8018
J. A m . Chem. SOC.1984, 106, 8018-8020
Aluminum Dicarbonyl: Matrix Isolation ESR Study Paul H. Kasai* and Paul M. Jones Contribution from IBM Instruments, Inc.. Orchard Park, Danbury, Connecticut 0681 0. Received July 9, 1984
Abstract: ESR spectra of an aluminum carbonyl generated' in argon matrices by co-condensation of aluminum atoms and carbon monoxide were observed and analyzed. It is shown that the aluminum carbonyl consists of one aluminum atom and two CO molecules. It has a bent structure OC-A1-CO and a semifilled orbital representing the back-donation from the A1 pr orbital into the antibonding x* orbitals of the CO moiety
A variety of metal atom carbonyls have been generated by co-condensation of metal atoms and carbon monoxide molecules in inert gas matrices. The list includes C O ( C O ) ~ -Cu(CO),-,, ~, Ag(CO),-,, Pd(C0)4,AI,(C0)2, and many others.'" All of these carbonyls have been identified and examined by their vibrational spectra (IR and Raman). For CO(CO)~, CU(CO)~, and Ag(CO),, ESR (electron spin resonance) spectra have also been ~bserved.l-~ Ogden and his co-workers reported that co-condensation of aluminum atoms and carbon monoxide in a krypton matrix led to formation of an aluminum carbonyL5 On the basis of the effect of C'*O upon the IR spectrum, they demonstrated that the species had the formula A1,(C0)2, but refrained from asserting the number of aluminum atoms involved. Ozin et aL3 suggested that it might be A12(CO)2. We report here the ESR spectra observed from an aluminum carbonyl generated in argon matrices by the co-condensation technique. It is shown that the carbonyl involves one aluminum atom and two carbon monoxide molecules. It has a bent structure (0C)-AI-(CO) and the semifilled orbital representing the back-donation from the A1 pT orbital into the antibonding T* orbitals of the carbon monoxide moiety.
Experimental Section A liquid-helium cryostat that would enable trapping of vaporized species in an inert gas matrix and examination of the resulting matrix by ESR has been described earlier.' In the present series of experiments aluminum atoms were generated from a tantalum cell resistively heated to 1400 O C and were trapped in argon matrices containing controlled amounts of carbon monoxide (1-20%). The ESR spectrometer used was an IBM Model ER2OOD system. A low-frequency (375 Hz) modulation was employed for the signal detection. All the spectra reported here were obtained with the matrix maintained at -4 K and the spectrometer frequency locked to the sample cavity at 9.4275 GHz. Research grade argon and CP grade carbon monoxide were obtained from Matheson, while "C-enriched (enrichment >90%) carbon monoxide was obtained from MSD Isotopes. Results The ground-state electronic configuration of Al atoms is 3s2 3p'. Thus, owing to the degeneracy of the p orbitals, the ESR signal of the A1 atoms situated at sites with a cubic symmetry would be broadened beyond detection. However, it has been shown that rare-gas matrices containing a high concentration (a few tenths of a percent) of AI atoms exhibit strong, well-defined ESR signals. The signals were designated as the AI-X signals* and were assigned to A1 atoms occupying axially distorted substitutional sites of the host m a t r i ~ . ~ (1) Hanlan, L. A.; Huber, H.; Kiindig, E. P.; McGarvey, B. R.; Ozin, G. A. J . Am. Chem. SOC.1975, 97, 7054. (2) Huber, H.; Kiindig, E. P.; Moskovits, M.; Ozin, G. A. J . Am. Chem. SOC.1975, 97, 2097. (3) McIntosh, D.; Ozin, G. A. J . Am. Chem. SOC.1976, 98, 3167. (4) Darling, J. H.; Ogden, J. S . Inorg. Chem. 1972, 11, 666. (5) Hinchcliffe, A. J.; Ogden, J. S.; Oswald, D. D. J . Chem. SOC.,Chem. Commun. 1972, 338. (6) See, for example, a review by Sheline and Slater: Sheline, R. K.; Slater, J. L. Angew. Chem., Int. Ed. Engl. 1975, 14, 309. (7) Kasai, P. H. Acc. Chem. Res. 1971, 4, 329. (8) Knight, L. B.; Weltner, W., Jr. J . Chem. Phys. 1971, 55, 5066.
0002-7863/84/1506-8018$01.50/0
For the Al/CO/Ar system of the current study, only the A1-X signals were observed when the C O concentration was low ( A,. From the observed 27Aland hfc constants the hf tensors of axial symmetry (with signs) are thus deduced as follows. ~ ~ ~ ( 2 =7 +52.0 ~1) G ~ l , ( ~ ~ A=l )f(JA21 + IA,1)/2 = f14.0 G AI,(l3C)= f1.5 G
A,(I3C) = -(1A21 + lA31)/2 = -9.0 G
Let us assumeA1(27Al) = +14.0 and All(I3C)= +1.5 G. Analysis of the tensors in terms of eq 3 yields the following Adip(27Al)= 12.7 G AiS0(*’Al)= 26.7 G AiS,3(l3C) = -5.5 G
&ip(l3C) = 3.5 G
(14) See, for example: Atkins, P. W.; Symons, M. C. R. “The Structure of Inorganic Radicals”; Elsevier: Amsterdam, 1967.
The positive signs were chosen for A,(27Al) and Ail(l3C),since they led to a spin-density distribution with a better overall agreement with that predicted by a molecular orbital calculation (vide infra). The atomic values Ais: and Adi: for a unit spin density in the valence s and p orbitals of atoms have been computed theoreti~ a l 1 y . l ~The results for A1 and C are Ais:(27Al) = 1400 G, Adi:(27Al) = 30 G, Ais:(I3C) = 1350 G, and Adip0(13C)= 38 G. The spin-density distribution in A1(C0)2 was thus determined as follows. p(3s)Al = 0.02 ~ ( 2 s =) ~-0.004
P(3Pz)~l= 0.42 p(2p,)c = 0.09
The balance of spin density must be at oxygens. Thus p ( 2 ~ , ) ~ = 0.19. The small but non-zero spin densities computed above for the AI 3s and C 2s orbitals should not be taken too literally. Small Ais,)s determined here are the sum attributes of the spin polarization in all the filled s orbitals of the respective atoms. However, Aisoof the I3C nucleus induced by spin polarization by an unpaired electron in the 2p orbital of the same atom is usually positive.I6 A negative spin density determined above for the C 2s orbital is hence particularly interesting. It can be best accounted for by polarization of electrons in the u dative bond by the large spin density in the A1 3p, orbital. In order to gain further insight into the carbonyl system and provide further support for the assignment presented above, an INDO molecular orbital calculation was ~ e r f 0 r m e d . l ~The structural parameters assumed were the following: OC-AI-CO angle = llOo, r(A1-C) = 1.80 A, and r(C-0) = 1.15 A. The theory predicted the semifilled orbital given by eq 1 and the highest doubly occupied orbital for the A1 lone-pair electrons. It also yielded the following spin-density distribution: p(3s)AI = 0.02, p(3P2),4l = 0.42, p(2S)c = 0.01, p(2p,)c = 0.15, and p(2p,)o = 0.14. Though it failed to generate a negative spin density in the C 2s orbital, the overall agreement is considered reasonable. The present ESR and earlier IR studies of aluminum atoms and carbon monoxide co-condensed in rare-gas matrices thus completely affirmed the formation of bis(carbonyl)aluminum(O). The bis(carbony1) state is formed as the result of u-type dative interaction between the lone-pair electrons of C O and the vacant spXg2hybridized orbitals of A1 and *-type back-donation of the unpaired electron from the A1 p, orbital into the vacant antibonding rr*orbitals of CO. Synergism between the two types of dative interactions must be crucial; aluminum monocarbonyl does not appear to have a bound state. Registry No. (0C)-Al-(CO), 12691-52-0; AI, 7429-90-5; C O , 63008-0. (15) Morton, J. R.; Preston, K. F. J . Magn. Reson. 1978, 30, 577.
(16) See a review article by Morton: Morton, J. R. Chem. Reu. 1964, 64, 453. (17) Pople, J. A.; Beveridge, D. L. “Approximate Molecular Orbital Theory”; McGraw-Hill: New York, 1970.
