1072 Inorg. Chem. 1993, 32, 1072-1073
Photoluminescence of Tellurium(
IV)Chloride Complexes in Solution
Hans Nikol and Amd Vogler'lnstitut fur Anorganische Chemie, Universitat Regensburg, Universitatsstrasse 3 1, W-8400 Regensburg, Germany Received November 1 I , I992
We wish to describe and discuss our observations on the photoluminescence of TeC15- and TeC162- in solution. While low- temperature photoemissions of TeC162- and related Te(IV) complexes in the solid state have been observed b e f ~ r e , l - ~ here we report for the first time luminescence from a simple compound of an element of group 16 under ambient conditions. Moreover, our results are of general importance with regard to the nature of excited states of s2 complexes. In recent years we observed the solution luminescence of complexes of the type MC13"-, M C W , and MC16n- with the s2 metal ions Ge2+, Sn2+, Pb2+, Sb3+, and Bi3+.4.5 We developed a general concept in order to characterize the emitting sp excited states. The luminescence of TeC15- is the first example of an emission from a pentacoordinate s2 complex.
Our findings serve to check the validity of our previous assumptions.
The absorption spectrum of TeC162- in acetonitrile (Figure 1) agrees with that reported p r e v i o ~ s l y . ~ . ~ The long-wavelength absorptions of this octahedral complex are assigned to sp transitions: A band ('SO
-
3PI or 'Al,-
3TI, inoh
symmetry), A,,, = 407 nm (sh), t = 800 M-1 cm-I, and 385 nm, c = 1500;B band (ISo- 3P2 or 'AI,
-
3Eu, )Tzu), A,,, = 320 nm, c = 2500, C band ('SO-
'PI or 'Al,-
ITlu), A,,, = 298 nm, t = 7100,287 nm, c = 8200, and 273 nm, t = 7300. Upon dilution TeC1b2- was converted to TeC15-,6 which exists as a discrete ion with a square-pyramidal structure even in the solid in accord with the VSEPR model.I0 The A band of TeCl5- (A,,, = 291 nm, t = 1600) appears at much shorter wavelength compared to that of TeC162-.6 Addition of chloride to this dilute solution led to a complete recovery of the spectrum of TeC162- (Figure 1). The isosbestic points which occur during thespectralvariations indicate the presence of only two species, TeC15- and TeC&?. The equilibrium constant (TeC15-
+
C1- ~t TeC162-) was determined to beK
= 1.5 X l o 2 M-I.TeC1b2- in CH3CN shows a red emission at A,,, = 603 nm with + = 1 X 10-4atA,,,= 388nm(Figure2). Theexcitationspectrum matched the absorption spectrum rather well. The spectral features of TeC162- in solution (A,,, of the 'SO
-
3Pl transition in absorption and emission, Stokes shiftAE
= 9400 cm-I) are quite similar to those of TeC1b2- in the solid state (emission: A,,,= 632 nm at 150 K;
AE
= 10 100 cm-l).' In analogy to other s2 complexes and in accord with results on TeC162- in the solid state, the emission of TeC162- in solution is assumed to originate from themetal-centeredspexcitedstate3PI (3Tl,in Ohsymmetry) which undergoes a moderate excited-state distortion.TeC15- in CH3CN shows a green luminescence at A,,, = 538 n m w i t h 4 = 8 X lO--'at Ae,,=280nm(Figure2). Theexcitation spectrum agreed with the absorption spectrum. The emitting excited state is certainly again the sp triplet )PI. However, the
( I ) Wernicke, R.; Kupka, H.; Ensslin, W.; Schmidtke, H.-H. Chem. Phys.
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(4) Nikol, H.; Vogler. A. J . Am. C&m. SOC. Ikl, 113, 8988.
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(6) Stufkens. D. J . R e d . Trau. Chim. 1970, 89, 1185.
( 7 ) Couch,D.A.; Wi1kins.C. J.;Rossman,G. R.;Gray.H. B. J . A m . C h e m . (8) Schonherr. T. 2. Narurforsch. 1988, 436, 159.
(9) Ozin, G. A.; Vander Voet, A. J . Mol. Strucr. 1972, 13, 435.
SOC. 1970, 92, 307.
(IO) (a) Gillespie, R . J.; Nyholm, R. S . Q. Reo. Chem. SOC. 1957, 11, 339.
(b) Gillespie, R. J. Molecular Geomerry; Van Nostrand Reinhold:
London, 1972. (c) Gillespie, R. J.: Hargittai, I. The VSEPR Modelof Molecular Geomerry; Allyn and Bacon: Boston, M A , 1991.
A
0.20
0.10
0.00
300
Figure 1. Electronic absorption spectra of [NBu4][TeC15] in acetonitrile at room temperature (I-cm cell). Absorption: 2.18 X M without NBu4CI (a) and in the presence of 7.08 X
and 4.14 X IO-) M (e) NBu4CI. 1.39
X IO-), 2.08 X
I I I I I I
400 500 600 700 nm
Figure 2. Emission spectra of [NBu4J[TeClsJ ( c = 2.18 X M, bxc
= 280 nm (a)) and [NBu4]2[TeCI6] ( c = 1.58 X lo4 M, A,,, = 400 nm (b)) in acetonitrile at room temperature.
R _ I
M ML, L5
Figure 3. Qualitative MO scheme (Walsh diagram) of a trigonal- bipyramidal complex (&*) and its distortion to a square-pyramidal structure (CJ,.). The *-orbitals of the ligands are omitted.
0020-1669/93/1332-1072%04.00/0 0 1993 American Chemical Society
Communications
Stokes shift of the ‘SO * 3P1 transition
(AE
= 15 700 cm-I) is much larger than that of TeCls2-. Accordingly, T e c h - undergoes a much larger structural rearrangement in the excited state compared to Tech2-.TeC15- might beexpected to havea trigonal-bipyramidalground state structure (DM). However, the s2 electron pair would then occupy a strongly antibonding a!’* orbital (Figure 3). As a consequence, a distortion to a square-pyramidal structure (Cb) takes place, because it is associated with an sp hybridization which lowers the energy of TeCls- by configuration interaction of the al orbitals (Figure 3).Ii The HOMO is thus stabilized and becomes stereochemically active as a lone pair in agreement with the VSEPR model.
In the ala]* sp excited state this stabilization of TeCl5- is lost because the a ] * orbital becomes now the HOMO. We suggest that the complex relaxes then to the stereochemically less
( 1 1) Albright, T. A,; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in Chemistry; Wiley: New York, 1985.
Inorganic Chemistry, Vol. 32, No. 7, 1993 1073 demanding trigonal-bipyramidal structure. Such a geometrical change is in accordance with the large Stokes shift observed for TeCls-. This explanation is consistent with a general concept which has been developed to characterize the emitting sp excited states of s2 complexes.’2J3
Theoctahedral structure Of TeCls2- l 4 is an exception from the VSEPR rules possibly because there is nospace left for a distortion which must provide an open coordination site for the lone pair.
In the case of TeCl5- the lower coordination number facilitates a distortion in accord with the VSEPR model.
Acknowledgment. Support of this research by the
BMFT
is gratefully acknowledged.(12) Vogler, A.; Nikol, H. Pure Appl. Chem. 1992, 64, 131 1.
(13) Vogler, A.; Nikol, H. Comments Inorg. Chem. 1993, 14, 245.
(14) A slight dynamic ground-state distortion seems to occur in the solid state: Abriel, W. Z . Naturforsch. 1987, 426, 1273.
