, 100%lo Rh,I = and iridium powder p r d into a pellet approximately 1 mm X 10 mm diameter (99.95% purity, 38.5% l9IIr, I = 3/2, and 61.5% 1931r,I = 3/2) were purchased from
P
Present address: Am= Laboratory, Amos, I A 50010.
0022-3654/92/2096-7247S03.00/0
(B
1992 American Chemical Society
7248 The Journal of Physical Chemistry, Vol. 96, No. 18, 1992
Van Zee et al.
1931'9'Ir02 /Ne
59C002A / r 193 191
Ir I I r I
I
I
1
2000
4000
6ooo
B (GI Figure 1. ESR of the s9C002(I = 7/2) molecule in an argon matrix at 4 K ( u 9.5593 GHz).
Io3RhO2 /Ar
1
I
1
n
I
I 0
I
I I
IllI
I
I
2030
I
3000
UI
I
I
4000
B (GI Figure 3. ESR of the I9lIrO2and 1931r02molecules in a neon matrix at 4 K ( u = 9.5592 GHz). The natural abundance of l9'Ir (I = 3/2) and 1931r(I = 3/2) is 38.5% and 61.596,respectively. TABLE I: Obrcned uwd Cdculrted ESR Line Positions b an Argon MaMx at 4 K and Derived Magnetic Parameters for %PO, and
'@*bo*
line wsitions. G obsd calcd I 331 (51 329.3
'9030,
u
= 9.5593 GHz
I
I' I 3ooo
I
1
3400
1
I
3800
'I
gl
The ESR spectra of matrices formed by depositing laservaporized cobalt, rhodium, or iridium metals with a 1% 02/argon mixture are shown in Figures 1-3. In the case of iridium, the dioxide molecule was also generated directly during metal vaporization with no added oxygen. Presumably this was because of oxide on the surface of the metal powder used in preparing the pellet. However, adding oxygen increased the signal intensity enormously. Also, intense Ir02 spectra were obtained by laser vaporization of pellets made by pressing a 1:1 molar mixture of Ir and Ir02 powders. All of the ESR spectra are accounted for as spectra of linear molecules.37 The lines in Figure 1 attributed to hyperfine (hf) transitions of a S = 59Co02molecule are listed in Table I. Strong perpendicular lines at 331,844, and 5906 G and weaker, oppo-
{ 3640 (2)
847.3 5905:O 4781 .O 6988.4 3186.0 3503.1 3271.8 3639.7
magnetic parameters
Figwe 2. ESR of the Io3RhO2(I = molecule in an argon matrix at 4 K ( u = 9.5596 GHz). CH3and H atom impurity signals are indicated.
III. ESRSpeetra
(:;::{i;
(2) * { 3186 3503 (2) 3272 (2)
103~h0~ u = 9.5596 GHz
B(G)
Aesar or in the case of iridium from Poly Research Corp. Solid Ir02 (99.5% purity) was purchased from Aldrich, oxygen gas (99.995% pure) from Airco, and 1 7 0 2 (74.1 atom 96 enriched) from Merck, Sharp, and Dohme Isotopes. Neon and argon were o b tained from Airco and krypton from Matheson; all were 99.995% pure. Other experiments were performed specifically to allow formation of a relatively weakly-bonded metal-02 complex, if it existed, perhaps as a precursor to the valence-bonded form o b served here. In all of these trials, the metal atoms were produced by resistively heating 0.75-1 .OO-mm-diameter wire to reproduce the matrix infrared experiments where metal-02 complexes were reported to be formed.12 Cobalt was deposited in an argon matrix containing 10% 02. Also, the metal and O2were deposited in separate matrix layers and allowed to diffuse together. Finally, photolysis with a medium-pressure mercury lamp was employed. In all of these experiments, ESR evidence for a complex did not appear, only the spectra produced by the above lascr vaporization were detectable.
844 (5j I5906 (5)
59Co02 2.000 (1) '03Rh02 1.972 (1)
g,
IAy(M)I: MHz
IAA(M)I? MHz
2.028 (1) 3150 (3) 2553 (3) 2.037 (2) 1014 (3) 902 (3)
'At metal atom nucleus. "obalt stant.
Q:b
MHz 4.5 (3)
nuclear quadrupole coupling con-
sitaphase, parallel lines at 4784 and 6988 G are observed,centered approximately around g = 2 at 3400 G. Since I = 7/2 for 59C0, one might expect eight perpendicular and eight parallel limes, but because of the large hyperfine splitting, most of these transitions are not otxwed." whm 1 7 0 2 (I = 5/Jreplaced 1602in the argon, no additional hyperfine splittings were detected, indicating that they were small enough to lie within the line widths. This was not the case for Rh and Ir (see below). When lo3Rh (I = '/3 was vaporized into the W2/Ar mixture, a pair of doublets was observed in the g = 2 region (see Figure 2) and are listed in Table I. Perpendicular lines are observed at 3186 and 3503 G and weak parallel lims of opposite phase at 3272 and 3640 G. This is expected for a linear molecule with S = 1/2 containing one magnetic nucleus with I = 1/2. Substitution of I6J7O2causes each perpendicular line to be surrounded by many hf lines, but an 170hf structure could not be observed an the weaker parallel lines. The pattern about each line is essentially the same as observed for Ir16J702, which will be discussed thoroughly below. Here the spacings in these lines are 5.0 (1) and 2.5 (1) G; their signifcance will be made clear in that discussion. The remarkable spectrum of Ir1602in Figure 3 consists of two sets of lines for molecules containing 1911r (38.5% natural abundance) and 1931r(61.5% natural abundance). The separatespectra are easily recognized because 19% is the more abundant and ale0 has a slightly larger nuclear moment. Four perpendicular and four parallel lines are expected for each isotopic molecule with Z = 3 / 2 for each metal nucleus. Four perpendicular (upphase) lines for each molecule are clearly visible in Figure 3, but only one weak parallel line at 4436,4514 G is discernible. The other two down-phase lines at 2663.5 and 2750 G can be accounted for
The Journal of Physical Chemistry, Vol. 96, No. 18, 1992 7249
Co, Rh, and Ir Dioxide Molecules TABLE Ik Observed and Cdculrted4Liw Positions (in C) for I9PrO2 and %O, in Argon Matriced at 4 K (v = 9.5592 CHz) WIr) obsd (I) calcd (I)obsd (11) calcd
193 1716
I r ' O,/Ar
(11)
193~r02
3i2
1936 (2) 2488 3264 4196
If2
-'I2 -3/2
1936 2490 3267 4195
2020 2654 3499 4514 2663.5
4514(2) 2663.5
extra line at 8 = 21'
l9I1ro2
)I2
2038 (2) 2565 3292 4132
'12 -If2
-V2
2038 2567 3294 4130
2150 (2)
2146 2741 3516 4432 2755
4436 2750
extra line at 8 = 2 1' Parameters used are given in Table 111.
I936 G
Figure 4. ESR line of 1931r1602 at 1936 G accompanied by smaller lines split into 6 and 11 components. amounts of 1601931r170 and 1931r1702 as indicated by respectively, by hyperfine interaction with I7O (I= the "stick" pattern. The observed strong central line arises from adventitious presence of excess Ig31Po2.
(I
as "extra" lines (off-principal axis t r a n s i t i ~ n s ) . ~Three ~ . ~ missing parallel lines are either too weak to be observed or are overlapped by other lines (see Table 11). Similar spectra for I r 0 2 were observed in argon and krypton matrices. The substitution of 1 7 0 2 (74% isotopically enriched, Z = 5 / 2 ) in the argon produced a large number of hf lines around each strong central I6O line in the spectrum, as illustrated in Figure 4. The largest splitting is 6.5 (1) G on the perpendicular lines and 8.5 (1) G on the extra lines. Hyperfine splittings were not observed on the parallel lines.
is shown in Table 11, and the final derived parameters for both isotopically-substitutedmolecules in all three matrices are tabulated in Table 111. The quadrupole coupling term makes a significant contribution in Ir02 The isotropic lAh(lg3Ir)1 = (AH + 2A,)/3 and dipolar IAdip(1931r)l= (All - A,)/3 hyperfine components, calculated assuming All and A, have the same sign, are also given there. The independently-determined hf parameters for the two isotopic molecules should be related by their nuclear gyromagnetic ratios */191/*/193 = 0,9185, and this is tested by the ratios of observed hf constants given at the bottom of Table 111. The ratios of Q'values should agree with the ratios of the nuclear quadrupole moments, but I9lQand 193Qare only known to about 10% accu-
IV. Aoalysis Since all of the spectra are attributable to axially-symmetric S = molecules, an appropriate Hamiltonian is 7t = g l l k w z + g , B ( W x + Bpy) + 4SJZ + A,(SA
+ s j Y )+ QW - w
+11/31 (1)
Introduction of "0(Z = 5/2) by using partially-enriched (74.1 atom %) I7O2in argon leads to the formation of three oxygen isotopomers, Ir'Q, 1601r170, and Ir1702in relative amounts 0.068, 0.385, and 0.548, respectively. Then the presence of 1'QIr170 and Ir1'02 splits each IrI6O2line into 6 and 1 1 hf components to produce a total of 17 components with relative intensities 1:2:3:4.2:4:4.2:5:4.2:6 ...,as shown in the "stick" pattern in Figure 4. The outer three lines in that pattern are split by IAl(170)l = 6.5 (1) G and the remainder by 1.4,(170)1/2. This hf pattern also appears on the extra lines, but there the splitting is slightly larger, 8.5 (1) G. Table IV gives a comparison of the g and A tensors for 59Co02, Io3RhO2,and '931r02in argon matrices and the derived isotropic Ai, and dipolar Adip hyperfine components. From Ah and Adip, one derives the Fermi-contact and anisotropic contributions, respectively, to the electron-nuclear interaction. The su contribution to the wave function is also listed and will be discussed below for each case.
Here the symbols have their usual meaning.41 Q', the nuclear electric quadrupole coupling constant, was only necessary in the analysis of the Coo2 and I r 0 2 spectra. Spin matrices were generated from eq 1 and eigenvalues assigned by computer diagonalization. HC002md 103Rb02. For cobalt with Z(59C0)= 7/2, the spin matrix is 16 X 16. Parameters were varied until the energy differences were in resonance with hv. The lowest field line was accounted for as an NMR transition (AM, = *l).38 Comparison of the observed and calculated line positions is given in Table I, along with the derived magnetic parameters. only a 4 X 4 matrix needed For Io3RhO2with Z(lo3Rh)= to be solved, and this nucleus has no quadrupole moment. The line positions and derived magnetic parameters are also given in Table I. 191J931r16J70 2' The 1911r1602 and 1931r1602 spectra could be analyzed separately, each involving an 8 X 8 matrix. The four perpendicular lines were fit using g,, A,, All, and Q'(assuming ga= 2.0023). Then in fitting the extra line and one or two parallel lines, A, and gll were varied and the perpendicular lines fit again, varying g,, A,, and Q! The goodness of fit for the argon spectra TABLE III: Derived Magnetic Parameters for
1g191931r02
V. Discwsioo The ESR spectra of all three oxides are those of S = molecules with axially symmetric g tensors, i.e., with 22ground
in Neon,Argon, and Krypton Matrices
argon
neon 81 g,
IArl, M H z lA1lv M H z IQ 1, M H z
IAh(1931r)l! M H z IAdip(1931r)1,"M H z
A1(191)lA1(193) A d 1 9 1) l A ~ ( 1 9 3 ) Q'(191)/Q'( 193y
krypton
191
193
191
193
191
193
2.0055 (5)o 2.1312 (5) 2057 (9) 1190 (3) 280 (3)
2.0055 (5)o 2.1295 (5) 2256 (9) 2148 (3) 280 (3) 2184 (5) 36 ( 5 )
2.0055 (5) 2.1332 (5) 2102 (3) 2012 (3) 306 (3)
2.0065 (5) 2.1325 (5) 2288 (3) 2178 (3) 294 (3) 2215 (2) 37 (2)
2.0055 (5)4 2.1184 (5) 2124 (9) 2008 (3) 215 (3)
2.0055 ( 5 ) O 2.1181 (5) 2292 (9) 2166 (3) 214 (3) 2208 (5) 42 ( 5 )
0.9 12 0.926 1.OO
0.919 0.924 1.04
0.927 0.927 1.00
4Assumed, taken from argon results. bAssuming signs of A,, and A, are the same. CRatioof Q(191)/Q(193) = 1.1 (from ref 42).
7250 The Journal of Physical Chemistry, Vol. 96, No. 18, 1992
TABLE I V Derived Magnetic Parameters for sC01702, lQ3Rhi7O29 a d 19W702 in Argon Matrices at 4 K gl, g,
IAii(M)I$ MHz lA,(M)l, MHz 1~,(170)1,
MHZ
lAl,(M)I," MHz IAdip(M)lt(l MHz W2(0)l(M), au ((3 cos2 0 - 1)/9)(M), au % SU(M)~
S9C01702
l03Rh17O2
1931r170
2.000 (1) 2.078 (1) 3150 (3) 2553 (3)
