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Excited state properties of (acetylacetonato)dicarbonyliridium(I)
Inorganic Chemistry Communications 8 (2005) 119–121 www.elsevier.com/locate/inoche
Excited state properties of (acetylacetonato)dicarbonyliridium(I) Horst Kunkely, Arnd Vogler
*
Institut fu¨r Anorganische Chemie, Universita¨t Regensburg, Universita¨tsstrasse 31, D-93040 Regensburg, Germany Received 18 August 2004; accepted 31 August 2004 Available online 8 December 2004
Abstract Monomeric Ir(CO)2acac as it occurs in dilute solutions (<10 3 M) absorbs only in the UV spectral region and shows a greenish luminescence (kmax = 480 nm) which originates from the lowest-energy intraligand (acac) triplet. At higher concentrations a yellow coloration is observed which is attributed to the formation of oligomers. At 77 K these solutions turn blue owing to the presence of oligomers which are characterized by strong metal–metal interactions. These blue oligomers display an intense red luminescence which consists of a fluorescence at kmax = 662 nm and a phosphorescence at kmax = 815 nm. 2004 Elsevier B.V. All rights reserved. Keywords: Electronic spectra; Luminescence; Iridium; Acetylacetonate
A variety of square-planar complexes of transition metals with a d8 electron configuration has been shown to be luminescent [1–4]. Diverse potential applications have stimulated the interest in these compounds [5]. Mononuclear d8 complexes frequently emit from metalto-ligand charge transfer (MLCT) or intraligand (IL) excited states. In this context, the excited state properties of complexes which contain the bidentate anionic ligand acac (acetylacetonate) [6] might be of particular interest. Since acac is not a p-accepting ligand it cannot serve as a CT acceptor for low-energy MLCT transitions.
CH3 O M O CH3 *
Corresponding author. Tel.: +49 941 9434485; fax: +49 941 9434488. E-mail address: [email protected] (A. Vogler). 1387-7003/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2004.08.026
However, acac complexes are well known to emit from their pp* (acac ) IL triplets provided they are the lowest-energy states of the complex [6–9]. The emission color can be easily tuned between blue and green depending on the nature of the substituents at the carbon atoms which carry methyl groups in the parent ligand acac . As a promising d8 complex which was expected to show an acac IL emission we selected the complex IrI(CO)2acac for this study. Owing to the presence of the strong-field ligand CO an interference by low-energy ligand field (LF) states may be avoided. Indeed, it is known that in contrast to RhI(CO)2acac, our target complex is much less light sensitive [10,11]. In the Rh(I) complex reactive LF states are apparently low enough to dominate the excited state behavior of the complex [12]. At lower concentrations (<10 3 M) and at r.t. solutions of Ir(CO)2acac are colorless. The electronic spectrum (Fig. 1) in CH3CN shows absorptions at kmax = 375 nm (sh, e = 800 M 1 cm 1), 336 (sh, 3800), 298 (9500), 262 (12,200) 237 (13,200) and 225 (sh, 12,600). At higher concentrations (>10 2 M) additional bands appear at 425, 480 and 560 nm. Such solutions are yellow. Solutions of Ir(CO)2acac display a distinct
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H. Kunkely, A. Vogler / Inorganic Chemistry Communications 8 (2005) 119–121
greenish photoluminescence (Fig. 1) at kmax = 480 nm which is independent of the concentration. The excitation spectrum matches roughly the absorption spectrum which is obtained at low concentrations. The emission quantum yield is approximately / = 3 · 10 5 at kexc = 360 nm. Solutions of Ir(CO)2acac are light sensitive. The photolysis leads to insoluble products. In the beginning the precipitate remains suspended in the solution and simply causes light scattering which increases towards shorter wavelengths. The concomitant spectral changes are shown in Fig. 2. The progress of the photolysis is monitored by measuring the decrease of the optical density at 298 nm taking into account the apparent light absorption of the suspended particles. The photolysis proceeds with / = 3 · 10 5 at kirr = 313 nm. When concentrated solutions of Ir(CO)2acac in ethanol are cooled to 77 K the yellow color changes to a deep blue. This process is completely reversible. The blue ethanol glass exhibits a rather intense red luminescence (Fig. 3) with an emission maximum at 662 nm and
Fig. 1. Electronic absorption and emission spectrum of 1.13 · 10 4 M IrI(CO)2acac in CH3CN under argon at room temperature, 1-cm cell. Emission: kexc = 360 nm, intensity in arbitrary units.
Fig. 2. Spectral changes during the photolysis of 8.71 · 10 5 M dicarbonylacetylacetonato iridium(I) in CH3CN under argon at room temperature after 0 min (a), 5 and 15 min (b) irradiation times with kirr = 313 nm (Osram HBO 200 W/2 lamp; Schott interference filter UV-PIL 313), 1-cm cell.
Fig. 3. Electronic excitation (exc, kem = 662 nm) and emission (em, kexc = 400 nm) spectrum of 1.2 · 10 2 M IrI(CO)2acac in EtOH under argon at 77 K, intensity in arbitrary units.
a shoulder at 815 nm. The excitation spectrum (Fig. 3) displays a long-wavelength band at kmax = 570 nm. It follows that the red emission is caused by the blue species. Solid Ir(CO)2acac which is dark brown is not luminescent (up to 900 nm) at r.t. or 77 K. In dilute solutions Ir(CO)2acac is assumed to exist as a mononuclear square-planar complex. The absorption maximum at kmax = 298 nm is assigned to the pp* (acac) IL transition in agreement with corresponding assignments for other acac complexes [6–9]. The longer-wavelength shoulders at 336 and 375 nm (Fig. 1) are suggested to be vibronic satellites of the IL band. This is supported by the fact that the absorption spectra of Ir(CO)2acac and Rh(CO)2acac in solution are very similar [13]. If this part of the spectrum would be dominated by other electronic transitions (e.g., LF or CT), the spectra of both complexes should be quite different. Nevertheless, LF bands of Ir(CO)2acac are expected to appear in the same spectral region, but owing to their low intensity they are probably obscured by the IL band. The excited state behavior of Ir(CO)2acac is apparently determined by the IL (acac) state as the lowest excited state of the complex and a reactive LF state at somewhat higher energies. The green emission (Fig. 1) is assumed to originate from the lowest-energy IL triplet. Various other acac complexes show also an IL phosphorescence at comparable energies [6–9]. The population of the LF state at higher energies may occur directly or by thermal activation from the IL state. As a consequence Ir(CO)2acac releases a CO ligand in the primary photochemical step [10,11] which may be followed by various reactions including the formation of polynuclear ligand-bridged complexes. Since the LF splitting of metals of the second transition row is smaller than that of metals of the third row the reactive LF state of Rh(CO)2acac may be pushed close to or below the acac IL state. Indeed, the photodissociation of Rh(CO)2acac is much more efficient than that of Ir(CO)2acac [10,11].
H. Kunkely, A. Vogler / Inorganic Chemistry Communications 8 (2005) 119–121
Moreover, solutions of Rh(CO)2acac have not been reported to be emissive. However, solid Rh(CO)2acac shows the acac IL emission [12] since the rigid lattice apparently prevents a photodissociation. The new longwavelength absorptions of Ir (CO)2acac at higher concentrations which appear above 400 nm are probably caused by an association. The nature of these oligomers (or dimers) is not clear but metal–metal interaction or the formation of acac bridges as they occur in [Pt(CH3)3(acac)]2 [14] are conceivable. These oligomers [Ir(CO)2acac]n are not emissive. However, when these solutions are cooled to 77 K a new species is formed as indicated by the appearance of an intense blue coloration. Simultaneously, a bright red emission occurs. It has a long-wavelength excitation maximum at 570 nm which clearly belongs to the blue species. We suggest that this blue compound is the oligomer [Ir (CO)2acac]n (n = 2 or larger) which is characterized by a rather strong Ir–Ir interaction. This blue color seems to be typical for a variety of dimeric or oligomeric Rh(I) and Ir(I) complexes [15–21]. The corresponding absorption which mostly appears between 550 and 600 nm is assigned to an electronic transition from a r-antibonding (M–M) MO to a r-bonding (M–M) MO. Whereas the formal M–M bond order is zero in the ground state it is one in the excited state. Generally, these blue complexes show a red emission which consists of a shorter-wavelength fluorescence and a weaker phosphorescence at longer wavelength [17–21]. The fluorescence overlaps with the longest-wavelength absorption band. The same observations are made for Ir(CO)2acac. However, the blue oligomeric Rh(I) and Ir(I) complexes which have been previously studied, are stable at room temperature. In contrast, [Ir(CO)2acac]n is apparently formed only at low temperatures. The reason for this difference is not clear, but might be related to the presence of the acac ligand which facilitates the formation of acac bridged complexes [22]. In this case a direct metal–metal interaction could be prevented. The yellow color of Ir(CO)2acac in concentrated solutions may be attributed to the formation of such a ligandbridged dimer or oligomer. It could then rearrange to a metal–metal bonded oligomer which is energetically favored at lower temperatures. In this context it is of interest that solid Ir(CO)2acac forms a polymeric chain with short M–M distances. This material shows an intense long-wavelength absorption at kmax = 568 nm [13] which matches nicely the excitation maximum of Ir(CO)2acac as it occurs in the blue ethanol glass at 77 K. However,
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solid Ir(CO)2acac has not been observed to emit up to 900 nm at r.t. or 77K. In conclusion, in dilute solution Ir(CO)2acac exists as a colorless monomer which emits a green emission from the lowest-energy intraligand (acac) triplet. At higher concentrations and low temperatures (77 K) an oligomer is formed. Its blue color is caused by metal–metal interaction. This oligomer displays a red emission at kmax = 662 (fluorescence) and kmax = 815 nm (phosphorescence). References [1] A.J. Lees, Chem. Rev. 87 (1987) 711. [2] D.M. Roundhill, Photochemistry and Photophysics of Metal Complexes, Plenum Press, New York, 1994. [3] M. Hissler, J.E. McGarrah, W.B. Connick, D.K. Geiger, S.D. Cummings, R. Eisenberg, Coord. Chem. Rev. 208 (2000) 115. [4] A. Vogler, H. Kunkely, Top. Curr. Chem. 213 (2001) 143. [5] J. Brooks, Y. Babayan, S. Lamansky, P.I. Djurovich, I. Tsyba, R. Ban, M.E. Thompson, Inorg. Chem. 41 (2002) 3055, and references cited therein. [6] R.L. Lintvedt, in: A.W. Adamson, P.D. Fleischauer (Eds.), Concepts of Inorganic Photochemistry, Wiley, New York, 1975, p. 299 (Chapter 7). [7] A. Strasser, A. Vogler, J. Photochem. Photobiol. A 165 (2004) 115, and references cited therein. [8] A. Strasser, A. Vogler, Inorg. Chem. Commun. 7 (2004) 528. [9] A. Strasser, A. Vogler, Inorg. Chim. Acta 357 (2004) 2345. [10] T.P. Dougherty, W.T. Grubbs, E.J. Heilweil, J. Phys. Chem. 98 (1994) 9396. [11] A.M.F. Brouwers, A. Oskam, R. Narayanaswamy, A.J. Rest, J. Chem. Soc., Dalton Trans. (1982) 1777. [12] N. Dunwoody, S.-S. Sun, A.J. Lees, Inorg. Chem. 39 (2000) 4442. [13] T.A. Dessent, R.A. Palmer, S.M. Horner, in: L.V. Interrante (Ed.), Extended Interactions between Metal Ions in Transition Metal Complexes, ACS Symposium Series, vol. 5, 1974, p. 301. [14] A.G. Swallow, M.R. Truter, Proc. R. Soc. Lond. 254 (1960) 205. [15] G.L. Geoffroy, M.S. Wrighton, Organometallic Photochemistry, Academic Press, New York, 1979, p. 271. [16] G.L. Geoffroy, M.G. Bradley, M.E. Keeney, Inorg. Chem. 17 (1978) 777. [17] V.M. Miskowski, G.L. Nobinger, D.S. Kliger, G.S. Hammond, N.S. Lewis, K.R. Mann, H.B. Gray, J. Am. Chem. Soc. 100 (1978) 485. [18] K.R. Mann, J.A. Thich, R.A. Bell, C.L. Coyle, H.B. Gray, Inorg. Chem. 19 (1980) 2462. [19] S.J. Milder, R.A. Goldbeck, D.S. Kliger, H.B. Gray, J. Am. Chem. Soc. 102 (1980) 6761. [20] W.A. Fordyce, G.A. Crosby, J. Am. Chem. Soc. 104 (1982) 985. [21] C.-M. Che, W.-M. Lee, H.-L. Kwong, V.W.-W. Yam, K.-C. Cho, J. Chem. Soc., Dalton Trans. (1990) 1717. [22] J.P. Fackler, Prog. Inorg. Chem. 7 (1966) 361.
Excited state properties of (acetylacetonato)dicarbonyliridium(I) Horst Kunkely, Arnd Vogler
*
Institut fu¨r Anorganische Chemie, Universita¨t Regensburg, Universita¨tsstrasse 31, D-93040 Regensburg, Germany Received 18 August 2004; accepted 31 August 2004 Available online 8 December 2004
Abstract Monomeric Ir(CO)2acac as it occurs in dilute solutions (<10 3 M) absorbs only in the UV spectral region and shows a greenish luminescence (kmax = 480 nm) which originates from the lowest-energy intraligand (acac) triplet. At higher concentrations a yellow coloration is observed which is attributed to the formation of oligomers. At 77 K these solutions turn blue owing to the presence of oligomers which are characterized by strong metal–metal interactions. These blue oligomers display an intense red luminescence which consists of a fluorescence at kmax = 662 nm and a phosphorescence at kmax = 815 nm. 2004 Elsevier B.V. All rights reserved. Keywords: Electronic spectra; Luminescence; Iridium; Acetylacetonate
A variety of square-planar complexes of transition metals with a d8 electron configuration has been shown to be luminescent [1–4]. Diverse potential applications have stimulated the interest in these compounds [5]. Mononuclear d8 complexes frequently emit from metalto-ligand charge transfer (MLCT) or intraligand (IL) excited states. In this context, the excited state properties of complexes which contain the bidentate anionic ligand acac (acetylacetonate) [6] might be of particular interest. Since acac is not a p-accepting ligand it cannot serve as a CT acceptor for low-energy MLCT transitions.
CH3 O M O CH3 *
Corresponding author. Tel.: +49 941 9434485; fax: +49 941 9434488. E-mail address: [email protected] (A. Vogler). 1387-7003/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2004.08.026
However, acac complexes are well known to emit from their pp* (acac ) IL triplets provided they are the lowest-energy states of the complex [6–9]. The emission color can be easily tuned between blue and green depending on the nature of the substituents at the carbon atoms which carry methyl groups in the parent ligand acac . As a promising d8 complex which was expected to show an acac IL emission we selected the complex IrI(CO)2acac for this study. Owing to the presence of the strong-field ligand CO an interference by low-energy ligand field (LF) states may be avoided. Indeed, it is known that in contrast to RhI(CO)2acac, our target complex is much less light sensitive [10,11]. In the Rh(I) complex reactive LF states are apparently low enough to dominate the excited state behavior of the complex [12]. At lower concentrations (<10 3 M) and at r.t. solutions of Ir(CO)2acac are colorless. The electronic spectrum (Fig. 1) in CH3CN shows absorptions at kmax = 375 nm (sh, e = 800 M 1 cm 1), 336 (sh, 3800), 298 (9500), 262 (12,200) 237 (13,200) and 225 (sh, 12,600). At higher concentrations (>10 2 M) additional bands appear at 425, 480 and 560 nm. Such solutions are yellow. Solutions of Ir(CO)2acac display a distinct
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H. Kunkely, A. Vogler / Inorganic Chemistry Communications 8 (2005) 119–121
greenish photoluminescence (Fig. 1) at kmax = 480 nm which is independent of the concentration. The excitation spectrum matches roughly the absorption spectrum which is obtained at low concentrations. The emission quantum yield is approximately / = 3 · 10 5 at kexc = 360 nm. Solutions of Ir(CO)2acac are light sensitive. The photolysis leads to insoluble products. In the beginning the precipitate remains suspended in the solution and simply causes light scattering which increases towards shorter wavelengths. The concomitant spectral changes are shown in Fig. 2. The progress of the photolysis is monitored by measuring the decrease of the optical density at 298 nm taking into account the apparent light absorption of the suspended particles. The photolysis proceeds with / = 3 · 10 5 at kirr = 313 nm. When concentrated solutions of Ir(CO)2acac in ethanol are cooled to 77 K the yellow color changes to a deep blue. This process is completely reversible. The blue ethanol glass exhibits a rather intense red luminescence (Fig. 3) with an emission maximum at 662 nm and
Fig. 1. Electronic absorption and emission spectrum of 1.13 · 10 4 M IrI(CO)2acac in CH3CN under argon at room temperature, 1-cm cell. Emission: kexc = 360 nm, intensity in arbitrary units.
Fig. 2. Spectral changes during the photolysis of 8.71 · 10 5 M dicarbonylacetylacetonato iridium(I) in CH3CN under argon at room temperature after 0 min (a), 5 and 15 min (b) irradiation times with kirr = 313 nm (Osram HBO 200 W/2 lamp; Schott interference filter UV-PIL 313), 1-cm cell.
Fig. 3. Electronic excitation (exc, kem = 662 nm) and emission (em, kexc = 400 nm) spectrum of 1.2 · 10 2 M IrI(CO)2acac in EtOH under argon at 77 K, intensity in arbitrary units.
a shoulder at 815 nm. The excitation spectrum (Fig. 3) displays a long-wavelength band at kmax = 570 nm. It follows that the red emission is caused by the blue species. Solid Ir(CO)2acac which is dark brown is not luminescent (up to 900 nm) at r.t. or 77 K. In dilute solutions Ir(CO)2acac is assumed to exist as a mononuclear square-planar complex. The absorption maximum at kmax = 298 nm is assigned to the pp* (acac) IL transition in agreement with corresponding assignments for other acac complexes [6–9]. The longer-wavelength shoulders at 336 and 375 nm (Fig. 1) are suggested to be vibronic satellites of the IL band. This is supported by the fact that the absorption spectra of Ir(CO)2acac and Rh(CO)2acac in solution are very similar [13]. If this part of the spectrum would be dominated by other electronic transitions (e.g., LF or CT), the spectra of both complexes should be quite different. Nevertheless, LF bands of Ir(CO)2acac are expected to appear in the same spectral region, but owing to their low intensity they are probably obscured by the IL band. The excited state behavior of Ir(CO)2acac is apparently determined by the IL (acac) state as the lowest excited state of the complex and a reactive LF state at somewhat higher energies. The green emission (Fig. 1) is assumed to originate from the lowest-energy IL triplet. Various other acac complexes show also an IL phosphorescence at comparable energies [6–9]. The population of the LF state at higher energies may occur directly or by thermal activation from the IL state. As a consequence Ir(CO)2acac releases a CO ligand in the primary photochemical step [10,11] which may be followed by various reactions including the formation of polynuclear ligand-bridged complexes. Since the LF splitting of metals of the second transition row is smaller than that of metals of the third row the reactive LF state of Rh(CO)2acac may be pushed close to or below the acac IL state. Indeed, the photodissociation of Rh(CO)2acac is much more efficient than that of Ir(CO)2acac [10,11].
H. Kunkely, A. Vogler / Inorganic Chemistry Communications 8 (2005) 119–121
Moreover, solutions of Rh(CO)2acac have not been reported to be emissive. However, solid Rh(CO)2acac shows the acac IL emission [12] since the rigid lattice apparently prevents a photodissociation. The new longwavelength absorptions of Ir (CO)2acac at higher concentrations which appear above 400 nm are probably caused by an association. The nature of these oligomers (or dimers) is not clear but metal–metal interaction or the formation of acac bridges as they occur in [Pt(CH3)3(acac)]2 [14] are conceivable. These oligomers [Ir(CO)2acac]n are not emissive. However, when these solutions are cooled to 77 K a new species is formed as indicated by the appearance of an intense blue coloration. Simultaneously, a bright red emission occurs. It has a long-wavelength excitation maximum at 570 nm which clearly belongs to the blue species. We suggest that this blue compound is the oligomer [Ir (CO)2acac]n (n = 2 or larger) which is characterized by a rather strong Ir–Ir interaction. This blue color seems to be typical for a variety of dimeric or oligomeric Rh(I) and Ir(I) complexes [15–21]. The corresponding absorption which mostly appears between 550 and 600 nm is assigned to an electronic transition from a r-antibonding (M–M) MO to a r-bonding (M–M) MO. Whereas the formal M–M bond order is zero in the ground state it is one in the excited state. Generally, these blue complexes show a red emission which consists of a shorter-wavelength fluorescence and a weaker phosphorescence at longer wavelength [17–21]. The fluorescence overlaps with the longest-wavelength absorption band. The same observations are made for Ir(CO)2acac. However, the blue oligomeric Rh(I) and Ir(I) complexes which have been previously studied, are stable at room temperature. In contrast, [Ir(CO)2acac]n is apparently formed only at low temperatures. The reason for this difference is not clear, but might be related to the presence of the acac ligand which facilitates the formation of acac bridged complexes [22]. In this case a direct metal–metal interaction could be prevented. The yellow color of Ir(CO)2acac in concentrated solutions may be attributed to the formation of such a ligandbridged dimer or oligomer. It could then rearrange to a metal–metal bonded oligomer which is energetically favored at lower temperatures. In this context it is of interest that solid Ir(CO)2acac forms a polymeric chain with short M–M distances. This material shows an intense long-wavelength absorption at kmax = 568 nm [13] which matches nicely the excitation maximum of Ir(CO)2acac as it occurs in the blue ethanol glass at 77 K. However,
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solid Ir(CO)2acac has not been observed to emit up to 900 nm at r.t. or 77K. In conclusion, in dilute solution Ir(CO)2acac exists as a colorless monomer which emits a green emission from the lowest-energy intraligand (acac) triplet. At higher concentrations and low temperatures (77 K) an oligomer is formed. Its blue color is caused by metal–metal interaction. This oligomer displays a red emission at kmax = 662 (fluorescence) and kmax = 815 nm (phosphorescence). References [1] A.J. Lees, Chem. Rev. 87 (1987) 711. [2] D.M. Roundhill, Photochemistry and Photophysics of Metal Complexes, Plenum Press, New York, 1994. [3] M. Hissler, J.E. McGarrah, W.B. Connick, D.K. Geiger, S.D. Cummings, R. Eisenberg, Coord. Chem. Rev. 208 (2000) 115. [4] A. Vogler, H. Kunkely, Top. Curr. Chem. 213 (2001) 143. [5] J. Brooks, Y. Babayan, S. Lamansky, P.I. Djurovich, I. Tsyba, R. Ban, M.E. Thompson, Inorg. Chem. 41 (2002) 3055, and references cited therein. [6] R.L. Lintvedt, in: A.W. Adamson, P.D. Fleischauer (Eds.), Concepts of Inorganic Photochemistry, Wiley, New York, 1975, p. 299 (Chapter 7). [7] A. Strasser, A. Vogler, J. Photochem. Photobiol. A 165 (2004) 115, and references cited therein. [8] A. Strasser, A. Vogler, Inorg. Chem. Commun. 7 (2004) 528. [9] A. Strasser, A. Vogler, Inorg. Chim. Acta 357 (2004) 2345. [10] T.P. Dougherty, W.T. Grubbs, E.J. Heilweil, J. Phys. Chem. 98 (1994) 9396. [11] A.M.F. Brouwers, A. Oskam, R. Narayanaswamy, A.J. Rest, J. Chem. Soc., Dalton Trans. (1982) 1777. [12] N. Dunwoody, S.-S. Sun, A.J. Lees, Inorg. Chem. 39 (2000) 4442. [13] T.A. Dessent, R.A. Palmer, S.M. Horner, in: L.V. Interrante (Ed.), Extended Interactions between Metal Ions in Transition Metal Complexes, ACS Symposium Series, vol. 5, 1974, p. 301. [14] A.G. Swallow, M.R. Truter, Proc. R. Soc. Lond. 254 (1960) 205. [15] G.L. Geoffroy, M.S. Wrighton, Organometallic Photochemistry, Academic Press, New York, 1979, p. 271. [16] G.L. Geoffroy, M.G. Bradley, M.E. Keeney, Inorg. Chem. 17 (1978) 777. [17] V.M. Miskowski, G.L. Nobinger, D.S. Kliger, G.S. Hammond, N.S. Lewis, K.R. Mann, H.B. Gray, J. Am. Chem. Soc. 100 (1978) 485. [18] K.R. Mann, J.A. Thich, R.A. Bell, C.L. Coyle, H.B. Gray, Inorg. Chem. 19 (1980) 2462. [19] S.J. Milder, R.A. Goldbeck, D.S. Kliger, H.B. Gray, J. Am. Chem. Soc. 102 (1980) 6761. [20] W.A. Fordyce, G.A. Crosby, J. Am. Chem. Soc. 104 (1982) 985. [21] C.-M. Che, W.-M. Lee, H.-L. Kwong, V.W.-W. Yam, K.-C. Cho, J. Chem. Soc., Dalton Trans. (1990) 1717. [22] J.P. Fackler, Prog. Inorg. Chem. 7 (1966) 361.