The influence of nitrosyl and allyl ligands on the formation of metalmetal bonds in bimetallic complexes containing S2CPCy3 bridges

www.elsevier.nl/locate/ica Inorganica Chimica Acta 307 (2000) 20 – 25

The influence of nitrosyl and allyl ligands on the formation of metalmetal bonds in bimetallic complexes containing S2CPCy3 bridges Daniel Miguel a,1, Vı´ctor Riera a, Mei Wang b,* a

Instituto Uni6ersitario de Quı´mica Organometa´lica ‘Enrique Moles’, Unidad Asociada del CSIC, Uni6ersidad de O6iedo, E-33071 O6iedo, Spain b Open Laboratory of Comprehensi6e Utilization of Carbon Resources, Department of Chemistry, Dalian Uni6ersity of Technology, Zhongshan Road 158 -46, Dalian 116012, PR China Received 11 February 2000; accepted 12 May 2000

Abstract The reactions of the anion [X(CO)2Mo(m-S2CPCy3)Mo(CO)3]− (X =NO, h3-C3H5) with Ph2PCl afforded the products [(ON)(CO)2Mo(m-PPh2)(m-S2CPCy3)Mo(CO)3] (3) and [(h3-C3H5)(CO)Mo(m-PPh2)(m-S2CPCy3)Mo(CO)3] (4), respectively. The chemical shifts of the resulting m-PPh2 ligands in the 31P{1H} NMR spectra indicate that the two molybdenum atoms are not bonded in 3, but are directly bonded in 4. The selenido-bridged binuclear complexes [X(CO)2Mo(m-SePh)(m-S2CPCy3)M(CO)3] (X= NO, M= Mo, 5a; M = W, 5b; X= h3-C3H5, M=Mo, 6a; M = W, 6b) were synthesized by the reactions of the corresponding anions [X(CO)2Mo(m-S2CPCy3)MCO)3]− with PhSeI. The influence of the nitrosyl and allyl ligands on the formation of metalmetal and metalcarbon bonds in bimetallic complexes containing both S2CPCy3 and PPh2 bridges is discussed. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Molybdenum complexes; Tungsten complexes; Nitrosyl complexes; Phosphido bridge complexes; Selenido bridge complexes

1. Introduction Diorganophosphide (PR2) groups display a high tendency to act as bridging ligands. These robust bridges have been used very often for the synthesis of homoand heterobimetallic complexes [1 – 13]. In addition to their stability, PR2 groups are able to act as bridges between metals with (type A and B in Scheme 1) or without (type C and D) metalmetal bonds [14 – 19]. In several instances, under conditions of thermolysis or UV irradiation, the C or D type binuclear complexes may change to the A or B type complexes, often with the accompanying dissociation of a CO ligand [20 –25]. The two bonding situations can be diagnosed by means of 31P{1H} NMR spectroscopy, since the chemical shift of the PR2 groups changes dramatically depending on the absence or presence of a direct metalmetal bond

[26]. Examples of bimetallic compounds with the related organoselenide (SeR) ligands are scarce [27 –29]. We have previously reported the preparation and structures of the manganese –molybdenum complexes containing both S2CPR3 and phosphido or selenido bridges [30]. It has been found that in solution the D

* Corresponding author. Fax: + 86-411-263 3041. E-mail address: [email protected] (M. Wang). 1 Present address: Departamento de Quı´mica Inorga´nica, Universidad de Valladolid, Real de Burgos s/n, E-47071 Valladolid, Spain. 0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0020-1693(00)00208-5

Scheme 1.

Scheme 2.

D. Miguel et al. / Inorganica Chimica Acta 307 (2000) 20–25

21

[X(CO)2Mo(m-SePh)(m-S2CPCy3)M(CO)3] (X=NO, M= Mo, 5a; M= W, 5b; X=h3-C3H5, M= Mo, 6a; M= W, 6b), and discuss the influence of the ancillary ligands on the formation of metalmetal bonds.

2. Results

2.1. Synthesis and spectroscopic characterization of [(ON)(CO)2Mo(v-PPh2)(v-S2CPCy3)Mo(CO)3] (3) and [(p 3-C3H5)(CO)Mo(v-PPh2)(v-S2CPCy3)Mo(CO)3] (4)

Scheme 3.

type MnMo complex 1 smoothly transformed into the B type complex 2 by the rearrangement of the central carbon atom of the S2CPR3 ligand from Mo to Mn and the dissociation of a CO ligand on Mn (Scheme 2). Recently, we have extended some of the chemistry initially carried out on the S2CPR3-bridged MnMo compounds to related bimolybdenum species. In particular, we prepared [(X)(CO)2Mo(m-Br)(m-S2CPCy3)Mo(CO)3] compounds differing only in the nature of X, which can be NO or h3-C3H5. Analogous to our previous work with the MnMo compounds, the reduction of these bromide-bridged complexes by sodium amalgam allowed the preparation of bimetallic anions, and subsequent reactions with electrophiles afforded novel neutral bimetallic complexes. In this paper we report the preparation and characterization of novel phosphido and selenido-bridged bimetallic derivatives, [(ON)(CO)2Mo(m-PPh2)(m-S2CPCy3)Mo(CO)3] (3), [(h3C3H5)(CO)Mo(m-PPh2)(m-S2CPCy3)Mo(CO)3] (4) and

The highly reactive and thermally unstable bimolybdenum anion, [(ON)(CO)2Mo(m-S2CPCy3)Mo(CO)3]−, was prepared by the reduction of [(ON)(CO)2Mo(mBr)(m-S2CPCy3)Mo(CO)3] [31] with sodium amalgam (1%) in THF at −40°C and used immediately. The analogous anion [(h3-C3H5)(CO)2Mo(m-S2CPCy3)Mo(CO)3]− was made in the same way by using [(h3C3H5)(CO)2Mo(m-Br)(m-S2CPCy3)Mo(CO)3] at room temperature. The anionic species were identified in situ by IR spectroscopy, which showed four w(CO) bands for the nitrosyl anions, located at higher frequencies (w(CO) 1989 1747 cm − l) than those for their allyl anionic analogs (w(CO) 19171737 cm − l). Complexes 3 and 4 were prepared by treatment of 1 equiv of Ph2PCl with the solutions of the corresponding anions in THF at − 60°C (Scheme 3). The IR and 31P{1H} NMR monitoring showed that complexes 3 and 4 were the only detectable products in each crude reaction solution. By careful workup at low temperature, 3 and 4 were obtained in low yields as dark green (3) and dark red –brown (4) crystalline compounds, respectively. Complexes 3 and 4 were stable as solids under a nitrogen atmosphere, but in solution they slowly decomposed at room temperature. In both cases, the low yields can be attributed to partial decomposition during workup. Both 3 and 4 are less stable than the previously reported compounds 1 and 2 with PPh2 and S2CPR3 bridges between Mn and Mo atoms [30].

Table 1 IR and 31P{1H} NMR data for the novel phosphido- and selenido-bridged binuclear complexes No.

3 4 5a 5b 6a 6b a

Compound

[(ON)(OC)2Mo(m-PPh2)(m-S2CPCy3)Mo(CO)3] [(h3-C3H5)(OC)Mo(m-PPh2)(m-S2CPCy3)Mo(CO)3] [(ON)(OC)2Mo(m-SePh)(m-S2CPCy3)Mo(CO)3] [(ON)(OC)2Mo(m-SePh)(m-S2CPCy3)W(CO)3] [(h3-C3H5)(OC)2Mo(m-SePh)(m-SCPCy3)Mo(CO)3] [(h3-C3H5)(OC)2Mo(m-SePh)(m-S2CPCy3)W(CO)3] Measured in CD2Cl2.

P{1H} NMR a l (ppm)

31

IR (THF) (cm−1)

w(CO)

w(NO)

S2CP

PPh2

2012m, 1998s, 1932s, 1875m 1998s, 1928vs, 1865m, 1845m 2023s, 2008s, 1640s, 1945vs, 1874s 2023s, 2003vs, 1945vs, 1926s, 1869s 2009s, 1939vs, 1866s, 1851s 2003s, 1940s, 1923vs, 1858s

1630m

38.0 34.2 37.3 35.4 36.6 34.9

−111.6 235.1

1642s

D. Miguel et al. / Inorganica Chimica Acta 307 (2000) 20–25

22

Scheme 4.

The novel phosphido-bridged bimolybdenum complexes 3 and 4 were characterized by IR, 31P{1H}, 1H and 13C NMR spectroscopy (Tables 1 and 2) and elemental analysis (see Section 4). The most important spectroscopic features of 3 and 4 are the 31P{1H} NMR spectra, in which the resonances attributed to the S2CPCy3 ligands have similar chemical shifts for 3 (l 38.0) and 4 (l 34.2), while the resonances assigned to the PPh2 bridges of 3 and 4 differ by 345.9 ppm. Complex 3 displayed an upfield 31P{1H} resonance at l − 111.6 (l = − 164.8 for 1) [30], indicating that the phosphido ligand bridges two molybdenum atoms, which are not directly bonded. In contrast, complex 4 displayed a downfield 31P{1H} resonance at l 234.3, similar to the 31P chemical shift found for complex 2 (l 253.1), implying that the phosphido ligand bridges two molybdenum atoms, which are directly bonded. The correlation between the 31P chemical shift of the m-PPh2 bridge and the presence or absence of a metalmetal bond has been well established [1,13,21,26]. In the 13C NMR spectra, complex 3 displayed three signals for the five carbonyls, while complex 4 displayed four signals for the four carbonyls. Doublet resonances assigned to the central carbon of the bridging S2CPCy3 ligand, at l 102.0 (lJ(PC) = 40 Hz) for 3 and at l 93.6 (lJ(PC) = 54 Hz) for 4, were observed in the 13C NMR spectra. In addition, the allyl ligand of 4 displayed three singlet resonances at l 90.0, 57.6 and 40.1, which, together with the resonances of the allyl ligand observed in the 1 H NMR spectrum of 4, are consistent with the coordination of an h3-allyl ligand.

complexes (6a and 6b, 2010 1846 cm − 1). In the 31 P{1H} NMR spectra of the four complexes, the singlet resonances assigned to the S2CPCy3 ligands fell within a narrow range of l 34.9 37.3. The interesting spectroscopic feature of the novel selenido-bridged binuclear complexes is their 13C NMR spectra, in which they each displayed five signals for the five carbonyl ligands. These spectra showed that, in solution and at room temperature, all five carbonyls in each molecule are inequivalent on the NMR time scale. In contrast, their only analogue, a selenido-bridged manganese – molybdenum complex [(CO)3Mn(m-SePh)(m-S2CPPri3)Mo(CO)3], showed three signals for the six carbonyls in the 13C NMR spectrum [30]. These different 13C NMR features of the carbonyls can be rationalized by assuming that the ligands, nitrosyl and h3-allyl in the selenido-bridged complexes, may disturb trigonal-twist processes of the carbonyls. Another feature of the 13C NMR spectra of 6a and 6b deserving to be mentioned is that two broad signals centered at l 61.6 61.7 and 55.0 55.2 attributed to the CH2 groups of the h3-allyl ligand were observed besides the sharp singlet assigned to the CH group. These spectra are different from those of the related h3-allyl binuclear complexes, which we have prepared previously, that showed only a sharp singlet for the two CH2 groups of the h3-allyl ligand [32,33]. Considering the structure of the analogous binuclear complex [(CO)3Mn(m-SePh)(m-S2CPPri3)Mo(CO)3], in which the phenyl group of the SePh bridge is out of the plane formed by MnMoSe [30], for the same case the phenyl group may induce a steric impediment in complexes 6a and 6b to freeze the fluxional process of the h3-allyl ligand on the NMR time scale. In addition, the expected resonances for the h3-allyl, phenyl and cyclohexyl groups with a rational ratio of integrations were observed in the 1H NMR spectra of 6a and 6b.

3. Discussion

2.2. Synthesis and spectroscopic characterization of [X(CO)2Mo(v-SePh)(v-S2CPCy3)M(CO)3] (X =NO, M =Mo, 5a; M =W, 5b; X =p 3-C3H5, M= Mo, 6a; M = W, 6b) These four selenido-bridged binuclear complexes were prepared in good yields by allowing PhSeI to react with solutions of the appropriate anions [X(CO)2Mo(mS2CPCy3)M(CO)3]− in THF (Scheme 3). Complexes 5 and 6 were stable at room temperature under a nitrogen atmosphere both in solid state and in solution. All spectroscopic data for complexes 5 and 6 are presented in Tables 1 and 2, and the analytical data are given in Section 4. In the IR spectra the frequencies of the w(CO) bands for the nitrosyl complexes (5a and 5b, 20231869 cm − 1) are higher than those for the allyl

The experimental results show that the different ligands, nitrosyl and h3-allyl, have no apparent influence on the structures of the products from the reactions of [X(CO)2Mo(m-S2CPCy3)M(CO)3]− (X=NO, h3-C3H5) with PhSeI. The similarity of the spectroscopic data of 6a and 6b, together with their analytic data, shows that they are isostructural with the previously reported complex [(CO)3Mn(m-SePh)(m-S2CPPri3)Mo(CO)3] [30]. But a significant influence on the structures of the products from the reactions of [X(CO)2Mo(m-S2CPCy3)Mo(CO)3]− with Ph2PCl has been observed. The reaction of the nitrosyl-coordinated anion with Ph2PCl gave only product 3 (D type). Under conditions of heating or UV irradiation, complex 3 gave decomposed compounds, instead of forming a metalmetal bond to give

Table 2 1 H and 13C{1H} NMR data for the novel phosphido- and selenido-bridged binuclear complexes a H NMR, l (ppm)

C {1H} NMR, l (ppm)

1

13

3

7.687.36 [m, 10H, Ph], 2.69 [m, 3H, CH of Cy] 2.061.39 [m, 30H, CH2 of Cy]

4

7.867.12 [m, 10H, Ph], 4.97 [m, 1H, CH of allyl], 3.75 [m, 2H, H syn of allyl], 2.53 [m, 3H, CH of Cy], 2.171.16 [m, 32H, CH2 of Cy and H anti of allyl]

5a

7.777.21 [m, 5H, Ph], 2.68 [m, 3H, CH of Cy], 1.981.39 [m, 30H, CH2 of Cy]

5b

7.747.20 [m, 5H, Ph], 2.62 [m, 3H, CH of Cy], 2.001.39 [m, 30H, CH2 of Cy]

6a

7.877.17 [m, 5H, Ph], 3.54 [s(br), 1H, CH of allyl], 3.20 [s(br), 2H, H syn of allyl], 2.62 [m, 3H, CH of Cy], 2.011.39 [m, 30H, CH2 of Cy], 1.12 [s(br), 2H, H anti of allyl]

6b

7.827.19 [m, 5H, Ph], 3.52 [s(br), 1H, CH of allyl], 3.17 [s(br), 2H, H syn of allyl], 2.56 [m, 3H, CH of Cy], 2.001.39 [m, 30H, CH2 of Cy], 1.11 [s(br), 2H, H anti of allyl]

218.0 [d(10), 3 Mo0CO], 212.6 [s(br), MoIICO], 205.8 [s(br), MoIICO], 139.1 [d(29), ipso-C of Ph], 134.8 [d(10), CH of Ph], 128.5 [d(10), CH of Ph], 127.7 [s, CH of Ph], 102.0 [d(40), S2CP], 33.6 [d(40), CH of Cy], 27.5 [d(34), CH2 of Cy], 27.1 [s, CH2 of Cy], 25.8 [s, CH2 of Cy] 232.9 [d(13), MoICO], 226.4 [d(19), MoICO], 224.3 [d(24), MoICO], 223.2 [s, MoICO], 141.6 [d(32). ipso-C of Ph], 134.2 [d(9), CH of Ph], 128.6 [d(10), CH of Ph, 127.8 [s, CH of Ph], 93.6 [d(54), S2CP], 90.0 [s, CH of allyl], 57.6 [s, CH2 of allyl], 40.1 [s, CH2 of allyl], 34.5 [d(41), CH of Cy], 28.1 [d(31), CH2 of Cy], 27.4 [d(7), CH2 of Cy], 26.0 [s, CH2 of Cy] 238.6 [d(5), Mo0CO], 219.9 [s, MoIICO], 217.7 [s, MoIICO], 217.1 [s, Mo0CO], 216.8 [s, Mo0CO], 136.2 [s, CH of Ph], 132.0 [s, ipso-C of Ph], 128.7 [s, CH of Ph], 126.4 [s, CH of Ph], 102.7 [d(38), S2CP], 33.6 [d(39), CH of Cy], 27.7 [s, CH2 of Cy], 27.1 [d(12), CH2 of Cy], 25.8 [s, CH2 of Cy] 230.0 [d(5), W0CO], 216.8 [s, MoIICO], 216.7 [s, MoIICO], 215.8 [s, W0CO], 213.1 [s, W0CO], 135.8 [s, CH of Ph], 131.7 [s, ipso-C of Ph], 128.8 [s, CH of Ph], 126.8 [s, CH of Ph], 95.1 [d(38), S2CP], 33.7 [d(40), CH of Cy], 27.8 [s, CH2 of Cy], 27.1 [d(12), CH2 of Cy], 25.8 [s, CH2 of Cy] 238.2 [d(4), Mo0CO], 226.7 [s, MoIICO], 225.2 [s, MoIICO], 220.4 [s, Mo0CO], 218.5 [s, Mo0CO], 136.4 [s, CH of Ph], 133.7 [s, ipso-C of Ph], 128.3 [s, CH of Ph], 125.5 [s, CH of Ph], 88.2 [d(44). S2CP], 75.2 [s, CH of allyl], 61.6 [s(br), CH2 of allyl], 55.2 [s(br), CH2 of allyl], 33.8 [d(40), CH of Cy], 27.8 [s, CH2 of Cy], 27.2 [d(12), CH2 of Cy], 25.7 [s, CH2 of Cy] 229.9 [d(3), W0CO], 226.4 [s, MoIICO], 225.7 [s, MoIICO], 216.1 [s, W0CO], 213.9 [s, W0CO], 136.0 [s, CH of Ph], 133.4 [s, ipso-C of Ph], 128.4 [s, CH of Ph], 126.1 [s, CH of Ph], 81.3 [d(46). S2CP], 75.1 [s, CH of allyl], 61.7 [s(br), CH2 of allyl], 55.0 [s(br), CH2 of allyl], 33.8 [d(40), CH of Cy], 27.8 [s, CH2 of Cy], 27.1 [d(12), CH2 of Cy], 25.7 [s, CH2 of Cy]

a

D. Miguel et al. / Inorganica Chimica Acta 307 (2000) 20–25

Compound

Measured in CD2Cl2. Coupling constants in parentheses are in Hz.

23

24

D. Miguel et al. / Inorganica Chimica Acta 307 (2000) 20–25

an isostructrural species like 4 (Scheme 4). In contrast, the reaction of the h3-allyl-coordinated anion with Ph2PCl afforded an exclusive product 4 (B type) instantaneously. It has been observed in the similar reaction of [(CO)3Mn(m-S2CPPri3)Mo(CO)3]− with ClPPh2 that the formation of the metalmetal bond was accompanied by the rearrangement of the central carbon of the S2CPPri3 ligand (Scheme 2). By reference to this fact, it is logical to propose that in complex 3 the central carbon of the S2CPCy3 ligand keeps binding to the Mo(0) atom of the ‘Mo(CO)3’ fragment, just as in the case of the starting complex [(ON)(CO)2Mo(m-Br)(m-S2CPCy3)M(CO)3], and that in complex 4 the central carbon of the S2CPCy3 ligand has migrated to the Mo center containing an h3-allyl ligand so as to gear both molybcenums to six coordination number configurations. Although no Dtype species has been observed by IR monitoring in the reaction of [h3-C3H5)(CO)2Mo(m-S2CPCy3)Mo(CO)3]− with Ph2PCl, the B-type complex 4 might be formed via a D-type transient intermediate. The change of the ligands from allyl to nitrosyl in the same unit should not cause any significant steric effect on the formation of the metalmetal bond or the rearrangement of the central carbon of the S2CPCy3 ligand. The migration appears to be driven or thwarted by purely electronic factors. Although both of the nitrosyl and h3-allyl ligands act as 3e donors, nitrosyl is a much stronger electron-withdrawing ligand than h3-allyl. Thus, the Mo center of the ‘Mo(CO)2(NO)’ fragment is more electron-deficient in comparison with the Mo center of the ‘Mo(CO)2(h3-C3H5)’ fragment. On the other hand, all the previously reported bimetallic complexes containing S2CPR3 ligands have shown that the central carbon atom of the S2CPR3 ligand is electrophilic to transition-metal centers. Therefore, one can expect that the rearrangement of the central carbon of the S2CPCy3 ligand to the Mo center, to which NO is coordinated, is unfavorable for complex 3, and the formation of the metalmetal bond in these reactions seems closely related to the regioselectivity of the metalcarbon bond formation. The distinct influences of the nitrosyl and h3-allyl ligands in these reactions suggest that the ligands can be used as a fine tuning button to predominate the regioselectivity of the metalcarbon bond formation and to control the formation of the metalmetal bond in the preparation of these types of phosphido-bridged bimetallic complex. However, more studies are needed to further prove this argument.

4. Experimental

4.1. General procedures All reactions and manipulations were carried out under a nitrogen atmosphere by using Schlenk tech-

niques. Solvents were freshly distilled under nitrogen according to standard methods before use. All the column chromatography was performed under nitrogen by use of alumina (activity I, 2× 15 cm column). Reagents Ph2PCl and PhSeI were purchased and used without further purification. [(h3-C3H5)(CO)2Mo(mBr)(m-S2CPCy3)M(CO)3] and [(ON)(CO)2Mo(m-Br)(mS2CPCy3)M(CO)3] (M = Mo, W) were prepared according to the published procedures [31,32], and their anions [(h3-C3H5)(CO)2Mo(m-S2CPCy3)M(CO)3]− and [(ON)(CO)2Mo(m-S2CPCy3)M(CO)3]− were prepared prior to use as our previous descriptions [34]. Infrared spectra were recorded on a Perkin –Elmer Paragon 1000 FT-IR spectrophotometer. NMR spectra were recorded on a Bruker AC-300 instrument. The 1H and 13C spectra were referenced to internal TMS and the 31 P spectra to external 85% H3PO4. Elemental analyses were performed on a Perkin –Elmer 240B microanalyser.

4.2. Preparation of [(ON)(CO)2Mo(v-PPh2)(v-S2CPCy3)Mo(CO)3] (3) Ph2PCl (45 ml, 0.25 mmol) was added to a THF solution (15 ml) of the anion [(ON)(CO)2Mo(mS2CPCy3)Mo(CO)3]− (ca. 0.25 mmol) at − 60°C. The solution changed instantly to dark green. After the mixture was stirred at − 60°C for 20 min, the solvent was removed in vacuo at − 10°C until dryness. The dark green residue was extracted with a minimum amount of pre-cooled CH2Cl2 and chromatographed through an alumina column at − 10°C. The collected solution was concentrated at 0°C until about 5 ml and then hexane (10 ml) was added. Slow diffusion of the solvents at −20°C gave 3 as dark green crystals in 49% yield (110 mg). Anal. Calc. for C36H43NO6S2P2Mo2·CH2Cl2: C, 44.99; H, 4.56; N, 1.42. Found: C, 45.06; H, 4.54; N, 1.46%.

4.3. Preparation of [(p 3-C3H5)(CO)Mo(v-PPh2)(v-S2CPCy3)Mo(CO)3] (4) Ph2PCl (45 ml, 0.25 mmol) was added to a THF solution (15 ml) of the anion [(h3-C3H5)(CO)2Mo(mS2CPCy3)Mo(CO)3]− (ca. 0.25 mmol) at −60°C, and IR monitoring showed that the reaction was completed instantly. After removal of the solvent at − 20°C, the red-brown residue was extracted with a minimum amount of pre-cooled CH2Cl2 and chromatographed through a short alumina column (2× 5 cm) at −20°C. The filtrate was concentrated at − 20°C to about 5 ml and then hexane (10 ml) was added. A red-brown crystalline solid was obtained in 36% yield (82 mg) by slow concentration of the solution at −20°C. Anal. Calc. for C38H48O4S2P2Mo2: C, 51.47; H, 5.42. Found: C, 50.52; H, 4.97%.

D. Miguel et al. / Inorganica Chimica Acta 307 (2000) 20–25

4.4. Preparation of [X(CO)2Mo(v-SePh)(v-S2CPCy3) M(CO)3] (X=NO, M =Mo, 5a; M =W, 5b; X = p 3-C3H5, M= Mo, 6a; M =W, 6b) PhSeI was added to a THF solution (10 ml) of the anion [(ON)(CO)2Mo(m-S2CPCy3)Mo(CO)3]− (ca. 0.20 mmol) at −40°C. After the mixture was stirred at low temperature for 10 min, it was allowed to reach room temperature. When the IR monitoring showed complete reaction (about 20 min), the solvent was evaporated in vacuo and the residue was chromatographed on an alumina column. The filtrate was concentrated until about 10 ml and hexane (15 ml) was added. Slow diffusion of the solvents at room temperature gave 5a as red-brown crystals in 76% yield (146 mg). Anal. Calc. for C30H38NO6S2PSeMo2·CH2Cl2: C, 38.79; H, 4.17; N, 1.46. Found: C, 38.45; H, 4.18; N, 1.48%. Essentially the same procedure was followed for the preparation of 5b, 6a and 6b, by using the THF solutions of the corresponding anions (ca. 0.20 mmol), respectively. Complex 5b was isolated in 72% yield (150 mg). Anal. Calc. for C30H38NO6S2PSeMoW·CH2Cl2: C, 35.56; H, 3.85; N, 1.34. Found: C, 35.58; H, 3.82; N, 1.36%. Complex 6a: 78% yield (138 mg). Anal. Calc. for C33H43O5S2PSeMo2: C, 44.79; H, 4.90. Found: C, 44.61; H, 4.88%. Complex 6b: 75% yield (146 mg). Anal. Calc. for C33H43O5S2PSeMoW: C, 40.74; H, 4.45. Found: C, 40.39; H, 4.50%.

Acknowledgements We are grateful to the Spanish Ministerio de Educacion for the award of a grant to M.W. The authors wish to thank Dr Julio A. Pe´rez-Martı´nez for helpful discussion.

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