Monocyclopentadienyl complexes of niobium, tantalum and tungsten containing heterodifunctional P,O ligands

Abstract

The reactions of P,O type ligands with the half-sandwich complexes [(η-C₅R₅)MCl₄] (R₅ = H₅, Me₅, ⁱPrH₄; M = Nb, Ta, W) have been investigated. Monodentate P-adducts were obtained with the β-amidophosphine Ph₂PCH₂C(O)NPh₂, whereas in the case of the keto ligand Ph₂PCH₂C(O)Ph a spontaneous HCl elimination occurred to give direct access to the corresponding phosphinoenolate complexes. The crystal structures of [(η-C₅H₅)Nb[upper bond 1 start]Cl₃{PPh₂CH[horiz bar, triple dot above]C([horiz bar, triple dot above]O[upper bond 1 end])Ph}], [(η-C₅H₅)Ta[upper bond 1 start]Cl₃{PPh₂CH[horiz bar, triple dot above]C([horiz bar, triple dot above]O[upper bond 1 end])Ph}] and [(η-C₅Me₅)Ta[upper bond 1 start]Cl₃{PPh₂CH[horiz bar, triple dot above]C([horiz bar, triple dot above]O[upper bond 1 end])Ph}] have been determined. Interestingly, the acetamido derived phosphine Ph₂PNHC(O)Me afforded O-adducts, which is an unusual bonding mode for a P,O ligand.

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Introduction

Bifunctional P,O ligands, which combine a soft phosphine moiety and a hard oxygen function, such as ester, amide or ketone, remain the subject of numerous studies, owing to the various properties and/or applications of their transition-metal complexes.^1–4 In metal complexes with these P,O molecules both P-monocoordination and P,O chelation are commonly found. The P,O chelate ligands give stable metal complexes and due to the different properties of the P and O ligand atoms they can strongly influence the stereoelectronic properties at the metal centre.³ Furthermore, in late transition metal complexes, the greater lability of the O-ligand atom favours fluxional processes resulting from reversible O-atom dissociation and recoordination.^1–4 The O-atom dissociation in these hemilabile ligands creates a vacant coordination site, and conversely weak O-association can give rise to stabilisation of an otherwise coordinatively unsaturated species. For this reason complexes with P,O ligands have been studied for their catalytic properties,^5–13 especially in reactions involving the activation of small molecules.^14–19

In contrast to the wealth of late transition metal complexes synthesized, which contain a β-keto-, ester- or amidophosphine ligand, there have been only a few studies on the early transition-metal chemistry of these ligands and they have mainly focused on molybdenum. Thus, the half-sandwich complexes of the β-amidophosphine Ph₂PCH₂C(O)NPh₂1, [(η-C₅H₅)MoCl₂{1-κ²P,O}] or [(η-C₅Me₅)MoCl₂{1-κ²P,O}][BF₄],²⁰ and of the keto-functionalised N-pyrrolylphosphine Ph₂PNHC₄H₃{C(O)Me}-2 (PpyrO), [(η-C₅H₅)MoCl(CO)₂{(PpyrO)-κP}] and [(η-C₅H₅)Mo(CO)₂{(PpyrO)-κ²P,O}][BF₄],²¹ have been reported. More recently, we have described the synthesis of η-arene-molybdenum complexes, such as [(η-C₆H₅R)Mo(η-C₃H₅){Ph₂PXHC(O)R′-κ²P,O}][PF₆] (R = H, Me; R′ = Ph, NPh₂, Me; X = N, C).^22,23 In these complexes, the P,O ligands act either as P-monodentate phosphine or as a P,O-chelate, whereas O-monocoordination, which may have been anticipated owing to the relatively hard nature of molybdenum vs. late transition metals, was not observed. Although ligands with carbonyl functions (ketones,^24–26 amides,^27,28 acids/esters^29,30) are known to coordinate readily to groups 5 and 6 metals, there has been no example reported, to the best of our knowledge, when a phosphine donor is also present in the ligand. This also applies to somewhat related ligands, such as R₂PNHP(O)R₂,³¹ or even when the P atom is not available for coordination as in R₃P=NC(O)R′.^32,33 We will present here a rare example of a P,O phosphine acting as a O-monodentate ligand in half-sandwich Nb, Ta and W complexes.

Anionic phosphino-enolates, formed from ketophosphines, are also a well-developed group of P,O ligands.^34–49 Interest in this chemistry partly stems from the fact that nickel phosphino-enolate complexes catalyze the oligomerisation of ethylene in α-olefins (Shell Higher Olefin Process).^34,40,50,51 Substitutional modification of the phosphino-enolate ligands can modify catalyst activity and the chain length distribution.^49–52 Numerous phosphino-enolate complexes of group 8–10 metals have been synthesized and their reactivity studied.^{35–39,41–46,49,53–58} For example, Rh(I) complexes in this class are efficient catalysts for the transfer dehydrogenation of alkanes to alkenes.⁵⁹

As for their neutral counterparts, the early transition metal chemitry of anionic P,O ligands has been little investigated. The η-arene-molybdenum complexes [(η-C₆H₅R)Mo(η-C₃H₅){Ph₂PX[horiz bar, triple dot above]C([horiz bar, triple dot above]O)Ph-κ²P,O}] or [(η-C₆H₅R)Mo{Ph₂PX[horiz bar, triple dot above]C([horiz bar, triple dot above]O)Ph-κ²P,O}₂] (R = H, Me; R′ = Ph, NPh₂, Me; X = N, C) represent, to the best of our knowledge, rare examples of early transition metal complexes, containing a chelating phosphinoenolate ligand.^22,23 Very recently, Edwards et al. reported the compound [(η-C₅H₅)₂Ti{OC(O)CH₂PPh₂}₂], prepared by treatment of [(η-C₅H₅)₂TiCl₂] with the anion Ph₂PCH₂C(O)O⁻.⁶⁰ The titanocene and zirconocene complexes of a camphor-derived phosphino-enolate have also been described.⁶¹ Further zirconocene phosphino-enolates derived from the α-ketophosphine Ph₂PC(O)R (R = Me, Et) have been prepared by Floriani et al.^62,63

Here, we report the first investigations into the coordination chemistry of P,O ligands, namely Ph₂PCH₂C(O)NPh₂1, Ph₂PCH₂C(O)Ph 2 and Ph₂PNHC(O)Me 3, with the half-metallocene complexes [(η-C₅R₅)MCl₄] 4 (a: R₅ = H₅, M = Nb; b: R₅ = H₅, M = Ta; c: R₅ = Me₅, M = Ta; d: R₅ = ⁱPrH₄, M = W). The latter were chosen because they represent versatile precursors for new materials, the design of highly efficient catalysts for organic transformations and polymerization reactions^64–68 or for W≡W triply bonded complexes (4d),⁶⁹ which have a remarkably rich chemistry.⁷⁰ We will show that under similar reaction conditions, the P,O ligands 1–3 lead to complexes in which they adopt either a P-monodentate, an anionic P,O-chelating or, quite unexpectedly, a neutral O-monodentate coordination mode.

Results and discussion

Half-sandwich complexes [(η-C₅R₅)MCl₄] 4a–d have Lewis acid character and form mono-adducts [(η-C₅R₅)MCl₄(L)] with neutral ligands such as L = amines, phosphines or phosphites.⁷¹ Early studies led to the conclusion that adducts of 4a, [(η-C₅H₅)NbCl₄(L)] where L = a tertiary phosphine or phosphite, were labile in contrast to their tantalum analogues.^72–74 The niobium adducts [(η-C₅H₄Me)NbCl₄(L)] (L = PMe₃, PEt₃ or P(OMe)₃) slowly decompose in solution.⁷⁵ Recently, Poli et al. showed that adducts of 4a with PMe_nPh_3−n (n = 3, 2 or 1) could be isolated provided that no excess of the phosphine, which may cause reduction side-products, was present.⁷⁶ The adducts of 4c, [(η-C₅H₄ⁱPr)WCl₄(L)] where L = vinylPH₂, allylPH₂ or PMe₃, are stable.^77,78

Formation of P-monoadducts

Treatment of [(η-C₅H₅)NbCl₄] 4a with one mol. equiv. of 1 gave the P-monoadduct [(η-C₅H₅)NbCl₄{PPh₂CH₂C(O)NPh₂}] 5a as a brown solid. The proposed structure is supported by the presence, in the ³¹P{¹H} NMR spectrum (CD₂Cl₂), of a very broad peak at ca.δ 33.1 (w_1/2 > 2500 Hz), owing to the quadrupolar broadening due to Nb (spin 9/2), and by the occurrence of a ν_C=O vibration at 1658 cm⁻¹ in the IR spectrum, which indicates non-coordination of the amide moiety (free ligand: 1657 cm⁻¹).⁴² In the ¹H NMR spectrum (CD₂Cl₂), the η-C₅H₅ and PCH₂ protons are observed at δ 6.89 (singlet) and 3.88 (doublet, ²J(P,H) = 7 Hz), respectively. The absence of coupling between the ³¹P nucleus and the Cp protons was also noted for [(η-C₅H₅)NbCl₄(PMe_nPh_3−n)] (n = 3, 2 or 1).⁷⁶ Complexes [(η-C₅H₅)TaCl₄{PPh₂CH₂C(O)NPh₂}] 5b and [(η-C₅H₄ⁱPr)WCl₄{PPh₂CH₂C(O)NPh₂}] 5c were prepared from 4b,c and 1. The spectroscopic data of the tantalum complex 5b mirror those of 5a. The paramagnetic tungsten complex 5c was characterized by IR spectroscopy (ν_C=O 1662 cm⁻¹), mass spectroscopy (m/z: 793 [M − Cl]) and elemental analysis. Complexes 5a–c were stable in solution for several hours and in the solid state for a few days, in the absence of oxygen. Contrary to the situation reported for the Mo complexes mentioned in the introduction,^20,22,23 there was no observed tendency for chelation for 1 in complexes 5a–c.

Formation of P,O-enolate complexes

Surprisingly, attempts to form the P-monodentate analogues of 5a–c from [(η-C₅H₅)NbCl₄] 4a and the β-ketophosphine Ph₂PCH₂C(O)Ph 2 failed. Instead, the reaction followed a different pathway, which resulted in the formation of a deep red solution from which HCl fumes emanated (the acidic nature of the gas was confirmed with pH-paper). After work up, brick-red [(η-C₅H₅)Nb[upper bond 1 start]Cl₃{PPh₂CH[horiz bar, triple dot above]C([horiz bar, triple dot above]O[upper bond 1 end])Ph}] 6a was isolated (Scheme 1). The ¹H NMR spectrum of 6a contains a sharp singlet at δ 6.95 (Cp) and a doublet at δ 6.25 [J(P,H) = 3.5 Hz], which is ascribed to the enolate proton. There were also signals arising from the aromatic hydrogens. The ³¹P{¹H} NMR spectrum (CD₂Cl₂) shows a very broad hump at ca.δ 36.5, as a result of the P–Nb coupling (w_1/2 > 2500 Hz). In the IR spectrum, a ν_{C[horiz bar, triple dot above]C} + ν_{C[horiz bar, triple dot above]O} vibration is observed at 1547 cm⁻¹ (vs 1670 cm⁻¹ for the ν_C=O vibration of uncoordinated 2).³⁵ This value lies within the range of those already reported for the same phosphino-enolate moiety in other metal complexes.^{35,37–39,41,43–46,49} The proposed structure of 6a was confirmed by X-ray diffraction (vide infra). Note that 6a was also obtained as the sole product when an excess of the ketophosphine ligand was used (2 to 4 mol. equiv.), no side-reaction being observed.

Scheme 1 Reaction conditions: Room temperature; solvent: M = Nb: DME; M = Ta: CH₂Cl₂; M = W: THF (1), toluene (2) or CH₂Cl₂/toluene 1∶1 (3).

Spontaneous elimination of HCl was also observed when 2 was reacted with 4b–d, thus leading to the formation of the phosphino-enolate derivatives 6b–d, respectively (Scheme 1). The spectroscopic data of the tantalum complex 6b mirror those of 6a. In the case of the Cp* complex 6d, the ³¹P{¹H} resonance occurs at slightly higher field (δ 31.3), whereas, in the ¹H NMR spectrum, the enolate and η-C₅Me₅ protons are observed at δ 6.08 [J(P,H) = 3.6 Hz] and 2.38 (singlet), respectively. The paramagnetic complex 6c was characterized by IR spectroscopy (ν_{C[horiz bar, triple dot above]C} + ν_{C[horiz bar, triple dot above]O} 1542 cm⁻¹), mass spectroscopy (m/z: 701 [M]) and elemental analysis. It was isolated as a burgundy solid, whereas phosphine adducts of the type [(η-C₅H₄ⁱPr)WCl₄(PR₃)] range from yellow ochre to brown.^77,78 The X-ray structures of the tantalum complexes 6b,d have been determined (vide infra).

To the best of our knowledge, such a spontaneous and facile HCl elimination in the absence of base has not been described for transition metal complexes containing 2 or any similar ligands. Yet, thermal activation or treatment with I₂ of Ru clusters of the type [Ru₃(CO)_12−nL_n] (L = 2, n = 1–3) have led to phosphino-enolate Ru complexes,^43,44 whereas a Rh(III) phosphino-enolate complex has been obtained by deprotonation of a co-ordinated ketophosphine 2 by an excess of this ligand present in the reaction mixture, the latter being a slow process (ca. 1 week).⁴⁸ Although the mechanism of the formation of 6 has not been established, we suggest, on the basis of the formation of complexes 5, that P-coordination of 2 at the metal centre occurs first. This would result in an enhanced acidity of the PCH₂ protons and thus favour HCl elimination. This is consistent with our recent observation of an H/D exchange involving the methylene protons of 2 in the Mo(II) complexes [(η-C₆H₅R)Mo[upper bond 1 start](η-C₃H₅){Ph₂PCH₂C(O[upper bond 1 end])Ph}][PF₆] (R = H, Me).²³ The dehydrochlorination may also be facilitated by intra- and/or inter molecular H⋯Cl interactions involving the methylene protons, a bonding situation which has been encountered in the crystal structure of the Mo(II) complex [(η-C₆H₅Me)Mo(η-C₃H₅)Cl{Ph₂PCH₂C(O)Ph}].²³

The complexes 6 exhibit remarkable stability both in solution and in the solid state. In the latter case they can be kept for weeks in the air. Note that when 6 was treated with an excess of PMe₃, no opening or complete displacement of the P,O chelate was observed. The straightforward formation of these phosphino-enolate complexes, under the same reaction conditions as those used for the preparation of 5a–c, emphasizes a marked difference of behaviour between the β -ketophosphine 2 and its β-amido counterpart 1. This is related to the more acidic character of the CH₂ protons in 2, owing to the different electronic influence of its Ph substituent vs. the NPh₂ group in 1.

Formation of O-monoadducts

The reaction of 4a–c with the acetamido-phosphine Ph₂PNHC(O)Me 3 led to the new complexes 7a–c in which the P,O ligand is O-monocoordinated and this is an unsual coordination mode for such a P,O ligand (Scheme 1). This proposed stucture is supported by the ν_C=O vibration at 1646 cm⁻¹ (7a,b) or 1651 cm⁻¹ (7c), to be compared to 1715 cm⁻¹ for the free ligand.⁷⁹ The fact that the P atom does not coordinate to the metal centre is suggested by the ³¹P{¹H} NMR resonances of 7a (δ 33.1) and 7b (δ 40.6), which occur in the same region as that of 3 (δ 31.1). These values suggest that the electronic properties of the phosphorus centre are slightly more affected by O-coordination of the ligand in the case of M = Ta (7b) than in the case of M = Nb (7a). Furthermore, the slightly broadened ³¹P singlet (w_1/2 = 65 Hz) observed for 7a contrasts with the very broad signals (w_1/2 > 2500 Hz) observed for 5a and 6a, in which the P atoms are coordinated to Nb. The ¹H NMR spectra of 7a,b show all the expected signals, including those of the NH protons at ca.δ 9 (free ligand: δ 6.15).⁷⁹ Complexes 7 are stable for a few days in the solid state, in the absence of air and moisture. They represent rare examples of transition metal derivatives in which a neutral functionalized phosphine is coordinated to the metal via the alternative donor atom instead of the P atom. Precedents include N- and O-adducts of MnI₂ of 2-(diphenylphosphino)pyridine and 2, respectively.⁸⁰ In the case of phosphino-enolate zirconocene and titanocene complexes, O-monocoordination of the anionic P,O ligands results from the formation of a covalent M–O bond.^61–63 Previous investigations on the coordination properties of 3 concerned the synthesis of neutral and cationic Pd,⁷⁹ Mo²² and Rh complexes,⁸¹ for which O-monocoordination was never encountered. Comparative studies showed that 3 is a better P,O-chelate than the keto- and amidophosphines 1 and 2, respectively.⁷⁹

Crystal structures of 6a,b,d

Crystal data and data collection parameters are given in Table 1. There are two crystallographically different but very similar molecules in the asymmetric unit of 6b whose selected bond lengths and angles are given in Table 2 together with those of complexes 6a,d. ORTEP views of 6a, 6b and 6d are pictured in Fig. 1, 2 and 3, respectively. If one considers the Cp centroid of the cyclopentadienyl ligands as an apex of the metal coordination polyhedron, then the three complexes exhibit a distorted pseudooctahedral geometry, with the three chlorine atoms and the oxygen atom lying in the equatorial plane and the P atom trans to the cyclopentadienyl ring.⁸² The Cp_cent–M–P(1) angles range from 175.76° (6a) to 178.48° (6d) and are slightly more obtuse than those found in the ylide complex [(η-C₅Me₅)TaCl₄(CH₂=PMePh₂)] (173°)⁸³ or in [(η-C₅H₅)NbCl₄(PMePh₂)] (174.6°),⁷⁶ the only phosphine adduct of [(η-C₅H₅)NbCl₄] that has been structurally characterized to date. The chlorine and oxygen atoms are bent away from the cyclopentadienyl ring towards the P ligand. The molecular structures show M–Cl [2.3873(9)–2.446(1) Å] bond lengths, Cp_cent–M–Cl [103.35–105.28°] and Cl–M–Cl [87.08(4)–90.31(3)° cis; 150.95(3)–152.37(8)° trans] bond angles which are consistent with those reported for related structures.^76,83–85 The M–O(1) bond lengths are similar to those found in other transition metal complexes with the same phosphino-enolate ligand^{35–39,41,43–46,49} or in [(η-C₅Me₅)Ta[upper bond 1 start]Cl₃{OC(SiMe₃)NN[upper bond 1 end]CPh₂}] [1.98(1) Å].⁸⁵ In the case of [(η-C₅Me₅)Ta[upper bond 1 start]Cl₃{PPh₂CH[horiz bar, triple dot above]C([horiz bar, triple dot above]O[upper bond 1 end])Ph}] 6d [2.024(2) Å] it is slightly longer than in the other complexes 6a,b [1.977(2)–1.983(3) Å]. The M–P(1) bond lengths of ca. 2.66 Å are shorter than those found in [(η-C₅H₄Me)TaCl₄{PH₂(C₆H₂ⁱPr₃)}] [2.710 Å]⁸⁶ or in [(η-C₅H₅)NbCl₄(PMePh₂)] [2.7844(9) Å]⁷⁶ and similar to those reported for [Nb(O)Cl₃(PMe₃)₃] [2.640(3) Å].⁸⁷ The other bond lengths and angles of the phosphino-enolate and cyclopentadienyl ligands are within the expected range.

Table 1 Selected crystallographic data for complexes 6a,b,d

	6a	6b	6d
Formula	C25H21Cl3OPNb·0.5C7H8	C25H21Cl3OPTa	C30H31Cl3OPTa
FW	613.75	655.72	725.86
Colour	Red	Yellow	Yellow
Crystal system	Monoclinic	Triclinic	Trigonal
Space group	P21/c	P1̄	R3̄
T/K	150(2)	150(2)	150(2)
a/Å	7.876(1)	8.4611(1)	42.5519(5)
b/Å	26.392(1)	16.6441(3)	42.5519(5)
c/Å	12.650(1)	16.8443(3)	8.7407(5)
α/°	90	92.6314(8)	90
β/°	97.01(1)	99.3326(9)	90
γ/°	90	94.1166(11)	120
V/Å^3	2609.8	2330.7	13 706.1
Z	4	4	18
μ/mm^−1	0.851	5.145	3.945
Unique reflections	5001	10 582	6943
Reflections	4100 with I > 3σ(I)	8244 with I > 2σ(I)	5794 with I > 3σ(I)
R	0.0515	0.0314	0.0281
R w	0.0543	0.0366	0.0298

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Table 2 Selected bond lengths (Å) and angles (°) for 6a, 6b (2 molecules) and 6d; Cp_cent refers to the computed cyclopentadienyl ring centroids

	6a	6b (molecule 1)	6b (molecule 2)	6d
M–P1	2.667(1)	2.670(1)	2.674(1)	2.6596(9)
Cpcent–M	2.132	2.127	2.142	2.168
M–Cl1	2.4186(9)	2.415(1)	2.410(1)	2.4209(8)
M–Cl2	2.4057(9)	2.4131(11)	2.402(1)	2.3873(9)
M–Cl3	2.446(1)	2.422(1)	2.413(1)	2.3934(8)
M–O1	1.977(2)	1.983(3)	1.982(3)	2.023(2)
P1–C1	1.779(4)	1.779(5)	1.792(4)	1.773(3)
C1–C2	1.345(5)	1.352(6)	1.342(6)	1.349(5)
C2–O1	1.346(4)	1.349(5)	1.352(3)	1.342(4)
Cpcent–M–P1	175.76	175.80	177.40	178.48
Cpcent–M–Cl1	104.02	103.59	105.28	104.39
Cpcent–M–Cl2	104.24	104.05	104.60	103.99
Cpcent–M–Cl3	103.44	104.02	103.35	103.81
Cpcent–M–O1	101.83	103.17	101.78	103.82
Cl1–M–Cl2	88.92(3)	88.25(4)	87.41(4)	89.49(3)
Cl1–M–Cl3	152.37(3)	151.26(3)	152.35(4)	150.95(3)
Cl1–M–O1	87.60(8)	85.21(9)	85.50(9)	82.04(7)
Cl2–M–Cl3	87.26(3)	87.08(4)	87.16(4)	90.31(3)
Cl2–M–O1	153.80(8)	153.56(8)	152.76(9)	152.12(7)
Cl3–M–O1	83.921(8)	86.47(9)	87.04(9)	84.70(7)
P1–M–O1	73.97(7)	74.10(8)	74.23(8)	74.83(7)
P1–C1–C2	115.4(3)	115.3(3)	114.7(3)	116.0(3)
C1–C2–O1	121.3(3)	121.5(4)	122.3(4)	122.6(3)
C2–O1–M	131.8(2)	131.6(2)	131.2(3)	129.2(2)

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Fig. 1 Molecular structure of 6a. Hydrogen atoms, except H(11), and the solvent molecule (toluene) are omitted for clarity.

Fig. 2 Molecular structure of 6b. Only one of the two independent molecules of the unit cell is shown. Hydrogen atoms, except H(11), are omitted for clarity.

Fig. 3 Molecular structure of 6d. Hydrogen atoms, except H(11), are omitted for clarity.

Conclusion

We have described here the first investigations on the chemistry of half-sandwich complexes of the type [(η-C₅R₅)MCl₄] (M = Nb, Ta or W) with β-carbonylphosphines. Depending on the nature of the P,O ligand three different coordination modes have been observed. With Ph₂PCH₂C(O)NPh₂1 P-monoadducts were obtained, whereas Ph₂PNHC(O)Me 3 led to O-monoadducts, which is an unsual coordination mode for P,O ligands. The reactions with the β-ketophosphine Ph₂PCH₂C(O)Ph 2 followed another pathway. In this case, no phosphine adduct was isolated, instead HCl elimination readily occurred giving the corresponding phosphino-enolate complexes. To the best of our knowledge, such a facile formation of transition metal phosphino-enolate complexes, in the absence of a base, has not been reported previously. The crystal structures of the Nb and Ta complexes 6a,b,d have been determined. The striking difference between 1 and 3 in bonding cannot be of steric origin since P-coordination of 3 would have led to a very similar steric situation at the metal centre. Therefore, the bonding difference must be of electronic origin, which has not been quantified at the moment and deserves further investigations.

Experimental

Reagents and physical measurements

All manipulations of air- and/or moisture sensitive materials were performed under an inert atmosphere of argon using standard Schlenk line techniques, or in an inert atmosphere dry box containing dinitrogen. Solvents were dried over the appropriate drying agent and distilled under nitrogen. Deuterated solvents were dried over the appropriate drying agent and vacuum distilled prior to use. The compounds [(η-C₅H₅)NbCl₄],⁷⁵ [(η-C₅H₅)TaCl₄],⁷⁵ [(η-C₅Me₅)TaCl₄],⁸⁸ [(η-C₅H₄ⁱPr)WCl₄],⁸⁹ Ph₂PCH₂C(O)Ph,³⁵ Ph₂PCH₂C(O)NPh₂⁴² and Ph₂PNHC(O)Me⁷⁹ were prepared according to previously published methods. NMR spectra were recorded on a Varian Mercury 300 (¹H, ¹³C, and ³¹P at 300.17, 75.48 and 121.51 MHz, respectively) spectrometer. They were referenced internally using the residual protio solvent (¹H) and solvent (¹³C) resonances and measured relative to tetramethylsilane (¹H and ¹³C; δ 0 ppm). ³¹P NMR was referenced externally to 85% H₃PO₄ (δ 0 ppm). Elemental analyses were provided by the microanalytical department, Inorganic Chemistry Laboratory, University of Oxford or by the Service de Microanalyses, Université Louis Pasteur, Strasbourg. Infra-red spectra were recorded on a Perkin Elmer 1600 Series FTIR or a Bruker IFS66 FTIR spectrometer. Mass spectra were recorded at the Inorganic Chemistry Laboratory, on a Macromass LC Tof spectrometer.

Synthesis

[(η-C₅H₅)NbCl₄{PPh₂CH₂C(O)NPh₂}] 5a. Solid [(η-C₅H₅)NbCl₄] (0.200 g, 0.66 mmol) and Ph₂PCH₂C(O)NPh₂ (0.261 g, 0.66 mmol) were placed in a Schlenk flask and DME (10 mL) was added. The solution was stirred for 3 h. The volatiles were then removed under reduced pressure and the residue washed with Et₂O (10 mL) and pentane (10 mL). Complex 5a was obtained as a pale brown air-sensitive solid (0.417 g, yield 90%). ¹H NMR (CD₂Cl₂, 300.18 MHz): δ 3.89 (d, 2H, ²J_PH = 7 Hz, PCH₂); 6.90–7.79 (m, 25H, Cp and aromatics). ³¹P{¹H} NMR (CD₂Cl₂, 121.5 MHz): δ 33.1 (vbr, w_1/2 > 2500 Hz). IR (CH₂Cl₂): 1658 cm⁻¹ (ν_C=O). Anal. calc. for C₃₁H₂₇Cl₄NNbOP: C, 53.55; H, 3.91; N, 2.01. Found: C, 53.81; H, 4.06; N, 1.95%.

[(η-C₅H₅)TaCl₄{PPh₂CH₂C(O)NPh₂}] 5b. This compound was prepared in a similar manner to 5a from [(η-C₅H₅)TaCl₄] (0.150 g, 0.38 mmol) and Ph₂PCH₂C(O)NPh₂ (0.152 g, 0.38 mmol) in CH₂Cl₂ (10 mL). Complex 5b was obtained as a yellow solid (0.257 g, 85%). ¹H NMR (CD₂Cl₂, 300.18 MHz): δ 3.69 (d, 2H, ²J_PH = 5.5 Hz, PCH₂); 6.77 (s, 5H, Cp); 6.95–7.97 (m, 20H, aromatics). ³¹P{¹H} NMR (CD₂Cl₂, 121.5 MHz): δ 27.9. IR (CH₂Cl₂): 1658 cm⁻¹ (ν_C=O). Anal. calc. for C₃₁H₂₇Cl₄NOPTa: C, 47.54; H, 3.47; N, 1.79. Found: C, 47.69; H, 3.38; N, 1.92%.

[(η-C₅H₄ⁱPr)WCl₄{PPh₂CH₂C(O)NPh₂}] 5c. This compound was prepared in a similar manner to 5a from [(η-C₅H₄ⁱPr)WCl₄] (0.600 g, 1.386 mmol) and Ph₂PCH₂C(O)NPh₂ (0.548 g, 1.386 mmol) in THF (30 mL). Complex 5c was obtained as a green-brown solid (0.845 g, 74%). IR (KBr): 1662 cm⁻¹ (ν_C=O). MS m/z: 793 (M⁺ − Cl showing the expected isotopic pattern), 758 (M⁺ − 2Cl). Anal. calc. for C₃₄H₃₃Cl₄OPW: C, 49.30; H, 4.02; N, 1.69. Found: C, 48.87; H, 4.27; N, 1.53%.

[(η-C₅H₅)Nb[upper bond 1 start]Cl₃{PPh₂CH[horiz bar, triple dot above]C([horiz bar, triple dot above]O[upper bond 1 end])Ph}] 6a. Solid [(η-C₅H₅)NbCl₄] (0.400 g, 1.33 mmol) and Ph₂PCH₂C(O)Ph (0.405 g, 1.33 mmol) were placed in a Schlenk flask and DME (30 mL) was added at ambient temperature. Rapidly, the solution turned deep red, and HCl emanated from it. The mixture was then stirred for 2 h and the volatiles were removed under reduced pressure. The red-brown residue was washed with Et₂O (2 × 20 mL) and pentane (2 × 20 mL) and dried in vacuo affording 6a as a red-brown solid (0.704 g, yield 93%). ¹H NMR (CD₂Cl₂, 300.18 MHz): δ 6.25 (d, 1H, ²J_PH = 3.5 Hz, PCH); 6.95 (s, 5H, Cp); 7.24–7.88 (m, 15H, aromatics). ³¹P{¹H} NMR (CD₂Cl₂, 121.5 MHz): δ 36.5 (vbr, w_1/2 > 2500 Hz). IR (Nujol): 1547 cm⁻¹ (ν_{C[horiz bar, triple dot above]C} + ν_{C[horiz bar, triple dot above]O}). Anal. calc. for C₂₅H₂₁Cl₃NbOP: C, 52.90; H, 3.72. Found: C, 53.21; H, 3.82%.

X-Ray quality single crystals were grown by slow diffusion of Et₂O into a toluene solution of 6a.

[(η-C₅H₅)Ta[upper bond 1 start]Cl₃{PPh₂CH[horiz bar, triple dot above]C([horiz bar, triple dot above]O[upper bond 1 end])Ph}] 6b. Solid [(η-C₅H₅)TaCl₄] (0.400 g, 1.03 mmol) and Ph₂PCH₂C(O)Ph (0.313 g, 1.03 mmol) were placed in a Schlenk flask and CH₂Cl₂ (20 mL) was added at room temperature. The mixture was stirred for 3 h, during which its colour changed from yellow to orange. The volatiles were then removed under reduced pressure. The dark yellow residue was extracted with toluene (15 mL) at 60 °C. The resulting clear yellow solution was evaporated under reduced pressure affording 6b as a yellow powder (0.563 g, yield 83%). ¹H NMR (CDCl₃, 300.18 MHz): δ 6.34 (d, 1H, ²J_PH = 3.0 Hz, PCH); 6.72 (s, 5H, Cp); 7.20–7.91 (m, 15H, aromatics). ³¹P{¹H} NMR (CDCl₃, 121.5 MHz): δ 37.2. IR (Nujol): 1552 cm⁻¹ (ν_{C[horiz bar, triple dot above]C} + ν_{C[horiz bar, triple dot above]O}). Anal. calc. for C₂₅H₂₁Cl₃OPTa: C, 45.79; H, 3.63. Found: C, 46.01; H, 3.46%.

X-Ray quality single crystals were grown by slow diffusion of petroleum ether (bp 40–60 °C) into a CDCl₃ solution of 6b.

[(η-C₅H₄ⁱPr)W[upper bond 1 start]Cl₃{PPh₂CH[horiz bar, triple dot above]C([horiz bar, triple dot above]O[upper bond 1 end])Ph}] 6c. Solid [(η-C₅H₄ⁱPr)WCl₄] (0.600 g, 1.38 mmol) and Ph₂PCH₂C(O)Ph (0.420 g, 1.38 mmol) were placed in a Schlenk flask and toluene (15 mL) was added at room temperature. The mixture was stirred overnight, affording a claret solution. The volatiles were then removed under reduced pressure and the residue was washed with Et₂O (15mL) and pentane (2 × 10 mL). It afforded 6c as a purple solid, which was dried in vacuo (0.786 g, yield 81%). IR: 1539 cm⁻¹ (Nujol), 1542 cm⁻¹ (KBr) (ν_{C[horiz bar, triple dot above]C} + ν_{C[horiz bar, triple dot above]O}). MS m/z: 701 (M⁺, showing the expected isotopic pattern), 666 (M⁺ − Cl). Anal. calc. for C₂₈H₂₇Cl₃OPW: C, 48.00; H, 3.88. Found: C, 47.83; H, 3.72%.

[(η-C₅Me₅)Ta[upper bond 1 start]Cl₃{PPh₂CH[horiz bar, triple dot above]C([horiz bar, triple dot above]O[upper bond 1 end])Ph}] 6d. This complex was prepared in a similar manner to 6b from [(η-C₅Me₅)TaCl₄] (0.400 g, 0.873 mmol) and Ph₂PCH₂C(O)Ph (0.266 g, 0.873 mmol). It was obtained as a yellow solid (0.544 g, yield 86%). ¹H NMR (CDCl₃, 300.18 MHz): δ 2.38 (s, 15H, Me); 6.08 (d, 1H, ²J_PH = 3.6 Hz, PCH); 6.90–7.90 (m, 15H, aromatics). ³¹P{¹H} NMR (CDCl₃, 121.5 MHz): δ 36.5 (vbr, w_1/2 > 2500 Hz). IR (Nujol): 1552 cm⁻¹ (ν_{C[horiz bar, triple dot above]C} + ν_{C[horiz bar, triple dot above]O}). Anal. calc. for C₃₀H₃₁Cl₃OPTa: C, 49.64; H, 4.30. Found: C, 49.87; H, 4.18%.

X-Ray quality single crystals were grown by slow diffusion of Et₂O into a CDCl₃ solution of 6d.

[(η-C₅H₅)NbCl₄{O=C(Me)NHPPh₂}] 7a. This complex was prepared in a similar manner to 5a from [(η-C₅H₅)NbCl₄] (0.500 g, 1.67 mmol) and Ph₂PNHC(O)Me (0.354 g, 1.67 mmol) in DME (20 mL). It was obtained as an orange-pink solid (0.751 g, yield 88%). ¹H NMR (CD₂Cl₂, 300.18 MHz): δ 2.16 (s, 3H, Me); 7.18 (s, 5H, Cp); 7.22–7.95 (m, 10H, aromatics); 9.64 (br s, 1H, NH). ³¹P{¹H} NMR (CD₂Cl₂, 121.5 MHz): δ 33.1 (w_1/2 = 65 Hz). IR (CH₂Cl₂): 1646 cm⁻¹ (ν_C=O). Anal. calc. for C₁₉H₁₉Cl₄NNbOP: C, 42.02; H, 3.53; N, 2.58. Found: C, 42.26; H, 3.81; N, 2.61%.

[(η-C₅H₅)TaCl₄{O=C(Me)NHPPh₂}] 7b. This complex was prepared in a similar manner to 5b from [(η-C₅H₅)TaCl₄] (0.150 g, 0.38 mmol) and Ph₂PNHC(O)Me (0.080 g, 0.38 mmol) in CH₂Cl₂ (10 mL). It was obtained as a yellow-brown solid (0.185 g, yield 80%). ¹H NMR (CDCl₃, 300.18 MHz): δ 2.61 (s, 3H, Me); 6.91 (s, 5H, Cp); 6.80–8.02 (m, 10H, aromatics); 8.85 (br s, 1H, NH). ³¹P{¹H} NMR (CD₂Cl₂, 121.5 MHz): δ 40.6. IR (CDCl₃): 1646 cm⁻¹ (ν_C=O). Anal. calc. for C₁₉H₁₉Cl₄NOPTa: C, 36.16; H, 3.03; N, 2.22. Found: C, 36.28; H, 3.05; N, 2.26%.

[(η-C₅H₄ⁱPr)WCl₄{O=C(Me)NHPPh₂}] 7c. This complex was prepared in a similar manner to 5c from [(η-C₅H₄ⁱPr)WCl₄] (1.000 g, 2.30 mmol) and Ph₂PNHC(O)Me (0.490 g, 2.30 mmol) in a 1∶1 CH₂Cl₂–toluene mixture (30 mL). It was obtained as a green-brown solid (1.212 g, yield 81%). IR (Nujol): 1651 cm⁻¹ (ν_C=O). MS m/z: 641 (M⁺ − Cl, showing the expected isotopic pattern), 606 (M⁺ − 2Cl). Anal. calc. for C₂₂H₂₅Cl₄NOPW: C, 39.08; H, 3.73; N, 2.07. Found: C, 38.70; H, 3.78; N, 2.03%.

Crystallography

Data were collected on an Enraf-Nonius DIP2000 Image Plate diffractometer (6a) and on an Enraf-Nonius KappaCCD diffractometer (6b,d) (graphite-monochromated MoK_α radiation λ = 0.71073 Å). Intensity data were processed using the DENZO-SMN package⁹⁰ running on a Silicon Graphics Indy computer. The structures were solved using the direct-methods program SIR-92.⁹¹ Subsequent full-matrix least-squares refinements were carried out using the SHELXL-93⁹² (6a) or the CRYSTALS⁹³ programs. The structure of 6b consists of two “pseudo-dimers” representing two slightly different mononuclear complexes, referred to as molecules 1 and 2 in Table 2. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were positioned geometrically after each cycle of refinement. A 3-term Chebychev polynomial weighting scheme was applied. Selected crystal data and refinement details are given in Table 1.

CCDC reference numbers 190864–190866.

See http://www.rsc.org/suppdata/nj/b2/b202983a/ for crystallographic data in CIF or other electronic format.

Acknowledgements

For providing financial support, we would like to thank the Centre National de la Recherche Scientifique/Royal Society cooperation programme (XM), the RSC for a travel grant (XM, JGA No. 0103 306), the AG Leventis Foundation (IV) and the EPSRC (LHR). We thank Dr. David Watkin and Andrew Cowley (Crystallography Laboratory, Oxford) for their generous assistance with the crystal structure determinations.

References

1. A. Bader and E. Lindner, Coord. Chem. Rev., 1991, 108, 27 and references cited therein.
2. E. Lindner, S. Pautz and M. Haustein, Coord. Chem. Rev., 1996, 155, 145.
3. C. S. Slone, D. A. Weinberger and C. A. Mirkin, Prog. Inorg. Chem., 1999, 48, 233 and references cited therein.
4. P. Braunstein and F. Naud, Angew. Chem., Int. Ed., 2001, 40, 680.
5. R. W. Wegman, A. G. Abatjoglou and A. M. Harrison, J. Chem. Soc., Chem. Commun., 1987, 1891.
6. G. J. P. Britovsek, W. Keim, S. Mecking, D. Sainz and T. Wagner, J. Chem. Soc., Chem. Commun., 1993, 1632.
7. E. Lindner, Q. Wang, H. A. Mayer, A. Bader, H. Kühbauch and P. Wegner, Organometallics, 1993, 12, 3291.
8. M. Alvarez, N. Lugan and R. Mathieu, J. Organomet. Chem., 1994, 468, 249.
9. C. Abu-Gnim and I. Amer, J. Chem. Soc., Chem. Commun., 1994, 115.
10. H. Yang, M. Alvarez, N. Lugan and R. Mathieu, J. Chem. Soc., Chem. Commun., 1995, 1721.
11. G. J. P. Britovsek, K. J. Cavell and W. Keim, J. Mol. Catal., 1996, 110, 77.
12. S. Mecking and W. Keim, Organometallics, 1996, 15, 2650.
13. H. Yang, M. Alvarez-Gressier, N. Lugan and R. Mathieu, Organometallics, 1997, 16, 1401.
14. P. Braunstein, D. Matt, Y. Dusausoy, J. Fischer, A. Mitschler and L. Ricard, J. Am. Chem. Soc., 1981, 103.
15. J. I. Dulebohn, S. C. Haefner, K. A. Berglund and K. R. Dunbar, Chem. Mater., 1992, 4, 506.
16. K. R. Dunbar, Comments Inorg. Chem., 1992, 13, 313.
17. H. Werner, M. Schulz and B. Windmüller, Organometallics, 1995, 14, 3659.
18. E. Lindner, B. Keppleler, H. A. Mayer, K. Gierling, R. Fawzi and M. Steinmann, J. Organomet. Chem., 1996, 526, 175.
19. H. Werner, J. Bank, B. Windmüller, O. Gevert and W. Wolfsberger, Helv. Chim. Acta, 2001, 84, 3163.
20. J.-M. Camus, D. Morales, J. Andrieu, P. Richard, R. Poli, P. Braunstein and F. Naud, J. Chem. Soc., Dalton Trans., 2000, 2577.
21. C. D. Andrews, A. D. Burrows, J. M. Lynam, M. F. Mahon and M. T. Palmer, New J. Chem., 2001, 25, 824.
22. N. G. Jones, M. L. H. Green, I. Vei, X. Morise and P. Braunstein, J. Chem. Soc., Dalton Trans., 2002, 1487.
23. N. G. Jones, M. L. H. Green, I. C. Vei, L. H. Rees, S. I. Pascu, D. Watkin, A. Cowley, X. Morise and P. Braunstein, J. Chem. Soc., Dalton Trans., 2002 , in the press.
24. J. A. Canich, F. A. Cotton and S. A. Duraj, Inorg. Chim. Acta, 1989, 41, 156.
25. S. Gambarotta, M. Pasquali, C. Floriani, A. Chiesi-Villa and C. Guastini, Inorg. Chem., 1981, 20, 1173.
26. F. E. Kuhn, A. D. Lopes, A. M. Santos, E. Herdtweck, J. J. Haider, C. C. Romao and A. G. Santos, J. Mol. Catal. A: Chem., 2000, 151, 147.
27. D. J. McCabe, E. N. Duesler and R. T. Paine, Inorg. Chem., 1987, 26, 2300.
28. V. S. Sergienko, N. A. Ovchinnikova, M. A. Porai-Koshits and M. A. Glushkova, Koord. Khim., 1986, 12, 1650.
29. D. G. L. Holt, L. F. Larkworthy, D. C. Povey, G. W. Smith and G. J. Leigh, J. Chem. Soc., Dalton Trans., 1990, 3229.
30. F. A. Cotton and G. W. Rice, Inorg. Chem., 1978, 17, 2004.
31. P. Bhattacharyya, A. M. Z. Slawin, M. B. Smith and J. D. Woollins, Inorg. Chem., 1996, 35, 3675.
32. L. R. Falvello, M. M. Garcia, I. Lazaro, R. Navarro and E. P. Urriolabeitia, New J. Chem., 1999, 227.
33. R. Navarro and E. P. Urriolabeitia, J. Chem. Soc., Dalton Trans., 1999, 4111.
34. W. Keim, A. Behr, B. Gruber, B. Hoffmann, F. H. Kowaldt, U. Kürschner, B. Limbäcker and F. P. Sistig, Organometallics, 1986, 5, 2356 and references cited therein.
35. S.-E. Bouaoud, P. Braunstein, D. Grandjean, D. Matt and D. Nobel, Inorg. Chem., 1986, 25, 3765.
36. F. Balegroune, P. Braunstein, D. Grandjean, D. Matt and D. Nobel, Inorg. Chem., 1988, 27, 3320.
37. S.-E. Bouaoud, P. Braunstein, D. Grandjean, D. Matt and D. Nobel, Inorg. Chem., 1988, 27, 2279.
38. P. Braunstein, D. Matt, D. Nobel, F. Balegroune, S.-E. Bouaoud, D. Grandjean and J. Fischer, J. Chem. Soc., Dalton Trans., 1988, 353.
39. P. Braunstein, T. M. Gomes-Carneiro, D. Matt, F. Balegroune and D. Grandjean, Organometallics, 1989, 8, 1737.
40. W. Keim, Angew. Chem., Int. Ed. Engl., 1990, 29, 235.
41. P. Berno, P. Braunstein, C. Floriani, A. Chiesi-Villa and C. Guastini, Inorg. Chem., 1991, 30, 1407.
42. J. Andrieu, P. Braunstein and A. D. Burrows, J. Chem. Res. (S), 1993, 380.
43. P. Braunstein, S. Coco-Cea, M. I. Bruce, B. W. Skelton and A. H. White, J. Chem. Soc., Dalton Trans., 1992, 2539.
44. P. Braunstein, S. Coco-Cea, M. I. Bruce, B. W. Skelton and A. H. White, J. Organomet. Chem., 1992, 423, C38.
45. P. Braunstein, L. Douce, F. Balegroune, D. Grandjean, D. Bayeul, Y. Dusausoy and P. Zanello, New J. Chem., 1992, 16, 925.
46. P. Braunstein, D. J. Kelly, A. Tiripicchio and F. Ugozzoli, Inorg. Chem., 1993, 32, 4845.
47. P. Braunstein, Y. Chauvin, J. Nähring, Y. Dusausoy, D. Bayeul, A. Tiripicchio and F. Ugozzoli, J. Chem. Soc., Dalton Trans., 1995, 851.
48. P. Braunstein, Y. Chauvin, J. Nähring, A. DeCian and J. Fischer, J. Chem. Soc., Dalton Trans., 1995, 863.
49. P. Braunstein, Y. Chauvin, J. Fischer, H. Olivier, C. Strohmann and D. V. Toronto, New J. Chem., 2000, 24, 437.
50. P. Braunstein, Y. Chauvin, S. Mercier, L. Saussine, A. DeCian and J. Fischer, J. Chem. Soc., Chem. Commun., 1994, 2203.
51. J. Pietsch, P. Braunstein and Y. Chauvin, New J. Chem., 1998, 22, 467.
52. K. A. O. Starzewski and J. Witte, Angew. Chem., Int. Ed. Engl., 1987, 26, 63.
53. C. J. Moulton and B. L. Shaw, J. Chem. Soc., Dalton Trans., 1980, 299.
54. J. Bank, P. Steinert, P. Windmüller, W. Wolfsberger and H. Werner, J. Chem. Soc., Dalton Trans., 1996, 1153.
55. P. Crochet and B. Demerseman, Organometallics, 1995, 14, 2173.
56. J. Vicente, M.-T. Chicote, M. A. Beswick and M. C. Ramirez-de-Arellano, Inorg. Chem., 1996, 35, 6592.
57. P. Crochet, B. Demerseman, M. I. Vallejo, M. P. Gamasa, J. Gimeno, J. Borge and S. Garcia-Granda, Organometallics, 1997, 16, 5406.
58. L. R. Falvello, S. Fernandez, R. Navarro, I. Pascual and E. P. Urriolabeitia, J. Chem. Soc., Dalton Trans., 1997, 763.
59. P. Braunstein, Y. Chauvin, J. Nähring, A. DeCian, J. Fischer, A. Tiripicchio and F. Ugozzoli, Organometallics, 1996, 15, 5551.
60. D. A. Edwards, M. F. Mahon and T. J. Paget, Polyhedron, 2000, 19, 757.
61. C. Mattheis, P. Braunstein and A. Fischer, J. Chem. Soc., Dalton Trans., 2001, 800.
62. P. Veya, C. Floriani, A. Chiesi-Villa and C. Guastini, Organometallics, 1991, 10, 2991.
63. P. Veya, C. Floriani, A. Chiesi-Villa and C. Guastini, Organometallics, 1994, 13, 208.
64. K. Mashima, Y. Nakayama and A. Nakamura, Adv. Polym. Sci., 1997, 133, 1.
65. A. Togni and R. L. Halterman, Metallocenes, Synthesis, Reactivity, Application, vol. 1 and 2, Wiley-VCH, Weinheim, Germany, 1998.
66. J. M. Decker, S. J. Geib and T. Y. Meyer, Organometallics, 1999, 18, 4417.
67. K. Mashima, Y. Matsuo, S. Nakamura and K. Tani, J. Organomet. Chem., 2000, 593–594, 69.
68. K. Mashima, Y. Matsuo and K. Tani, Angew. Chem., Int. Ed. Engl., 2001, 40, 960 .and references cited therein.
69. M. L. H. Green, P. C. McGowan and P. Mountford, J. Chem. Soc., Dalton Trans., 1995, 1207.
70. M. L. H. Green and P. Mountford, Chem. Rev., 1994, 94, 29.
71. R. Poli, Chem. Rev., 1991, 91, 509 and references cited therein.
72. R. J. Burt and J. G. Leigh, J. Organomet. Chem., 1978, 148, C19.
73. R. J. Burt, J. G. Leigh and D. L. Hughes, J. Chem. Soc., Dalton Trans., 1981, 793.
74. R. D. Sanner, S. T. Caster and W. J. Bruton, J. Organomet. Chem., 1982, 240, 157.
75. M. J. Bunker, A. DeCian, M. L. H. Green, J. J. E. Moreau and N. Siganporia, J. Chem. Soc., Dalton Trans., 1980, 2155.
76. J. C. Fettinger, D. W. Keogh and R. Poli, Inorg. Chem., 1995, 34, 2343.
77. X. Morise, M. L. H. Green, P. C. McGowan and S. J. Simpson, J. Chem. Soc., Dalton Trans., 1994, 871.
78. M. L. H. Green, X. Morise, A. H. Hamilton and A. N. Chernaga, J. Organomet. Chem., 1995, 488, 73.
79. P. Braunstein, C. Frison, X. Morise and R. D. Adams, J. Chem. Soc., Dalton Trans., 2000, 2205.
80. P. Braunstein, D. J. Kelly, A. Tiripicchio and F. Ugozzoli, Bull. Soc. Chim. Fr., 1995, 132, 1083.
81. P. Braunstein, B. H. Heaton, C. Jacob, L. Manzi and X. Morise, to be published.
82. P. Kubácek, R. Hoffmann and Z. Havlas, Organometallics, 1982, 1, 180.
83. R. Fandos, M. Gómez, P. Royo, S. Garcia-Blanco, S. Martinez-Carrera and J. Sanz-Aparicio, Organometallics, 1987, 6, 1581.
84. M. V. Galakhov, M. Gómez, G. Jiménez, P. Royo, M. A. Pellinghelli and A. Tiripicchio, Organometallics, 1994, 14, 1901.
85. J. Arnold, T. D. Tilley, A. L. Rheingold and S. J. Geib, J. Chem. Soc., Chem. Commun., 1987, 793.
86. G. A. A. Hadi, K. Fromm, S. Blaurock, S. Jelonek and E. Hey-Hawkins, Polyhedron, 1997, 16, 721.
87. V. Gibson, T. P. Kee, R. M. Sorrell, A. P. Bashell and M. McPartlin, Polyhedron, 1988, 7, 2221.
88. W. A. Herrmann, W. Kalcher, H. Biersack, I. Bernal and M. Creswick, Chem. Ber., 1981, 114, 3558.
89. M. L. H. Green, J. D. Hubert and P. Mountford, J. Chem. Soc., Dalton Trans., 1990, 3793.
90. Z. Otwinowski and W. Minor, Processing of X-ray Diffraction Data Collected in Oscillation Mode, in Methods Enzymol., eds. C. W. Carter and R. M. Sweet, Academic Press, 1997, vol. 276, p. 357.
91. A Program Solution of Crystal Structure by Direct Methods: A. Almatore, G. Cascarano, C. Giavocazzo and A. Guargliardi, J. Appl. Crystallogr., 1994, 27, 435.
92. G. M. Sheldrick, SHELXL93, Program for Crystal Structure Refinement, 1993, Intitüt für Anorganische Chemie der Universität, Tammanstrasse 4, D-3400 Göttingen, Germany.
93. D. J. Watkin, K. C. Prout, J. R. Carruthers, P. W. Betteridge and R. I. Cooper, CRYSTALS issue 11, Chemical Crystallography Laboratory, Oxford, UK, 2001.

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Footnote

† Dedicated to Prof. P. Royo on the occasion of his 65th birthday, with our warmest congratulations.

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