- Email: [email protected]
Coordination of the macrocyclic thioether 1,4,7-trithiacyclononane (TTCN) to chromium(3+) and to dimolybdenum(2+) di-acetato bridged molecules
Po/ykdm Vol. 8, No. 13/14, pp. 177C-1773, Printed in Great Britain
1989
0277-5387189 $3.00 + .0i1 Maxwell Per@~~~on Macmillan plc
COORDINATION OF THE MACROCYCLIC THIOETHER 1,4,7-TRITHIACYCLONONANE (TTCN) TO CHROMIUM(3+) AND TO DIMOLYBDENUM(2+) DI-ACETATO BRIDGED MOLECULES HEINZ-JOSEF
and KARL WIEGHARDT
KfZPPERS*
Ruhr-Universitgt
Bochum, D4630 Bochum, F.R.G.
Abstract-Although chromium(II1) is a hard acid and the macrocyclic thioether 1,4,7trithiacyclonone (TTCN) a soft base, the following compounds could be synthesized: [Cr(TTCN)Cl,], [Cr(TTCN)(SO,CF,),] and [Cr(TTCN),13+ (a rare example of a Cl”(thioether), coordinatjon sphere). Comparison with the analogous 1,4,7-triazacyclononane (TACN) complexes shows that the bonding is relatively weak and the ligand-field splitting small. This is in contrast to the bonding properties of these ligands in “late” transition metals and results from the lack of n-donor capability of the Cr3+ centre. TTCN forms several dimeric di-acetato bridged complexes with the (Mo~)~+ unit. The unique interaction of steric and electronic effects of TTCN causes the unusual coordination of a tridentate ligand to the [Mo~(~-OAC)~~+ unit.
The coordination chemistry of the crown thioether 1,4,7-trithiacyclononane (TTCN) has been intensively investigated in the last few years. Although TTCN forms stable complexes with nearly every transition metal, ’ only a few examples of the coordination to early transition metals are known.’ We report here the reaction of TTCN with chromium(II1) and with the [Mo~(~-OAC)~]~+unit.
TTCN
TACN
The strong coordination of TTCN to the “late” transition metals comes from both its steric (endoconformation) and electronic properties (7c-acceptor capability). The question arises, whether this soft base can coordinate to a hard acid such as Cr3+ (class A metal). The reaction of [Cr(H20)d]C13 (in MeCN) or [Cr(THF),Cl,] (in dioxane) with TTCN yields [Cr(TTCN)Cl,]. This blue-violet complex is inert and sparingly soluble in common solvents. Stirring
* Author to whom correspondence should be addressed.
in trifluoromethanesulphonic acid for some hours, at room temperature, results in the substitution of chloride by triflate to form blue-green [Cr(TTCN)(SO,CF,),]. The IR spectrum confirms monodentate coordination of triflate. In solution, solvent molecules such as CH,CN, (CH3)2CO, DMF, THF replace the labile triflate ligands. All attempts to synthesize the bis-TTCN-Cr3+ complex by the reaction of excess TTCN with Cr”’ salts of Cr(TTCN)X, (X = Cl- ; CF3SO; ; solvent) in solution, under various conditions, failed. Only the heating of a mixture of solid [Cr(H20),](C104), and solid TTCN results in the pink, very explosive [Cr(TTCN),](ClO,),, which decomposes in a few seconds in solution. All compounds gave suitable elemental analyses and IR spectra. The magnetic moments of P,_~= 4.1 BM ([Cr(TTCN)Cl,]) and pefl = 3.9 BM ([Cr(TTCN)(SO,CF,),]) are in the normal range of octahedral Cr3+ compounds. The electronic spectra allow the discussion of the binding properties in these compounds. For octahedral chromium(II1) complexes one expects three bands which correspond to the transitions 4AQ -, 4T29(vl), 4A2g-, 4T&?(v2) and 4A~g--, 4Tl,(P)(v3). In most cases v3 is obscured by chargetransfer bands. From the spectra the ligand-field splitting, lODq, can be calculated. The data for the Cr-TTCN complexes and some other Cr3+ compounds are summarized in Table 1.
1770
Coordination of 1,4,7-trithiacyclononane
1771
Scheme I.
The data can be summarized in the following Substitution of three Cl- by TTCN increases 1ODq only slightly. Further substitution of Clspectrochemical series: Cl- < S(CH3)r, TTCN, by a second TTCN molecule does not affect 1ODq. THT, THF < HZ0 < NH3 < TACN. This can be explained by the loss of charge neuIn case of Cr3+ coordination, TTCN binds tralization, which leads to an increasing positive weakly. Although the ring size and the rigidity do charge at the chromium centre. This in turn, effects not allow optimal interaction between the Cr3+ and a contraction of the chromium d-orbitals and, there- the ligand orbitals, [Cr(TTCN)J3’ is a rare case of fore, decreases their interaction with the ligand Cf”(thioether), coordination. The great difference in the bonding properties of TTCN of chroorbitals.’ The value of IODq also does not change if one mium(II1) and “late” transition metals seems to TTCN is replaced by three dimethylsulphide or be the lacking of p-donor capability of the metal tetrahydrothiophene ligands. In addition, the open centre. Another interesting reaction of TTCN with an chain ligand S(CH2)$CH3 causes a larger ligandearly transition metal is the reaction of TTCN with field splitting than TTCN. Accordingly, the macrothe dimeric molybdenum(II) unit, ~o&OAc)J*+. cyclic nature of TTCN does not enhance the apparent ligand-field strength (cf. /Ni(TTCN)J2+). The The chemistry of (Mo*)~+ has been intensively ring size and the rigidity of the TTCN-macrocycle studied in the last 25 years. However, only a few obviously does not allow optimal interaction compounds with tridentate coordinated ligand are between the thioether donor atoms and the rela- known. lo Is TTCN with its unique characteristics’** able to form such a class of coordination comtively large Cr3+ centre. pounds? The two pairs, [Cr(TTCN)J3+/[Cr(TACN)J3+ and [Cr(TT~~(S03CF3)31/[Cr~ACN)(S03CF3)31 In 1986, Garner and Clegg described the (which was synthesized by the reaction of [Cr synthesis of [(CH3CN)3Mogl-OAc)2Mo(CH3CN)3] (BF4), (1) [SCXFig. l(a)] from ~o~@-OAC)~] with (TACN)(Br) 31 9 with trifluoromethanesulphonic acid), allow a comparison of the different effect Me30BF4.’ ’ This compound proved to be an of TTCN and TACN (TACN = 1,4,7-triazacycloideal starting material for the synthesis of sevnonane). Therefore TTCN generates a smaller eral dimeric MO”-TTCN complexes. Stirring of 1 with excess TTCN for 12 h at room ligand-field splitting than does TACN for Cr3+ ; for temperature, in MeCN, leads to the red-violet Fe’+, Co3+ or Ni*+ the opposite is true.‘*
Table 1. Absorption maxima and IODqdata of some Cr3+ complexes Compound [Crcld3[Cr(TTCN)Cl 3]
[WWH3L)3C131 fat-[Cr(THT),Cl,]
[CrWF)3C131 [Cr(TTCN)(OSO,CF,),] ICr(TACN)(OSO~CFJJ [Cr(TTCN)J’+ [Cr(TACN)J3+
[WWHdWH3M3+ KWW3d3+ [Cr(NH~h13+ THT : tetrahydrothiophene.
vI (nm)
v2 (run)
760 690 690 685 675 635 523 685 439 636 575 464
535 493 523 480 494 475 509 340 436 406 350
IODq(kK) Reference 13.1 14.5 14.5 14.6 14.8 15.8 19.1 14.6 22.7 15.7 17.4 21.6
3 4 5 4
H.-J. Kf&PERS
1772
and K. WIEGHARDT
a. />~--,C’ MeCNYo---_Mo-NCMe MeCN’h MeCN’k C C Me Me [MO(CL- OAc),(MeCN) J”
Me
-
Flo,(~ - OAc),‘lTCN(MeCN@+
[Mo,(P - OAc),(?TCN),l*+ Fig. 1. The structures (b) and (c) are proposed, while (a) has been proved by X-ray analysis. ’ ’
[(TTCN)Mo@-OAC),MO(CH$N),I(BF~)~ (2) [see Fig. l(b)]. Substitution of the last MeCN ligands requires refluxing of 2 in CH2CC12 with excess TTCN. This procedure forms the blue-violet [(TTCN)Mo(yOAc),Mo(TTCN)](BF,), (3) [see Fig. l(c)]. Compound 2 reacts in aqueous solution
with KX to form the blue [(TTCN)Mo@-OAc)* MoWLl(X = Cl-,Br-,SCN-,OCN-)(4),(~,(6),(~, respectively. The IR spectra of these compounds prove the retention of the bridging [MO&-OAc)d unit. Also, the elemental analyses and the electronic spectra further support this structural suggestion. In electronic spectra a band at about 500 nm, corresponding to the transition 6 + 6* in (Mo*)~+ compounds, is expected. The data of these absorption bands are listed in Table 2. A comparison of the absorption bands of [Mo&-OAC)~(CH~CN~)~]~+ and [MO,@-OAc), W-WWd2+ shows a shift to lower energy. The reason for this is probably the weakening of the metal-metal bond by the coordination of axial MeCN ligands. The bonding of a ligand trans to the Mo-MO axis weakens the metal-metal o-bonding and therefore increases the metal-metal distance, which causes a smaller overlapping of the n- and b-orbitals. Substitution of the MeCN ligands by TTCN in 2 and 3 further weakens the metal-metal bonding because of the larger z-acceptor capability of the thioether donor atoms compared to the Natoms of MeCN. Coordination of ligands with even larger n-acceptor capability, e.g. CO and NO, cleaves the metal-metal bond.” In the solid state the compounds 2-7 are stable in air for several weeks. Dissolved in non-coordinating solvents, however, 3,6 and 7 rapidly react with air. The MO,@-OAc),(TTCN)] compounds are an example of the coordination of tridentate ligands to the [Mo2(p-OAc)d unit. This unusual coordination sphere further exemplifies the unique chelating ability of TTCN caused by the interaction of its steric and electronic properties. AcknowledgementsOne of us (H.J.K.) would like to thank Dr S. R. Cooper for helpful discussions S. Stbtzel for synthesizing compounds 4 and 7.
Table 2. Absorption maxima of [MO&-AC),] compounds
Compound
L, (nm) [E(dm3 mol- ’ cm- ‘)]
[(CH,CN)2Mo~-OA~)2Mo(CH,CN)2]2+ [(CH3CN)3Mo@-OA~)2Mo(CHsCN)3]2+ [(TTCN)MO~-OAC)~MO(CH~CN),]~+ [(TTCN)Mo@-OAC),MO(TTCN)]~+ [(-l-fCN)Mo&-OAC)~MO(SCN)A [(TTCN)Mo@-OAc),Mo(OCN),] [(TTCN)Mo(~-OAc),Mo(Br)J [(TTCN)Mo&-OAc),Mo(Cl)d
526(844) 53 l(925) 537( 1loo) 550(990)b 586 600’ 596’ 621’
a In MeCN. bIn acetone. ‘Solid state reflectance spectrum.
Reference 11 11
and
Coordination
of 1,4,7&thiacyclononane
REFERENCES 1. (a) K. Wieghardt, H.-J. Kiippers and J. Weiss, Znorg. Chem. 1985,24,3067 ; (b) H.-J. Ktippers, A. Neves, C. Pomp, D. Ventur, K. Wieghardt, B. Nuber and J. Weiss, Znorg. Chem. 1986, 25, 2400 ; (c) M. N. Bell, A. J. Blake, H.-J. Ktippers, M. Schroder and K. Wieghardt, Angew. Chem. Znt. Edn Engl. 1987, 26, 250 ; (d) H.-J. Ktippers, K. Wieghardt, B. Nuber and J. Weiss, Z. Anorg. Allg. Chem. 1989, in press ; (e) S. R. Cooper, Accts Chem. Res. 1988, 21, 141 ; (f) W. N. Setzer, C. A. Ogle, G. S. Wilson and R. S. Glass, Znorg. Chem. 1983, 22, 266; (g) A. J. Blake, A. J. Holder, T. I. Hyde and M. Schroder, J. Chem. Sot., Chem. Commun. 1987,987. 2. (a) M. T. Ashby and D. L. Lichtenberger, Znorg. Chem. 1984, 24, 636 ; (b) H. Elias, G. Schmidt, H.-J. Kiippers, M. Saher, K. Wieghardt, B. Nuber and J. Weiss, Znorg. Chem. 1989, in press.
1773
3. A. L. Hale and W. Levason, J. Chem. Sot., Dalton Trans. 1983,2569. 4. R. J. H. Clark and G. Natile, Znorg. Chim. Acta 1970, 4, 533. 5. J. Hughes and G. R. Willey, Znorg. Chem. Acta 1974, 11, L25. 6. K. Wieghardt, W. Schmidt, W. Herrmann and H.-J. Kiippers, Znorg. Chem. 1983, 22, 2953. 7. C. K. Jorgensen, Adv. Chem. Phys. 1963,5, 33. 8. L. R. Gray, A. L. Hale, W. Levason, F. P. McCullough and M. Webster, J. Chem. Sot., Dalton Trans. 1984,47. 9. P. Chaudhuri, M. Winter, H.-J. Ktippers, K. Wieghardt, B. Nuber and J. Weiss, Znorg. Chem. 1987, 26, 3302. 10. F. A. Cotton and R. A. Walton, Multiple Bonak Between Metal Atoms. Wiley, New York (1982). 11. G. Pimblett, C. D. Garner and W. Clegg, J. Chem. Sot., Dalton Trans. 1986, 1257.
1989
0277-5387189 $3.00 + .0i1 Maxwell Per@~~~on Macmillan plc
COORDINATION OF THE MACROCYCLIC THIOETHER 1,4,7-TRITHIACYCLONONANE (TTCN) TO CHROMIUM(3+) AND TO DIMOLYBDENUM(2+) DI-ACETATO BRIDGED MOLECULES HEINZ-JOSEF
and KARL WIEGHARDT
KfZPPERS*
Ruhr-Universitgt
Bochum, D4630 Bochum, F.R.G.
Abstract-Although chromium(II1) is a hard acid and the macrocyclic thioether 1,4,7trithiacyclonone (TTCN) a soft base, the following compounds could be synthesized: [Cr(TTCN)Cl,], [Cr(TTCN)(SO,CF,),] and [Cr(TTCN),13+ (a rare example of a Cl”(thioether), coordinatjon sphere). Comparison with the analogous 1,4,7-triazacyclononane (TACN) complexes shows that the bonding is relatively weak and the ligand-field splitting small. This is in contrast to the bonding properties of these ligands in “late” transition metals and results from the lack of n-donor capability of the Cr3+ centre. TTCN forms several dimeric di-acetato bridged complexes with the (Mo~)~+ unit. The unique interaction of steric and electronic effects of TTCN causes the unusual coordination of a tridentate ligand to the [Mo~(~-OAC)~~+ unit.
The coordination chemistry of the crown thioether 1,4,7-trithiacyclononane (TTCN) has been intensively investigated in the last few years. Although TTCN forms stable complexes with nearly every transition metal, ’ only a few examples of the coordination to early transition metals are known.’ We report here the reaction of TTCN with chromium(II1) and with the [Mo~(~-OAC)~]~+unit.
TTCN
TACN
The strong coordination of TTCN to the “late” transition metals comes from both its steric (endoconformation) and electronic properties (7c-acceptor capability). The question arises, whether this soft base can coordinate to a hard acid such as Cr3+ (class A metal). The reaction of [Cr(H20)d]C13 (in MeCN) or [Cr(THF),Cl,] (in dioxane) with TTCN yields [Cr(TTCN)Cl,]. This blue-violet complex is inert and sparingly soluble in common solvents. Stirring
* Author to whom correspondence should be addressed.
in trifluoromethanesulphonic acid for some hours, at room temperature, results in the substitution of chloride by triflate to form blue-green [Cr(TTCN)(SO,CF,),]. The IR spectrum confirms monodentate coordination of triflate. In solution, solvent molecules such as CH,CN, (CH3)2CO, DMF, THF replace the labile triflate ligands. All attempts to synthesize the bis-TTCN-Cr3+ complex by the reaction of excess TTCN with Cr”’ salts of Cr(TTCN)X, (X = Cl- ; CF3SO; ; solvent) in solution, under various conditions, failed. Only the heating of a mixture of solid [Cr(H20),](C104), and solid TTCN results in the pink, very explosive [Cr(TTCN),](ClO,),, which decomposes in a few seconds in solution. All compounds gave suitable elemental analyses and IR spectra. The magnetic moments of P,_~= 4.1 BM ([Cr(TTCN)Cl,]) and pefl = 3.9 BM ([Cr(TTCN)(SO,CF,),]) are in the normal range of octahedral Cr3+ compounds. The electronic spectra allow the discussion of the binding properties in these compounds. For octahedral chromium(II1) complexes one expects three bands which correspond to the transitions 4AQ -, 4T29(vl), 4A2g-, 4T&?(v2) and 4A~g--, 4Tl,(P)(v3). In most cases v3 is obscured by chargetransfer bands. From the spectra the ligand-field splitting, lODq, can be calculated. The data for the Cr-TTCN complexes and some other Cr3+ compounds are summarized in Table 1.
1770
Coordination of 1,4,7-trithiacyclononane
1771
Scheme I.
The data can be summarized in the following Substitution of three Cl- by TTCN increases 1ODq only slightly. Further substitution of Clspectrochemical series: Cl- < S(CH3)r, TTCN, by a second TTCN molecule does not affect 1ODq. THT, THF < HZ0 < NH3 < TACN. This can be explained by the loss of charge neuIn case of Cr3+ coordination, TTCN binds tralization, which leads to an increasing positive weakly. Although the ring size and the rigidity do charge at the chromium centre. This in turn, effects not allow optimal interaction between the Cr3+ and a contraction of the chromium d-orbitals and, there- the ligand orbitals, [Cr(TTCN)J3’ is a rare case of fore, decreases their interaction with the ligand Cf”(thioether), coordination. The great difference in the bonding properties of TTCN of chroorbitals.’ The value of IODq also does not change if one mium(II1) and “late” transition metals seems to TTCN is replaced by three dimethylsulphide or be the lacking of p-donor capability of the metal tetrahydrothiophene ligands. In addition, the open centre. Another interesting reaction of TTCN with an chain ligand S(CH2)$CH3 causes a larger ligandearly transition metal is the reaction of TTCN with field splitting than TTCN. Accordingly, the macrothe dimeric molybdenum(II) unit, ~o&OAc)J*+. cyclic nature of TTCN does not enhance the apparent ligand-field strength (cf. /Ni(TTCN)J2+). The The chemistry of (Mo*)~+ has been intensively ring size and the rigidity of the TTCN-macrocycle studied in the last 25 years. However, only a few obviously does not allow optimal interaction compounds with tridentate coordinated ligand are between the thioether donor atoms and the rela- known. lo Is TTCN with its unique characteristics’** able to form such a class of coordination comtively large Cr3+ centre. pounds? The two pairs, [Cr(TTCN)J3+/[Cr(TACN)J3+ and [Cr(TT~~(S03CF3)31/[Cr~ACN)(S03CF3)31 In 1986, Garner and Clegg described the (which was synthesized by the reaction of [Cr synthesis of [(CH3CN)3Mogl-OAc)2Mo(CH3CN)3] (BF4), (1) [SCXFig. l(a)] from ~o~@-OAC)~] with (TACN)(Br) 31 9 with trifluoromethanesulphonic acid), allow a comparison of the different effect Me30BF4.’ ’ This compound proved to be an of TTCN and TACN (TACN = 1,4,7-triazacycloideal starting material for the synthesis of sevnonane). Therefore TTCN generates a smaller eral dimeric MO”-TTCN complexes. Stirring of 1 with excess TTCN for 12 h at room ligand-field splitting than does TACN for Cr3+ ; for temperature, in MeCN, leads to the red-violet Fe’+, Co3+ or Ni*+ the opposite is true.‘*
Table 1. Absorption maxima and IODqdata of some Cr3+ complexes Compound [Crcld3[Cr(TTCN)Cl 3]
[WWH3L)3C131 fat-[Cr(THT),Cl,]
[CrWF)3C131 [Cr(TTCN)(OSO,CF,),] ICr(TACN)(OSO~CFJJ [Cr(TTCN)J’+ [Cr(TACN)J3+
[WWHdWH3M3+ KWW3d3+ [Cr(NH~h13+ THT : tetrahydrothiophene.
vI (nm)
v2 (run)
760 690 690 685 675 635 523 685 439 636 575 464
535 493 523 480 494 475 509 340 436 406 350
IODq(kK) Reference 13.1 14.5 14.5 14.6 14.8 15.8 19.1 14.6 22.7 15.7 17.4 21.6
3 4 5 4
H.-J. Kf&PERS
1772
and K. WIEGHARDT
a. />~--,C’ MeCNYo---_Mo-NCMe MeCN’h MeCN’k C C Me Me [MO(CL- OAc),(MeCN) J”
Me
-
Flo,(~ - OAc),‘lTCN(MeCN@+
[Mo,(P - OAc),(?TCN),l*+ Fig. 1. The structures (b) and (c) are proposed, while (a) has been proved by X-ray analysis. ’ ’
[(TTCN)Mo@-OAC),MO(CH$N),I(BF~)~ (2) [see Fig. l(b)]. Substitution of the last MeCN ligands requires refluxing of 2 in CH2CC12 with excess TTCN. This procedure forms the blue-violet [(TTCN)Mo(yOAc),Mo(TTCN)](BF,), (3) [see Fig. l(c)]. Compound 2 reacts in aqueous solution
with KX to form the blue [(TTCN)Mo@-OAc)* MoWLl(X = Cl-,Br-,SCN-,OCN-)(4),(~,(6),(~, respectively. The IR spectra of these compounds prove the retention of the bridging [MO&-OAc)d unit. Also, the elemental analyses and the electronic spectra further support this structural suggestion. In electronic spectra a band at about 500 nm, corresponding to the transition 6 + 6* in (Mo*)~+ compounds, is expected. The data of these absorption bands are listed in Table 2. A comparison of the absorption bands of [Mo&-OAC)~(CH~CN~)~]~+ and [MO,@-OAc), W-WWd2+ shows a shift to lower energy. The reason for this is probably the weakening of the metal-metal bond by the coordination of axial MeCN ligands. The bonding of a ligand trans to the Mo-MO axis weakens the metal-metal o-bonding and therefore increases the metal-metal distance, which causes a smaller overlapping of the n- and b-orbitals. Substitution of the MeCN ligands by TTCN in 2 and 3 further weakens the metal-metal bonding because of the larger z-acceptor capability of the thioether donor atoms compared to the Natoms of MeCN. Coordination of ligands with even larger n-acceptor capability, e.g. CO and NO, cleaves the metal-metal bond.” In the solid state the compounds 2-7 are stable in air for several weeks. Dissolved in non-coordinating solvents, however, 3,6 and 7 rapidly react with air. The MO,@-OAc),(TTCN)] compounds are an example of the coordination of tridentate ligands to the [Mo2(p-OAc)d unit. This unusual coordination sphere further exemplifies the unique chelating ability of TTCN caused by the interaction of its steric and electronic properties. AcknowledgementsOne of us (H.J.K.) would like to thank Dr S. R. Cooper for helpful discussions S. Stbtzel for synthesizing compounds 4 and 7.
Table 2. Absorption maxima of [MO&-AC),] compounds
Compound
L, (nm) [E(dm3 mol- ’ cm- ‘)]
[(CH,CN)2Mo~-OA~)2Mo(CH,CN)2]2+ [(CH3CN)3Mo@-OA~)2Mo(CHsCN)3]2+ [(TTCN)MO~-OAC)~MO(CH~CN),]~+ [(TTCN)Mo@-OAC),MO(TTCN)]~+ [(-l-fCN)Mo&-OAC)~MO(SCN)A [(TTCN)Mo@-OAc),Mo(OCN),] [(TTCN)Mo(~-OAc),Mo(Br)J [(TTCN)Mo&-OAc),Mo(Cl)d
526(844) 53 l(925) 537( 1loo) 550(990)b 586 600’ 596’ 621’
a In MeCN. bIn acetone. ‘Solid state reflectance spectrum.
Reference 11 11
and
Coordination
of 1,4,7&thiacyclononane
REFERENCES 1. (a) K. Wieghardt, H.-J. Kiippers and J. Weiss, Znorg. Chem. 1985,24,3067 ; (b) H.-J. Ktippers, A. Neves, C. Pomp, D. Ventur, K. Wieghardt, B. Nuber and J. Weiss, Znorg. Chem. 1986, 25, 2400 ; (c) M. N. Bell, A. J. Blake, H.-J. Ktippers, M. Schroder and K. Wieghardt, Angew. Chem. Znt. Edn Engl. 1987, 26, 250 ; (d) H.-J. Ktippers, K. Wieghardt, B. Nuber and J. Weiss, Z. Anorg. Allg. Chem. 1989, in press ; (e) S. R. Cooper, Accts Chem. Res. 1988, 21, 141 ; (f) W. N. Setzer, C. A. Ogle, G. S. Wilson and R. S. Glass, Znorg. Chem. 1983, 22, 266; (g) A. J. Blake, A. J. Holder, T. I. Hyde and M. Schroder, J. Chem. Sot., Chem. Commun. 1987,987. 2. (a) M. T. Ashby and D. L. Lichtenberger, Znorg. Chem. 1984, 24, 636 ; (b) H. Elias, G. Schmidt, H.-J. Kiippers, M. Saher, K. Wieghardt, B. Nuber and J. Weiss, Znorg. Chem. 1989, in press.
1773
3. A. L. Hale and W. Levason, J. Chem. Sot., Dalton Trans. 1983,2569. 4. R. J. H. Clark and G. Natile, Znorg. Chim. Acta 1970, 4, 533. 5. J. Hughes and G. R. Willey, Znorg. Chem. Acta 1974, 11, L25. 6. K. Wieghardt, W. Schmidt, W. Herrmann and H.-J. Kiippers, Znorg. Chem. 1983, 22, 2953. 7. C. K. Jorgensen, Adv. Chem. Phys. 1963,5, 33. 8. L. R. Gray, A. L. Hale, W. Levason, F. P. McCullough and M. Webster, J. Chem. Sot., Dalton Trans. 1984,47. 9. P. Chaudhuri, M. Winter, H.-J. Ktippers, K. Wieghardt, B. Nuber and J. Weiss, Znorg. Chem. 1987, 26, 3302. 10. F. A. Cotton and R. A. Walton, Multiple Bonak Between Metal Atoms. Wiley, New York (1982). 11. G. Pimblett, C. D. Garner and W. Clegg, J. Chem. Sot., Dalton Trans. 1986, 1257.