J . Phys. Chem. 1988, 92, 1060-1061
1060
Ga Atom-Ethylerre Complex: Matrix Isdation ESR Study Paul M. Jones and Paul H. Kasai* IBM Almaden Research Center, San Jose, California 951 20 (Received: July 6, 1987)
The Ga atom-ethylene complex, Ga(C2H4),was generated in argon matrices and its ESR spectrum was examined. The spectral analysis revealed that the complex has a side-on structure and is held by a dative interaction between the semifilled Ga 4p, orbital and the antibonding 7ry* orbital of ethylene. The g tensor and the 69Gahyperfine tensor were determined as follows: g, = 2.0031 ( 6 ) , g, = 1.9745 (6), g, = 2.0235 ( 6 ) , A, = 92 (1) G, A, = -40 (1) G, and A, = -30 (1) G.
Introduction Recently we reported on ESR spectra of aluminum atomethylene complex, Al(C2H4),generated in argon matrices by the cocondensation technique.’ The spectral analysis revealed that the complex has a “side-on” structure of the bonding scheme 1. H2C=CH2
HzC=CHz
Y
n
W (1)
TABLE I: g Tensors, Metal Nuclear Hf CoupUng Tensors, and Spin Densities, p(np,), of Ga(C2H,) and AI(CzH4)”
axis
1
(=d ~I
Ga(C2H4) g tensor A1(C2H4)
2.0031 ( 6 ) AP9Ga1. ,, G +92 111 g tensor 2.0025 ’(3) A(27Al),G +44.5 (5)
.
2
(=v)
1.9745 (6) -40 (1) 1.9965‘(3) -5 (1)
3 (=z) 2.0235 (6) -30 (1) 2.0097’(3) -5 (1)
hP.) ..
+0.60 +0.56
‘Transcribed from ref 1; based on the more recent Ab,,”(A1,3p)value given in ref 8, the signs of A, and A, were changed, and p(3p.J was recomputed.
(2)
The complex is held primarily by the semifilled orbital representing migration of the unpaired electron from the A1 p, orbital into the T,,* orbital of ethylene. The copper atom-ethylene complex, Cu(C2H4),has also been generated and examined in argon matrices.2 It also has a side-on structure. The unpaired electron of the latter complex, however, is in the spy orbital of Cu pointing away from the ligand and the complex is held primarily by the dative interaction between the filled d, orbital of Cu and the A,* orbital of the ligand (scheme 2). We report here the ESR spectra of Ga atom-ethylene complex, Ga(C2H4),similarly generated in argon matrices. The A1 atom has the 3s23p1 configuration whereas that of the Ga atom is 3dIo4s24p1. It follows that the Ga(C2H4)complex, if formed, should have a similar side-on structure, but the bonding schemes 1 and 2 could both be operative in its formation. The present study revealed that the complex indeed has a side-on structure but is held essentially by scheme 1 only.
Experimental Section The same experimental setup used to generate Al(C,H4) and Cu(C2H4)in argon matrices was used.lW2 The Ga atoms were vaporized from a resistively heated (- 1400 “C) tantalum cell and were trapped in matrices of argon-ethylene (5%) mixture at liquid helium temperature. The frequency of the spectrometer locked to the sample cavity was 9.436 GHz, and the magnetic field was measured by using the signals of inadvertently formed methyl radicals as intemal standards (for CH3in argon g = 2.0019 and A = 23.06 G).All the spectra were observed while the matrix was maintained at 4 K. Spectra and Assignment Figure l a shows the ESR spectrum observed from the Ga/ C,H4(5%)/Ar system. Ammeter et al. showed that rare gas matrices containing a high concentration (a few tenths of a percent) of Ga atoms often exhibit strong ESR signals due to isolated metal atoms at distorted sites in the host crysta1.j Under our experimental condition no ESR signal was seen from argon matrices containing Ga atoms only. The spectrum seen in Figure 1a is totally different from that reported for the isolated metal atoms. Based on this observation and the analysis of the semifilled (1) Kasai, P. H. J . Am. Chem. SOC.1982, 104, 1165. (2) Kasai, P. H.; McLeod, Jr., D.; Watanabe, T. J. Am. Chem. SOC.1980, 102, 179. (3) Ammeter, J. H.; Schlosnagle, D. C. J . Chem. Phys. 1973, 59, 4784.
0022-3654/88/2092-1060$01.50/0
orbital given below, the spectrum is assigned to the Ga atomethylene complex. There are two major gallium isotopes, 69Ga ( I = 3/2, natural abundance = 60%, 1.1 = 2.0108 On), and 71Ga ( I = 3/2, natural abundance = 40%, 1.1 = 2.5549 8,). In the observed spectrum of Ga(C2H4)quartet structures due to the hf (hyperfine) interaction with the metal nuclei are not very apparent; we surmised that the nominal quartet patterns were obscured by the anisotropy of the g tensor. Through careful analyses of the positions, shapes, and magnitudes of the resolved components, the g tensor of the complex, and the hf splittings along the respective principal axes were determined as indicated in the figure. Thus only in the g, direction the difference in the hf splittings of the 69Ga and 71Gaspecies is resolved. The “house figures” shown below the spectrum represent the absorption pattern due to the 69Ga species. The assessed principal values of the g tensor, and the 69Ga hf coupling tensor, are given in Table I. Figure l b is a computer-simulated spectrum based on these parameters. For the simulation a Lorentzian line shape with the line width of 15 G (the full width at half-height) was assumed, and the spectra of both the 69Gaand 71Gaspecies were considered and superimposedP The ESR spectrum of the Ga atom-perdeuterioethylene complex observed from the Ga/C2D4/Ar system was clearly sharper than that of the normal species; many components were half as wide as the corresponding components of the normal species. From this difference in the line width, and assuming the presence of four equivalent protons, the proton hf coupling constant was estimated at 4.0 f 1.0 G.
Discussion Taking cognizance of the possibility that both schemes 1 and 2 are involved in its formation, the semifilled orbital of Ga(C2H4) might be generally expressed as follows. Here &(4px), for example, represents the 4p, orbital of the metal atom and dL(7r,*) the antibonding A orbital of the ligand. It has been shown that, for a radical having a nondegenerate ground state IO), deviation of the g tensor from the spin only value g, (2.0023) is given by eq 2.5 Here i ( = x , y , z ) represents a
(4) The simulation program used was described in: Kasai, P. H. J . Am. Chem. SOC.1972, 94, 5950, and was run on an IBM main frame system.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 5, 1988 1061
Ga Atom-Ethylene Complex -A,(”Ga)-
I
I
r
1
I
3d, part would be symmetric about the z axis. The Ga hf tensor of the complex should then take the following form.’
-A,(’’Gal+
yl
1
I
I
IA.
0 I
Io
0 = Aiso A,I
0 0 - A,
o
+
120 0 0 -a
Io
o
0 0
I
+
-a/
I-b 0
Io
I
0
0
-b
0 2b1
o
(3)
Here A,,, represents the isotropic term induced by polarization of filled s orbitals, while a and b represent the dipolar terms given by the 4p, and 3d, parts of eq 1, respectively. The dominance of the 4p, orbital revealed by the g tensor dictates that A , > A, = A,. We hence conclude that the elements of the Ga hf tensor of the complex are either (1) all positive, or (2) such that A , is positive while A, and A, are negative. Analyses of these elements in terms of eq 3 yield the following. A,,, = +54.0 or +7.0 G
a = +17.3 or +44.0 G b = -3.3 or +3.3 G The first set is for case 1, and the latter for case 2. The atomic values, A,,: and Ad,;, expected from a unit spin density in the Ga 4s and 4p orbitals, respectively, had been computed from the Hartree-Fock wave functions.8 The Ad,: value for Ga 3d, orbital can be similarly c o m p ~ t e d . ~These theoretical values are A,,,O(Ga,4s) = +4361 G
M
AdlpO(Ga,4p) = gePdflg,Pfl(2/5)(1/P)4, = +73 G &,pO(Ga,3dx,) = gddflflfl(-2/7)( 1/r3)3d = -173
I
u
3,
3
2
1
2
Figure 1. (a) ESR spectrum observed from the Ga/ethylene(S%)/argon system. (b) Computer simulated spectrum based on the parameters given in Table I. The figures at the bottom represent the absorption patterns of the four hyperfine components of the 69Gaspecies, the numbers indicating the corresponding principal axes.
principal axis of the g tensor, L,the orbital angular momentum operator, and X the one electron spin-orbit coupling constant. The summation is performed over all the excited states. In evaluating eq 2 in terms of LCAO-MO’s, only one-centered integrals may be retained, and for each atomic integral the spih-orbit coupling constant of the particular atom is used. The one-electron spin-orbit coupling constants for the 4p and 3d orbitals of Ga and the 2p orbital of carbon are 551, 800, and 15 cm-*, respectively.6 It follows that the g tensor of Ga(C2H4) would be determined essentially by the Ga 4p, and/or 3d, parts of eq 1. It further follows that, if the Ga 4p, orbital dominates in eq 1 (scheme I), deviation of g, from g, would be nil, while admixture of the 3d, orbital (scheme 2) would generate g-value deviation in all three principal directions. The observed g, of Ga(C2H4) is almost exactly the free spin value. We hence conclude that the 3d, part in eq 1 is negligibly small and the complex is held primarily by scheme 1. In this situation the lowest unoccupied orbital of the complex would be the Ga 4pz orbital, while the highest doubly occupied orbital would be the G a 4sp, orbital of the nonbonding lone pair electrons. Equation 2 then stipulates that gy < g, and g, > g,. The experimentally determined g,, g2, and g3 are thus respectively identified with the g,, gy, and g, of the molecular coordinates of scheme 1. The Ga hf tensor of Ga(C2H4)rendered by the 4p, part of eq 1 would be symmetric about the x axis, while that given by the (5) Pryce, M. H.L. Proc. Phys. SOC.,London, Sect. A 1950, 63, 25. (6) Computed from the table of Moore, C. E. “Atomic Energy Levels’’; Natl. Bur. Stand. Circ. 467, Vol. 1, 1949, Vol. 2, 1952; Vol. 3, 1958.
Thus, in either case, the A,,, value of the complex is very small in comparison with the atomic value AWo(Ga,4s),and is consistent with its origin being the polarization of the filled s orbitals. The spin density distributions in the Ga 4p, and 3d, orbitals computed from a and b determined above are p(4p,) = +0.24 or +0.60 p(3d,)
= +0.02
or -0.02
for cases 1 and 2, respectively. The spin density in the Ga 4p, orbital given by the first set is unreasonably small; it places a spin density of -0.38 on each 2p, orbital of the carbon atoms. If the ethylene moiety in the complex remains planar, the McConnell relation then predicts the proton hf coupling constant of -9 G. The second case treated similarly predicts the proton hf coupling constant of - 5 G in close agreement with the value assessed from the observed spectra. The small negative density in the 3d, orbital is then attributed to a polarization effect. The g tensor, the metal nuclear hf coupling tensor, and the spin density distribution of Ga(C2H4) thus determined are compared with the corresponding quantities of Al(C2H4) in Table I. The similarity between the semifilled orbitals, and hence the bonding schemes of the two complexes, is immediately apparent. The ostentatious difference between the ESR spectra of the complexes is caused by the much larger deviations of g, and g, from the spin only value in Ga(C2H4). The one-electron spin-orbit coupling constant for the A1 3p orbital is 75 cm-’ as compared to 551 cm-’ for the Ga 4p orbitaL6 Registry No. Ga(C2H4),112087-89-5; Ga(C,D,), 112087-90-8; Ga, 7440-55-3; ethylene, 74-85-1; perdeuterioethylene,683-73-8. (7) For analyses of hf coupling tensors, see, for example: Atkins, P. W.; Symons, M . C. R. The Structures of Inorganic Radicals; Elsevier: Amsterdam, 1967. (8) Morton, J. R.; Preston, K. F. J . Magn. Reson. 1978, 30, 577. (9) Computed from the table of Herman, F.; Skillman, S. Atomic Structure Calculations; Prentice-Hall: Englewood Cliffs, NJ, 1963.
