Multiple Bonds Between Metal Atoms / 11-Iron, Cobalt and Iridium Compounds

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derivatives, respectively. The structure of the p-anisyl derivative given in Fig. 11.9 shows that only one of the two axial sites is occupied by an oxygen atom of the triflate anion. The EPR spectrum of Ir2(µ-DAniF)4(δ1-O2CCF3) in frozen CH2Cl2 solution at -100 °C is consistent with the presence of an unpaired electron; it shows a ground state of S = 1/2 with a giso of 2.14. The preparations of these compounds from the IrIΑIrIII precursor are summarized in the following chart:

There is also a compound with an Ir26+ core which does not have precedent in the chemistry of rhodium. This is the guanidinate compound Ir2(hpp)2Cl2 where hpp is the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidate. The compound has a paddlewheel structure with four bridging hpp ligands and two axial chlorine atoms and a short Ir–Ir distance of 2.495(1) Å.30 It is paramagnetic with two unpaired electrons.

Fig. 11.9. The structure of Ir2(µ-DAniF)4(δ1-O2CCF3).

As listed in Table 11.2, the Ir–Ir distances of the paddlewheel compounds with all-nitrogen donor ligands decrease slightly as the charge in the Ir2 core increases. This variation is consistent with the change from a single bond in the Ir24+ unit to a formal bond order of 1.5 in Ir25+ to a double bond in Ir26+. Unfortunately it is hard to make stronger correlations because there are not enough structurally characterized compounds to make comparisons and one cannot rule out that this correlation might be fortuitous. Therefore it is hard to tell the precise electronic configuration of these Ir2n+ cores solely on the basis of their structures without comprehensive theoretical calculations that are still lacking. However, the limited magnetic data are consistent with the electronic configurations μ2/4β2β*2/*4, μ2/4β2β*2/*3 and μ2/4β2β*2/*2 for n = 4, 5 and 6, respectively.

A large number of di-iridium compounds have mixed-valence cores stabilized by diphosphazane ligands of the type tfepma (tfepma is the neutral molecule bis(bis(trifluoro-

458Multiple Bonds Between Metal Atoms Chapter 11

ethoxy)phosphino)methylamine, MeN[P(OCH2CF3)2]2).31 The precursor Ir2(tfepma)3Cl2 has a core with Ir0 and IrII centers. These react with PhICl2 in CH2Cl2 to give IrI/IrIII compounds which upon heating with an excess of PhICl2 in CH3CN give complexes such as 11.3 which has two bridging tfepma groups and two IrII atoms with a long Ir–Ir distance of 2.752(1) Å.

11.3

11.4.2 Unsupported Ir–Ir bonds

Controlled thermal decomposition of di(acido)phthalocyaninatoiridates in an inert, highboiling solvent such as 1-chloronaphthalene or under reduced pressure at a temperature of less than 350 ºC produces a blue, diamagnetic di(iridiumphthalocyaninate(2-)), (Irpc2-)2.32 This is soluble in pyridine yielding a blue-violet, diamagnetic compound of composition Ir2(pc2-)2(py)2. This is a dimeric compound with an Ir–Ir distance of 2.707(1) Å. Each Ir atom is surrounded by a phthalocyaninato dianion and a pyridine molecule occupying the axial position. A differential pulse voltammogram shows four quasi-reversible one electron transfer processes at -1.34, -0.82, 0.55 and 0.82 V. The process at 0.55 V is assigned to the redox couple {Ir2(pc2−)2(py)2/ [Ir2(pc2−)(pc−)(py)2]+ by comparison to the electronic spectrum of the product of oxidation by iodine. The Ir–Ir stretching vibration at 135 cm−1 is selectively enhanced in the FT-Raman spectrum.

A compound that bears a close relationship is the di-iridium(II)octaethylporphyrin dimer [Ir(OEP)]2. While this has not been structurally characterized, it almost certainly possesses an unsupported Ir–Ir single bond. It is prepared by the photolysis of Ir(OEP)CH3 in C6D633 but an improved and convenient synthesis uses the reaction of M(OEP)H, M = Ir and Rh, with 2,2,6,6-tetramethyl-1-piperdinoxy (TEMPO):34

2M(OEP)H + 2TEMPO Α M2(OEP)2 + 2TEMPOH

The iridium compound reacts35 in a similar fashion to its dirhodium(II) analog (Section 12.4.3) including alkene insertion and the oxidative addition of alkyl C–H bonds. These reactions probably proceed through the intermediacy of the metalloradical [Ir(OEP)]• which is formed by homolytic dissociation of the Ir–Ir bond.

An early example that might have an unsupported Ir–Ir bond is the Ir24+ complex Ir2(Tcbiim)2(CO)4(NCCH3)2 (Tcbiim is the dianion of tetracyanobisimidazole) whose isolation was reported36 in 1985. It is prepared by the electrolysis of salts of the mononuclear iridium(I) species [Ir(CO)2(Tcbiim)]− in acetonitrile at a Pt anode. While the structure of this compound has not been determined, it can be derivatized by reaction with P(OEt)3 to give Ir2(Tcbiim)2- (CO)2(NCCH3)2[P(OEt)3]2, a compound with an unsupported Ir–Ir bond and a linear P–Ir–Ir–P unit. The Ir–Ir distance is 2.826(2) Å. The equatorial plane about each iridium atom contains cis sets of CO and CH3CN ligands; there is staggered rotation geometry with a C–Ir–Ir–C torsional angle of 44.4º.

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11.4.3 Other species with Ir–Ir bonds

In addition to the structurally characterized complex Ir2(Tcbiim)2(CO)2(NCCH3)2[P(OEt3)2] 2 mentioned at the end of the previous section, there are several other di-iridium(II) compounds that contain carbonyl ligands. Recent examples are those of the type 11.4 which has two cis bridging acetate groups, one chloride ion and a carbonyl group at the equatorial position of each iridium atom. Solvent molecules such as CH3CN, DMSO, pyridine and 4-isopropylpyridine (4-Pripy) can occupy axial positions.37 The first three compounds have moderately short Ir–Ir distances of 2.569(1), 2.5980(5) and 2.5918(5) Å, respectively. These have the formula [Ir2(µ- O2CCH3)2Cl2(CO)2L2] and they are prepared by a one-step reaction of H2IrCl6 with lithium acetate in the presence of O2 and a mixture of acetic acid and acetic anhydride. Compounds where

L = PPh3, PCy3, P(OPh)3, AsPh3 and SbPh3 have slightly longer Ir–Ir distances in the range 2.620(1)-2.694(1) Å.38 Cyclic voltammograms show a one-electron quasi-reversible oxidation wave. Electrolytic or radiolytic one-electron oxidations of the py and 4-Pripy compounds give cationic radicals, which show pseudo-axially symmetric EPR spectra suggesting that the odd electron is in the βIrIr* orbital. A somewhat related compound is the _-pyridonate-bridged (hp) 11.5 which has the formula HH-Ir2(hp)2(CO)4I2. In this compound the pyridonate (2-hy- droxypyridinate) ligands are cis and in a head-to-head arrangement.39 The Ir–Ir distances in two crystallographically independent molecules are 2.643(1) and 2.635(1) Å.

11.4

11.5

The reaction of Ir2Cl2(CO)2(µ-dppm)2 with H2 affords the dihydrido complex Ir2H2Cl2(CO)2(µ-dppm)2 in which the hydrido ligands are believed to be mutually cis on adjacent metal atoms.40 This complex reacts with 1 equiv of MeO2CC>CCO2Me to give Ir2HCl2(δ1-MeO2CC=CHCO2Me)(CO)2(µ-dppm)2 in which alkyne insertion into one of the Ir–H bonds has occurred. A double alkyne insertion occurs upon reacting MeO2CC>CCO2Me with [Ir2H2C1(CO)2(µ-dppm)2]BF4 in dichloromethane; a major product is the di-iridium(II) complex Ir2Cl2(MeO2CC=CHCO2Me)2(CO)2(µ-dppm)2. This compound has the structure depicted in 11.6 with a very long Ir–Ir separation (3.013(1) Å and 3.022(1) Å for the two crystallographically independent molecules within the asymmetric unit). These long separations have been attributed to steric crowding about the Ir atoms in a molecule in which there is an eclipsed rotational geometry.

Another carbonyl-containing di-iridium complex that has been structurally characterized is the tetracarbonyl derivative Ir2(µ-pyS)2(CH2I)I(CO)4, where pyS is the monanion of 2-mercap- topyridine.41 It is prepared by the oxidative-addition reaction of CH2I2 with Ir2(µ-pyS)2(CO)4 at room temperature upon exposure to direct sunlight or irradiation with a 150 W incandescent lamp. Similar reactions occur with I2 and with CH3I to give Ir2(µ-pyS)2I2(CO)4 and Ir2(µ- pyS)2(CH3)I(CO)4, respectively. The lability of the iodide ligand in Ir2(µ-pyS)2(CH2I)I(CO)4 has been demonstrated by the preparation of Ir2(µ-pyS)2(CH2I)X(CO)4 (X = C1 or Br). The structure of Ir2(µ-pyS)2(CH2I)I(CO)4, which has been determined by X-ray crystallography, is

460Multiple Bonds Between Metal Atoms Chapter 11

depicted in 11.7; there is a cisoid head-to-tail arrangement of pyS ligands and a relatively short Ir–Ir distance (2.695(2) Å).

11.6

Just as iodine oxidatively adds to Ir2(µ-pyS)2(CO)4 upon exposure to sunlight to give Ir2(µ- pyS)2I2(CO)4, a similar reaction of a dichloromethane solution of Ir2(µ-C7H4NS2)2(CO)4, where C7H4NS2 is the benzothiazole-2-thiolate anion, affords Ir2(µ-C7H4NS2)2I2(CO)4.42 Its structure is similar to 11.7 with an Ir–Ir single bond length of 2.676(2) Å. If the reaction with I2 is carried out in toluene the intermediate tetranuclear cluster Ir4(µ-C7H4NS2)4I2(CO)8 can be isolated (see Section 11.4.4). The tetranuclear complex reacts rapidly with another equivalent of I2 in dichloromethane by a light-assisted step to give the dinuclear species Ir2(µ-C7H4NS2)2I2(CO)4; this conversion also involves a switch in the coordination mode of the pairs of benzothiazole- 2-thiolate ligands from a head-to-head to a head-to-tail arrangement. A relevant review on controlling the molecular architecture of low nuclearity rhodium and iridium complexes using bridging N–C–X (X = N, O, S) ligands has appeared.43

11.7

A few di-iridium(II) complexes that contain the 2,5-di-isocyano-2,5-dimethylhexane ligand (abbreviated TMB) have been prepared and characterized. The compounds [Ir2(TMB)4X2](BPh4)2 (X = Cl, Br or I) are prepared by titrating acetonitrile solutions of the di-iridium(I) compounds [Ir2(TMB)4](BPh4)2 with X2.44,45 The X-ray structure of crystals of composition [Ir2(TMB)4I2]- (BPh4)2·1.5(CH3)2CO shows this compound to be isostructural with its rhodium analog [Rh2(TMB)4Cl2](PF6)2; the Ir–Ir distance is 2.803(4) Å and the C–Ir–Ir–C torsion angle is 31º, values that are close to those of 2.770(3) Å and 33°, respectively, which have been determined for the dirhodium analog. Evidence for the interconversion of ¨- and Ρ-type enantiomers has been obtained from 1H NMR spectroscopy, while detailed studies have been made of the vibrational and electronic absorption spectral properties of the [Ir2(TMB)4X2]2+ cations. When solutions that contain the di-iridium(I) cation [Ir2(TMB)4]2+ are irradiated in the presence of hydrogen atom donors such as 1,4-cyclohexadiene, the dihydrido species [Ir2(TMB)4H2]2+ is generated; this can be isolated as its crystalline BPh4− salt.46 A structure determination on a crystal of [Ir2(TMB)4H2](BPh4)2·C7H8 showed a close structural relationship to that of the di-iodo derivative but with a Ir–Ir distance (2.920(2) Å) that was longer by c. 0.1 Å than that of [Ir2(TMB)4I2]2+ although the rotational geometries are very similar. The linear H–Ir–Ir–H unit is characterized by ι(Ir–H) and ι(Ir–Ir) vibrational frequencies of 1940 and 136 cm−1,

Iron, Cobalt and Iridium Compounds 461

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respectively; the Raman-active ι(Ir–Ir) mode in the spectra of the chloride, bromide, and iodide complexes decreases from 140 to 128 to 116 cm−1.

While an assortment of other compounds that contain Ir–Ir single bonds are well documented, these do not possess the structural features that accord with the theme of this chapter. Examples include such structurally characterized complexes as (COD)IIr(µ-I)2IrI(COD)47 and (COD)ClIr(µ-SPh)2IrCl(COD),48 where COD = 1,5-cyclo-octadiene, which possess Ir–Ir distances of 2.914(1) and 2.800(1) Å, respectively, but with each Ir center exhibiting an approximately square-pyramidal metal–ligand coordination sphere. In other instances no Ir–Ir bond whatsoever may exist in di-iridium(II) complexes. Such an example is encountered in the case of Ir2{µ-1,8-(NH)2C10H6}(µ-CH2)I2(CO)2)PPh3)2 in which the Ir···Ir separation is 3.0306(4) Å.49 The absence of an Ir–Ir bond accords with the EAN rule.

11.4.4 Iridium blues

These compounds are named after the family of deeply colored platinum compounds known as platinum blues (Section 14.4.7). The term blue has been used to describe a class of compounds, independent of their color, that are mainly tetrametallic (or a multiple thereof) chains with at least one unsupported metal–metal bond, in which the metal atoms possess nonintegral oxidation numbers. For iridium (and also rhodium), most of the work has been done by the groups of Ciriano and Oro in Zaragoza and has been reviewed.50

In dichloromethane solution, iodine oxidatively adds to Ir2(µ-C7H4NS2)2(CO)4, where C7H4NS2 is the benzothiazole-2-thiolate anion, to afford Ir2(µ-C7H4NS2)2I2(CO)4. If the reaction with I2 is carried out in toluene the intermediate tetranuclear cluster Ir4(µ-C7H4NS2)4I2(CO)8 can be isolated. Its structure is shown in Fig. 11.10 and reveals that the outer Ir–Ir bonds are shorter than the inner, unsupported Ir–Ir bond (2.73l(2) Å versus 2.828(2) Å). The bridging benzothiazole-2-thiolate ligands are bound in a head-to-head fashion. Structurally this diamagnetic complex resembles the linear tetranuclear species [Rh4(1,3-di-isocyanopropane)8Cl]5+ (see Section 12.4.3) and certain platinum blues. It can be considered to arise from the coupling of two radical species [Ir2(µ-C7H4NS2)2I(CO)4]•. This tetranuclear dichroic (black, goldengreen) compound shows all the characteristics of the platinum blues (four metal atoms with an average fractional oxidation number of 1.5 and bound by an unsupported metal–metal bond).

Fig. 11.10. The structure of the linear molecule in iridium blue Ir4(µ-C7H4NS2)4I2(CO)8.

Bright purple, EPR silent solutions are obtained by mixing the pyrazolyl (pz) compounds Ir2(pz)2(CNBut)4 and [Ir2(pz)2(CNBut)4(CH3CN)2]PF6 of which there are also rhodium analogs. Oxidation of Ir2(pz)2(CNBut)4 with iodine (in a 1:1 molar ratio) in acetonitrile yields a neutral red complex [Ir2(pz)2(I)2(CNBut)4]2.51 This tetranuclear complex has iodine atoms at each of the

462Multiple Bonds Between Metal Atoms Chapter 11

axial positions. The outer Ir–Ir distances of 2.727(1) Å are crystallographically equivalent and the inner Ir–Ir distance of 2.804(1) Å is significantly longer than the outer distances.

Other iridium blues have been made using _-pyridonate (hp) bridging ligands according to the sequence:52

The precursor is the head-to-tail Ir2(hp)2(COD)2 complex which upon carbonylation gives a mixture of head-to-head and head-to-tail Ir2(hp)2(CO)4. Upon oxidation with iodine below 0 °C the unusual HT,HH-[Ir2(hp)2(I)(CO)4]2 iridium blue forms in 75% yield; it has an average oxidation number of 1.5+ per iridium atom. The outer Ir–Ir distances are 2.692 and 2.711(1) Å and the inner and unsupported Ir–Ir distance is 2.779(1) Å. If the oxidation is carried out at 50 °C, cis-[Ir2(hp)2(I)(CO)4]2 is obtained. This has the more common HH, HH arrangement and outer Ir–Ir distances of 2.702(2) Å and an inner and unsupported distance of 2.750(2) Å. In the two complexes, the two dinuclear moities are arranged in an almost transoid conformation.

Finally there is a hexanuclear iridium chain compound having the formula HH,HT,HH- [Ir6(hp)6(I)2(CO)12] in which the formal oxidation number of each iridium atom is +1.33. This is made by oxidation of Ir2(hp)2(CO)4 with iodine in a 3:1 molar ratio at 0 ºC which gives an EPR silent, dark-blue solution from which a crystalline solid having a copper-like aspect is isolated in 75% yield. The crystal structure shows a hexanuclear chain formed by an almost linear array in which two HH-[Ir2(hp)2(I)(CO)4] units sandwich an HT-[Ir2(hp)2(CO)4] complex as shown in 11.8. The six iridium atoms are linked by metal–metal bonds, two of which are unsupported by bridging ligands. The unsupported Ir–Ir distances in the range of 2.776(2) to 2.793(1) Å are longer than those with pyridonate bridges (range of 2.685(1) to 2.710(1) Å in two independent molecules). The relative conformation of the dinuclear units around the unsupported metal–metal bonds is staggered and almost transoid. These structural features are similar to those found in the tetranuclear complex HT,HH-[Ir2(hp)2(I)(CO)4]2.

11.8

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References

1.F. A. Cotton, L. M. Daniels, L. R. Falvello and C. A. Murillo, Inorg. Chim. Acta 1994, 219, 7.

2.F. A. Cotton, L. M. Daniels, L. R. Falvello, J. H. Matonic and C. A. Murillo, Inorg. Chim. Acta 1997, 256, 269.

3.F. A. Cotton, X. Feng and C. A. Murillo, Inorg. Chim. Acta 1997, 256, 303.

4.F. A. Cotton, L. M. Daniels and C. A. Murillo, Inorg. Chim. Acta 1994, 224, 5.

5.F. A. Cotton, L. M. Daniels, J. H. Matonic and C. A. Murillo, Inorg. Chim. Acta 1997, 256, 277.

6.D. Lee and S. J. Lippard, J. Am. Chem. Soc. 1998, 120, 12153.

7.C. R. Randall, L. Shu, Y.-M. Chiou, K. S. Hagen, M. Ito, N. Kitajima, R. J. Lachicotte, Y. Zang and L. Que, Jr., Inorg. Chem. 1995, 34, 1036.

8.D. Lee, J. Du Bois, D. Petasis, M. P. Hendrich, C. Krebs, B. H. Huynh and S. J. Lippard, J. Am. Chem. Soc. 1999, 121, 9893.

9.D. Lee, C. Krebs, B. H. Huynh, M. P. Hendrich and S. J. Lippard, J. Am. Chem. Soc. 2000, 122, 5000.

10.A. Klose, E. Solari, C. Floriani, A. Chiesi-Villa, C. Rizzoli and N. Re, J. Am. Chem. Soc. 1994, 116, 9123.

11.H. Müller, W. Seidel and H. Görls, J. Organomet. Chem. 1993, 445, 133.

12.F. A. Cotton and R. Poli, Inorg. Chem. 1987, 26, 3652.

13.F. A. Cotton, L. M. Daniels, X. Feng, D. J. Maloney, J. H. Matonic and C. A. Murillo, Inorg. Chim. Acta 1997, 256, 291.

14.L.-P. He, C.-L. Yao, M. Naris, J. C. Lee, J. D. Korp and J. L. Bear, Inorg. Chem. 1992, 31, 620.

15.J. Catterick, M. B. Hursthouse, P. Thornton and A. J. Welch, J. Chem. Soc., Dalton Trans. 1977, 223.

16.Y. Cui, F. Zheng and J. Huang, Acta Cryst. 1999, C55, 1067.

17.F. A. Cotton and X. Feng, Inorg. Chem. 1989, 28, 1180.

18.F. A. Cotton, L. M. Daniels, D. J. Maloney and C. A. Murillo, Inorg. Chim. Acta 1996, 249, 9.

19.F. A. Cotton, L. M. Daniels, D. J. Maloney, J. H. Matonic and C. A. Murillo, Inorg. Chim. Acta 1997, 256, 283.

20.G. L. Simon, A. W. Adamson and L. F. Dahl, J. Am. Chem. Soc. 1972, 94, 7654.

21.L. D. Brown, K. N. Raymond and S. Z. Goldberg, J. Am. Chem. Soc. 1972, 94, 7664.

22.F. A. Cotton, T. G. Dunne and J. S. Wood, Inorg. Chem. 1964, 3, 1495.

23.P. L. Bello, C. Benedicenti, G. Caglio and W. Manassero, J. Chem. Soc., Chem. Commun. 1973, 946.

24.R. Mason, I. Soetofte, S. D. Robinson and M. F. Uttley, J. Organomet. Chem. 1972, 46, C61.

25.F. A. Cotton and R. Poli, Polyhedron 1987, 6, 1625.

26.F. A. Cotton and R. Poli, Inorg. Chem. 1987, 26, 590.

27.F. A. Cotton and R. Poli, Organometallics 1987, 6, 1743.

28.K. R. Dunbar, S. O. Majors and J.-S. Sun, Inorg. Chim. Acta 1995, 229, 373.

29.F. A. Cotton, C. Lin and C. A. Murillo, Inorg. Chem. 2000, 39, 4574.

30.F. A. Cotton, C. A. Murillo and D. J. Timmons, Chem. Commun. 1999, 1427.

31.A. F. Heyduk and D. G. Nocera, J. Am. Chem. Soc. 2000, 122, 9415.

32.H. Hückstädt and H. Homborg, Z. anorg. allg. Chem. 1997, 623, 369.

33.K. J. Del Rossi and B. B. Wayland, J. Chem. Soc., Chem. Commun. 1986, 1653.

34.K. S. Chan and Y.-B. Leung, Inorg. Chem. 1994, 33, 3187.

35.K. J. D. Rossi, X.-X. Zhang, B. B. Wayland, J. Organomet. Chem. 1995, 504, 47.

36.P. G . Rasmussen, J. E. Anderson, O. H. Bailey, M. Tamres and J. C. Bayón, J. Am. Chem. Soc. 1985, 107, 279.

37.N. Kanematsu, M. Ebihara and T. Kawamura, J. Chem. Soc., Dalton Trans. 1999, 4413.

38.N. Kanematsu, M. Ebihara and T. Kawamura, Inorg. Chim. Acta 2001, 323, 96.

39.C. Tejel, M. A. Ciriano, B. E. Villarroya, J. A. López, F. J. Lahoz and L. A. Oro, Angew. Chem. Int. Ed. 2003, 42, 530.

40.B. R. Sutherland and M. Cowie, Organometallics 1985, 4, 1801.

464Multiple Bonds Between Metal Atoms Chapter 11

41.M. A. Ciriano, F. Viguri, L. A. Oro, A. Tiripicchio and M. Tiripicchio-Camellini, Angew. Chem., Int. Ed. Engl. 1987, 26, 444.

42.M. A. Ciriano, S. Sebastián, L. A. Oro, A. Tiripicchio, M. Tiripicchio-Camellini and F. J. Lahoz,

Angew. Chem., Int. Ed. Engl. 1988, 27, 402.

43.L. A. Oro, M. A. Ciriano, J. J. Pérez-Torrente and B. E. Villarroya, Coord. Chem. Rev. 1999, 193195, 941.

44.V. M. Miskowski, T. P. Smith, T. M. Loehr and H. B. Gray, J. Am. Chem. Soc. 1985, 107, 7925.

45.A. W. Maverick, T. P. Smith, E. F. Maverick and H. B. Gray, Inorg. Chem. 1987, 26, 4336.

46.D. C. Smith, R. E. Marsh, W. P. Schaefer, T. M. Loehr and H. B. Gray, Inorg. Chem. 1990, 29, 534.

47.F. A. Cotton, P. Lahuerta, M. Sanaú and W. Schwotzer, J. Am. Chem. Soc. 1985, 107, 8284.

48.F. A. Cotton, P. Lahuerta, J. Latorre, M. Sanau, I. Solana and W. Schwotzer, Inorg. Chem. 1988, 27, 2131.

49.M. J. Fernández, J. Modrego, F. J. Lahoz, J. A. López and L. A. Oro, J. Chem. Soc., Dalton Trans. 1990, 2587.

50.C. Tejel, M. A. Ciriano and L. A. Oro, Chem. Eur. J. 1999, 5, 1131.

51.C. Tejel, M. A. Ciriano, J. A. López, F. J. Lahoz and L. A. Oro, Angew. Chem. Int. Ed. 1998, 37, 1542.

52.C. Tejel, M. A. Ciriano, B. E. Villarroya, R. Gelpi, J. A. López, F. J. Lahoz and L. A. Oro, Angew. Chem. Int. Ed. 2001, 40, 4084.