1328
J. Phys. Chem. 1989, 93, 1328-1332
fit the assignedE fundamental frequencies. The symmetry force constant values are available as supplementary material. A trial structure for BCB was taken from the results for CCB and from the position of the C-Br peak in the experimental RD curve. Least-squares refinements” were carried out by fitting a theoretical intensity curve to the two averaged experimental intensity curves. All geometrical parameters could be refined independently, but some of the vibrational amplitudes had to be combined into groups. These are evident from Table I, which contains the final results together with those for CCB. The correlation matrix for the final model is given in Table 11. The theoretical intensity and RD curves are shown in Figures 2 and 3, respectively.
Discussion Table I shows that the chlorine-substituted and the brominesubstituted cyclobutene-3,4-dioneshave almost identical structures. r(C=O) is found to be slightly shorter and r(C3-C4) slightly longer in BCB than in CCB, but none of the differences are significant. Substituting C1 with the less electronegative but larger and more polarizable Br atoms therefore appears to have very little effect on the structure of the rest of the molecule, a behavior that extends also to 1,2-disubstitution with iodine atorns.lE This fact may be attributed on one hand to the ample positional freedom of the atoms external to the ring as a consequence of the small internal angles of the ring, and on the other to a remarkable compliance of the cyclobutene-3,4-dione moiety to substituents of different electronegativity. Such a feature is found in a rather extreme degree in more complex derivatives, which can be dianions, radical anions, and electron-deficient, neutral compounds, without changing t o p ~ l o g y . ’ ~ (17) Hedberg, K.; Iwasaki, M. Acta Crystallogr. 1964, 17, 529. (18) Lunelli, B.; Monari, M., submitted for publication. (1 9) Busetti, V.; Lunelli, B. J. Phys. Chem. 1986, 90,2052, and references therein.
In CCB we observed a very short carbon-halogen bond (r(C-CI) = 1.683 (2) A). The same is also found in BCB with r(C-Br) = 1.831 (4) A. This is significantly shorter than the value observed for a bond between an sp2 carbon atom and Br in other molecules (r(C-Br) = 1.881 (7) A in bromoethene,20 and 1.883 (8) A in 1.909 (4) A in 2,3-dibrom0-1,3-butadiene,~’ 2 bromopropena122). As far as we know the C-Br bond in BCB is the shortest bond of this t pe ever observed, but the results for 2-bromofuranZ3(1.849 (4) ) and 3-bromof~ran2~ (1.853 (7) A) also show short C-Br bonds. A possible explanation for such short distances may be diminished repulsion between the bond electrons of the C-Br and adjacent bonds and also between the bromine and the other atoms bonded to the same carbon atom, due to the large change of the standard sp2 carbon valence angles in the cyclobutenedione and furan derivatives.
K
Acknowledgment. We are grateful to H. V. Volden for running the electron diffraction experiments and to S. Gundersen for technical assistance. Financial support from the Norwegian Research Council for Science and the Humanities, the Italian Minister0 della Pubblica Istruzione, fondi 60%, and of Chemische Werke Huls through a generous gift of squaric acid, is acknowledged. Registry No. BCB, 110497-38-6.
Supplementary Material Available: Symmetry coordinates (Table 111), values of the symmetry force constants (Table IV), total scattered intensities, s41,, final backgrounds, and average molecular intensities (7 pages). Ordering information is given on any current masthead page. (20) Huisman, P. A. G.; Milhoff, F. C. J . Mol. Struct. 1979, 57, 83. (21) Neisess, J. A. Thesis, Oregon State University, 1971. (22) Hagen, K., to be published. (23) Belyakov, A. V.;Scherbak, G. A.; Vilkov, L. V. J. Mol. Struct. 1985, 131, 101. (24) Scherbak, G. A.; Vilkov, L. V.; Sadova, N. I.; Boiko, Yu. A. Zh. Srrukt. Khim. 1979, 20, 530.
Indtum Dicarbonyl: Matrix Isolation ESR and I R Study William G. Hatton, Nigel P. Hacker, and Paul H. Kasai* IBM Almaden Research Center, San Jose, California 951 20 (Received: July 28, 1988)
Indium atoms and CO molecules were co-condensed in argon matrices at -4 K. Examination of the resulting matrices by IR and ESR revealed the formation of the In(0) dicarbonyl complex. No evidence was obtained for the formation of In(0) monocarbonyl. The characteristic symmetric and antisymmetricstretching modes of the dicarbonyl system were observed hyperfine and quadrupole coupling tensors of the complex at 2037 and 1997 cm-l, respectively. The g tensor and the ’Ish were determined as follows: g, = 1.978, g, = 1.980, gu = 1.969; A, = +420, A, = -400, A, = -440 MHz; and P, = +7.2, P, = Pu = -3.6 MHz. The x and y axes lie within the molecular plane of the complex having a bent, planar structure. The unpaired electron density is essentially all in the In pr orbital perpendicular to the plane.
Introduction Recently, we reported on the ESR spectra of aluminum dicarbonyl, Al(CO)z, and gallium dicarbonyl, Ga(C0)2, generated in argon matrices by co-condensation of the metal atoms with carbon monoxide at near liquid helium temperature.’s2 Earlier, Ogden and co-workers showed, by IR spectroscopy, the formation of AI(CO)z in krypton matrices3 and Ga(CO)z in xenon matrices4 (1) Kasai, P. H.; Jones, P. M. J . Am. Chem. SOC.1984, 106, 8018. (2) Kasai, P. H.; Jones, P. M. J. Phys. Chem. 1985, 89, 2019. (3) Hinchcliffe, A. J.; Ogden, J. S.; Oswald, D. D. J. Chem. Soc., Chem. Commun. 1972. 338.
0022-3654/89/2093- 1328$01.50/0
at 20 K. More recently, Howard, Mile, and their co-workers demonstrated the formation of A1(C0)2 in adamantane matrices at 77 K, by both ESR and IR, and showed that the complex was stable, at least in the host matrix, close to room temperature.5-6 Most interestingly, no evidence was observed for the formation of the corresponding monocarbonyl species in any of these studies. (4) Ogden, J. S. In Cryochemistry; Moskovitz, M . , Ozin, G. A,, Eds.; Wiley: New York, 1976; p 247. ( 5 ) Chenier, J. H. B.; Hampson, C. A.; Howard, J. A,; Mile, B.; Sutcliffe, R.J . Phys. Chem. 1986, 90, 1524. (6) Chenier, J. H. B.; Hampson, C. A.; Howard, J. A,; Mile, B. J. Chem. Soc., Chem. Commun. 1986, 730.
0 1989 American Chemical Society
Matrix Isolation ESR and IR Study of III(CO)~
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1329
It was deduced that these dicarbonyls have a bent, planar structure, I. Thus, the metal atom is sp2 hybridized with its
I
lone-pair electrons in the orbital pointing away from the ligands and with its unpaired electron in the pz orbital perpendicular to the molecular plane. The complex is hence formally stabilized by the dative interaction of the lone-pair electrons of the carbonyls into the vacant sp2 orbitals of the metal atom and the back-donation'of the unpaired electron into the antibonding 7r* orbitals of the carbonyls. Recent ab initio calculations performed on AlCO and A1(CO)2 by Jordan and co-workers predicted that AlCO and A1(C0)2 were both bound, but the binding energy of the latter ( 17 kcal/mol) for the two C O molecules was 6 times larger than that of the single C O in the former.' We report here on the ESR and IR spectra of indium dicarbonyl, In(CO),, generated in argon matrices. The IR spectra demonstrated the presence of two carbonyls in the complex, while the ESR spectra showed the presence of a single indium atom in the complex. The present complex also has structure I, but the extent of the back-donation of the unpaired electron is much less. As in the cases of aluminum and gallium, no evidence was observed for the formation of the monocarbonyl complex.
-
Experimental Section A liquid helium cryostat designed for trapping vaporized species in an inert gas matrix and examination of the resulting matrix by ESR has been previously described.* In the present series of experiments indium atoms were generated from a resistively heated (- 1300 "C) tantalum cell and were trapped in argon matrices containing a controlled amount (-20%) of carbon monoxide. The metal atom concentration was estimated to be 0.1%. The ESR spectrometer used was an IBM Model ER2OOD system. All the ESR spectra reported here were obtained while the matrix was maintained at -4 K, and the spectrometer frequency locked to the sample cavity was 9.425 GHz. In the IR experiments matrices were deposited on a CsI disk cooled to 30 K by an Air Products Displex closed-cycle system, and the spectra were obtained with an IR-44 FTIR of IBM Instruments. Research grade argon and C P grade carbon monoxide were obtained from Matheson, while I3C-enriched (enrichment >90%) carbon monoxide was obtained from MSD Isotopes. Indium metal (99.999%) was obtained from Ventron Corp.
2500
3000
3500
4000
GAUSS
Figure 1. ESR spectrum observed from the In/CO (20%)/Ar system. The doublet due to inadvertently formed HCO radicals and the decet due to In(CO)? are indicated.
dicated. The major isotope of indium is ( I = 9/2, natural abundance = 96%, p = 5.50728,). The decet pattern was thence attributed to the hf (hyperfine) interaction with the 'I5In nucleus. Decreasing the C O concentration in the matrix resulted only in diminution of the signals seen in Figure 1. The In(0) monocarbonyl complex, if formed, would be linear, the dative interaction of the C O lone-pair electrons being the dominant factor of its formation. It would then be a 211radical, and its ESR spectrum would again be broadened beyond detection. Thus, in analogy with the cases of aluminum and gallium atoms, the decet in Figure 1 was assigned to In(C0)2 of structure I. The overall variation of the peak height and line shape of individual components of the decet indicates an anisotropic g tensor and an equally anisotropic hf coupling tensor of the complex. We also note, in the lower field half of the spectrum, an anomalous intensity variation and presence of additional signals midway between successive components of the decet. These features suggest the presence of nuclear quadrupole interaction. Thus, the spin Hamiltonian appropriate for the present complex is given by
Here (gx,gy,gz),(Ax,Ay,Az),and (Px,Py,Pz)are the diagonal elements of the principal g tensor, the hf coupling tensor, and the quadrupole coupling tensor, respectively. The most notable effect of the quadrupole terms, when small compared to the hf terms, is induction of forbidden transitions of the type AM, = f l and
ESR Spectra and Assignments The ground-state electronic configuration of In atoms is 4d'05s25p1. Thus, owing to the degeneracy of the p orbitals, the ESR signal of the indium atoms situated at sites of cubic symmetry would be broadened beyond detection. In fact, argon matrices containing the In atoms alone appeared white and showed no ESR signal. Argon matrices containing both the In atoms and C O (>sa)appeared bright red and showed strong ESR signals. Figure 1 shows the ESR spectrum observed from the In/CO (20%)/Ar system. Except for the sharp doublet due to inadvertently formed H C O radicals9 and several sharp peaks (near and centered about the g = 2.00 position) due to organic contaminants, the entire spectrum was recognized as a decet as in-
A treatment of Hamiltonian 1 by the second-order perturbation theory has been reported by several authors."*I2 Analytical expressions for the resonance positions of the normal and forbidden transitions and their transition probabilities have been reported, and a powder pattern simulation program based on these equations has also been described.12 A preliminary analysis of the decet revealed that the magnitude of the quadrupole terms was -1/50 of the hf terms so that the resonance positions of the normal hf components could be given accurately by the usual second-order solution of Hamiltonian 1
(7) Balaji, V.; Sunil, K. K.; Jordan, K. D. Chem. Phys. Left. 1987, 136, 309. (8) Kasai, P. H. Acc. Chem. Res. 1971, 4 , 329. (9) Adrian, F. J.; Cochran, E. L.; Bowers, V. A. J . Chem. Phys. 1962, 36, 1661.
(10) See, for example: Abragam, A,; Bleaney, B. Electron Paramagnetic Resonance of Transition Ions; Oxford: London, 1970; pp 178-186. (1 1 ) See ref 10 for solutions for an axially symmetric case. (12) Kasai, P. H.; McLeod, D., Jr.; Watanabe, T. J. Am. Chem. Soc. 1980, 102, 119.
*2.'0
1330 The Journal of Physical Chemistry, Vol. 93, No. 4 , 1989
Hatton et al.
H 200 G
Figure 2. (a) Computer simulated spectrum based on the g and hf tensors given in the text. (b) Computer simulated spectrum based on the same g and hf tensors and the quadrupole terms given in the text.
without the quadrupole terms. Thus, from the canonical resonance positions (i, j , k , and 1, m , n) of the lowest and highest field components indicated in the figure, the following g and hf coupling tensors were determined.I3 g, = 1.980
lAll = 144 G
g2 = 1.978
lAzl = 151 G
g3 = 1.969 /A3[= 161 G
Figure 2a is the simulated powder pattern based on these parameters and Hamiltonian l without the quadrupole terms. The agreement between the observed and simulated spectra in terms of the overall peak height and line-shape variations is reasonable. In determining the g and A tensors given above, it was assumed that the canonical resonance positions i, j , and k in the lowest field component were associated respectively with the positions n, m, and 1 of the highest field component. A much poorer agreement in the peak height and line-shape variations resulted when any of the five other possible associations was assumed. The quadrupole tensor of the complex was then determined, through a trial-and-error process for the best fit, with the following result: Pi = -1.3 G
P2 = + 2 . 6 G
P3 = -1.3 G
Figure 2b shows the simulated spectrum based on the g and hf tensors determined earlier and the quadrupole terms given above. The anomalous intensity variation and additional peaks midway between successive normal hf components in the lower field half of the decet are clearly demonstrated. The quadrupole tensor determined above has an apparent axial symmetry. A reasonable agreement was obtained only when the symmetry axis was identified with axis 2. The sensitivity of the simulated spectrum upon the quadrupole tensor is illustrated in Figure 3 where only the quadrupole terms were changed as indicated. The most conspicuous discrepancy between the observed and simulated (1 3) For analyses of ESR powder patterns, see, for example: Ayscough, P. B. Electron Spin Resonance in Chemistry; Mathuen: London, 1967; pp 323-332.
w
Figure 3. Computer simulated spectra based on the g and hf tensors given in the text and the following quadrupole terms. (a) P, = +2.2, P, = Py = -1.1 G ; (b) P, = +2.6, P, = Py = -1.3 G ; and (c) P, = +3.0, P, = Py = -1.5 G .
spectra is noted in the highest field component. It is ascribed to the inhomogeneity of both the g tensor and the hf tensor due to matrix effects. As stated earlier, when small compared to the hf terms, the quadrupole terms induce forbidden transitions but do not affect the resonance positions of the normal hf components. The quadrupole terms must affect the transition probabilities of the normal components, however. We hence wrote a computer program that would graphically illustrate the orientation dependencies of the resonance positions of the normal and forbidden transitions and their transition probabilities. Figure 4 is a result of such computation based on the g tensor, the hf tensor, and quadrupole terms determined above. Here the direction of the magnetic field is changed from axis 2 (6' = Oo) toward axis 1 (6' = 90°), and the transition probabilities are indicated by vertical bars at '3 intervals. For sake of clarity, only the lower field half of the pattern is shown, and the forbidden transitions of the type AM,= f 2 are omitted as their transition probabilities are negligibly small throughout the range. The figure reveals the following. (1) The transition probabilities of the lowest and highest field normal components ( M I = fgI2)and those of central components ( M I = f l / * ) are hardly affected by the quadrupole terms. (2) The forbidden transitions of the type. AM, = f l occur most conspicuously in the outer intervals at the expense of the transition probabilities of the and f5/z. The anomalous normal components MI = peak-height variation and additional peaks midway between the successive normal components in the lower field half of the
Matrix Isolation ESR and IR Study of In(C0)2
-27
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1331
-32
5
2
C
2 m
m
32w gdUSS
Figure 4. Orientation dependencies of the resonance positions and the transition probabilities of the normal and forbidden transitions (AM, = il)of In(C0)2. The magnetic field is moved from axis 2 (e = OO), toward axis I (e = 90°), and the transition probabilities are indicated by vertical bars at 3' intervals. Only the lower field half of the pattern is shown.
spectrum are thus accounted for. These features are less apparent in the higher field half due to larger orientation dependency of the resonance positions of both the normal and forbidden transitions. The experiment was also performed using 13C-enrichedcarbon monoxide (enrichment >go%). Other than slightly increased line width of individual components, the observed spectrum was essentially identical with that of the normal isotopic species. From the line-width increase the upper limit of the 13C hf coupling constant was estimated to be - 5 G. This figure is significantly smaller than the largest 13Chf coupling constants, 10 and 8 G, respectively resolved in the spectra of Al(13CO)2and Ga(13C0)2.1J Thus, unlike the situations for the A1 and Ga carbonyls, the number of carbonyls involved in the present complex could not be determined unequivocally from the ESR study. To this end we examined the IR spectra of In/CO/Ar matrices.
IR Spectra and Assignments Figure 5a shows the IR spectrum observed from the In/CO (10%)/Ar system. The bands due to isolated ' T O and I3CO were readily recognized as indicated. The two bands labeled S and A, respectively, were observed only when both C O and In atoms were co-condensed with relatively high C O concentration (>5%). The relative intensity ratio of the S and A bands remained fixed at The S and A bands at 2037 and 1997 cm-' were hence assigned to the symmetric and antisymmetric stretching modes of the dicarbonyl moiety of In(CO),. When the C O concentration was low ( g, (=g,, the free spin value, 2.0023) > g,. It is also expected that the metal hf tensor is approximately symmetric about the z axis. The observed, individual g values and hf coupling constants were assigned to the respective molecular axes of structure I based on these considerations. The hf coupling constant expected from a unit spin density in the In 5 s orbital is 20 180 MHz (-7200 G).I6 The unpaired
1332 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 electron density in the In 5s orbital in the present complex, In(CO),, is hence negligible. The ESR result obtained by using I3CO showed that the unpaired electron density in the carbon orbitals is also small. It must be that In(CO), also has structure I, and its semifilled orbital is given by eq 2. However, the observed g values and the hf coupling constants appear at variance with the expectations cited above. The observed g values of the complex are all less than the free spin value, and none of the hf coupling constants appear singularly unique representing the value along the symmetry axis. The one-electron spin-orbit coupling constants of the valence p orbitals of Al, Ga, and In atoms are 75, 551, and 1475 cm-I, respective1y.l’ Thus, if a similar energy relation exists between the semifilled orbital and the orbitals involving the metal p, and pv orbitals, a much larger positive shift for gv and an equally large negative shift for g, are expected for In(CO)> The observed, much smaller than expected g shifts of In(CO), can be accounted for if we postulate that the In atom in the complex remains essentially in its atomic configuration, 5s25p,’5p~5p~, and the two carbonyls datively interact with the vacant p, and py orbitals. The relation g,, > g, > g, is expected to remain due to some hybridization caused by the ligands. The observed g values of In(CO), were hence assigned to the respective molecular axes as given in Table I. The overall negative shift observed for the whole g tensor is attributed to the increased overlap of the semifilled orbital with the orbitals of the host matrix atoms. It has been shown that delocalization of an unpaired electron into orbitals of rare-gas matrices causes a negative g shift.I8 CuCO and AuCO have been generated in argon matrices and examined by ESR.19s20 The semifilled orbital of these complex is an sp, orbital pointing away from the ligand. The gzvalue of CuCO is 1.998 and that of AuCO is 1.979. The axial symmetry about the z axis for the ‘lsIn hf tensor can be attained if we postulate A I = A = -144 G, A , = A, = + I 51 G, and A3 = A, = -161 G. The llrIn nuclear quadrupole tensor should reflect the symmetry of all the valence electrons about the In nucleus. The configuration assumed by the In atom and the suggested scheme for the dative interaction by the two carbonyls predict that the quadrupole tensor should also be symmetric about the z axis as observed. The ‘lsIn hyperfine and quadrupole tensors thus determined are given in Table I in megahertz (converted by multiplication by gi/3). The principal components All and A , of an axially symmetric A tensor are related to the isotropic term, Ai,,, and the dipolar term, Adipt as All = Aiso + 2Adip
A , = Aiso - Adip
(3)
For an unpaired electron in the In 5p orbital, Ais,results from polarization of the filled s orbitals and should be extremely small in comparison with Ais, of an electron in the In 5s orbital. The dipolar part Adip for an electron in the In 5p orbital can be calculated as follows: Adip(InS~)= g J k P , 2/5(r-3)1n,sp= 284 MHz The numerical value is based on the Hartree-Fock solution of the In 5p orbital.16 . The analysis of the hf tensor of In(CO), according to eq 3 (letting A , = ( A , + A y ) / 2 )yields the following: Aiso = -142 M H z Adip = +280 MHz Ai, and Adipthus obtained correspond to a spin density of -0.007 in the In 5 s orbital and +0.99 in the In 5p orbital, respectively.
The former figure should not be taken literally since, as stated (16) Morton, J. R.; Preston, K. F. J . Magn. Reson. 1978, 30, 577. (17) Computed from the fine structure spacings of atomic energy levels given in: Moore, C. E. Natl. Bur. Stand. (US.) Circ. No. 467. 1949, 1 ; 1952, 2; 1958, 3. (18) Adrian, F. J. J . Chem. Phys. 1960, 32, 972. (19) Kasai, P. H.; Jones, P.M. J . Am. Chem. Soc. 1985, 107, 813. (20) Kasai, P. H.; Jones, P. M. J . Am. Chem. SOC.1985, 107, 6385. (21) Smith, W. V.; Sorokin, P. P.; Gelles, I . L.; Lasher, G. J. Phys. Reu. 1959, 115, 1546.
Hatton et al. above, A , here represents the sum attributes of spin polarization in all the filled indium s orbitals. Nearly a whole unpaired electron density is found in the In 5p, orbital; the back-donation from this orbital into the A orbitals of carbonyls, as given in eq 2, must hence be negligible. For AI(C0)2 and Ga(CO), spin densities in the metal p, orbitals were determined as $0.42 and +0.55, respectiveIy.lS2 The electric field gradient tensor e4 at the nucleus due to an electron in a valence p, orbital is given by eq,, = -2eq,, = -2eqvy = 4 / s e ( r - 3 ) pand, , if it is the only electron in the valence shell, the quadrupole tensor in the form of eq 1 would be as follows:
Substituting the known quadrupole moment of the llsIn nucleus (0.83 X cm2) and 6.05 au-3 for (r-3)ln,sp (the value obtained from the Hartree-Fock solution),I6 one obtains P, = 13 MHz. Recognizing that the ‘lsIn nuclear quadrupole tensor of In(CO), should reflect the net effect of the unpaired electron in the p, orbital and electrons in the p, and pv orbitals due to dative interaction of the carbonyls, the observed tensor (in megahertz) may be constituted as follows: r7.2
1:
o
-3.6 0
1
o i 0 -3.6
r13 = Czl: -6.5
o
06.5
o
-I
0 -6.5
113
]+ [
-6.5
Cy 0
0
0
-6.5
0
0 -6.5 0
0 0 13
]
where C,,C,, and Cy are the electron populations in the indicated p orbitals. From the analysis of the hf tensor C, zz 1.O. We hence obtain C, = Cy”= 0.5. The indicated degree of occupation of these orbitals by the lone-pair electrons of carbonyls seems reasonable. The exact numbers should be viewed with due caution, however, because of the uncertainty related to the (r-3)1n,5p value and the neglect of shielding by the core electrons. From the symmetric and antisymmetric bands, ug and vu, observed in the IR spectrum of In(12C0)2,based on the CottonKraihanzel approximation (weakly coupled carbonyl systems),14 the stretching force constant, kco, and the interaction constant, kcGco, were calculated as follows: kco = X p O 2 = 16.44 mdyn/A vg
-
kcMo = -kco
V”
= 0.33 mdyn/A
VO
Here uo = (up + u,)/2 and X = 5.8918 X IO-’ when u’s are given in cm-l and p, the reduced mass of CO, is given in amu. The values of kco and kcMo previously determined for Al(CO), in krypton matrix are 15.18 and 0.77 mdyn/A, respectively, and those of Ga(CO), in xenon matrix are 15.51 and 0.74 mdyn/A, respe~tively.~,~ It is apparent that the C O bonds in In(CO), are stronger due to less back-donation from the metal p, orbital into the antibonding A, orbitals of the carbonyls, and the interaction constant, kco,co is consequently significantly smaller. The stability or rather “inertness” of the valence s electrons of the group 111 elements is known to increase as the group is descended. Thus, no monovalent aluminum or gallium compounds are known, but both mono- and trivalent indium halides, for example, exist. As the group is descended, increasing bond length, and hence the decreasing bond strength, must favor less the hybridization required of the trivalent system. The g tensor of In(CO), indicating little sp2 hybridization of the metal atom, the I lSIn hf tensor indicating little migration of the unpaired electron into the carbonyl orbitals, and the IR spectra showing much less back-donation and weaker CO-CO interaction are all consistent with the inertness of the In 5s pair. The stretching force constant, kco (16.44 mdyn/A), of In(CO), is still significantly smaller than that of free C O (18.50 mdyn/A). The back-donation from the In d, orbital is most likely involved. Registry No. In(CO),, 117678-69-0; Ar, 7440-37-1.
