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Inorganica Chimica Acta 359 (2006) 1006–1011

(C5Me5)2NbBH4, a new reagent for the synthesis of Ru and Cocarbonyl clusters with interstitial boron or carbon

Andreas Lange, Walter Meier, Joachim Wachter *, Manfred Zabel

Institut fur Anorganische Chemie der Universitat Regensburg, D-93040 Regensburg, Germany

Received 3 August 2005; accepted 16 September 2005Available online 14 November 2005

Ruthenium and Osmium Chemistry Topical Issue

Abstract

The reaction of Cp*2NbBH4 (Cp* = g5-C5Me5) with Ru3(CO)12 gave a mixture of compounds, from which only [Cp*2Nb(CO)2]2-[Ru6(CO)16C] (1) could be characterized by spectroscopic and crystallographic methods. 11B NMR spectroscopy proved that interstitialboron may be present in other Ru6 clusters, but these compounds did not crystallize. The reaction of Cp*2NbBH4 with Co2(CO)8 gaveamong others the salts [Cp*2Nb(CO)2]2[Co6(CO)15C] (4) and [Cp*2Nb(CO)2]3[Co13(CO)24C2] (5), which were examined by X-ray diffrac-tion studies. The true nature of the interstitial atoms in 5 was deduced from electrochemical investigations, which reveal similar redoxproperties as for the already known [Co13(CO)24C2]

3� anion.� 2005 Elsevier B.V. All rights reserved.

Keywords: Cobalt; Ruthenium, Clusters; Carbide; Boride

1. Introduction

Transition metal carbonyl clusters containing intersti-tial main group atoms are an important link betweenorganometallic chemistry and inorganic solid state chem-istry [1]. Whereas there are numerous examples for theelements C and N, either in octahedral [2] or trigonal-prismatic [3] cavities, there are only a limited numberof examples for boron. Examples for interstitial borideclusters are nearly exclusively restricted to the Ru6B skel-eton [4].

The usual synthetic route to Ru6B clusters is theexpansion of Ru3B or Ru4B building blocks upon reac-tion with appropriate complex fragments. The directsynthesis from Ru3(CO)12 and a boron containingreagent like BH3 Æ THF gives HRu6B(CO)17 in 10% yield(Eq. 1(a)) [5].

0020-1693/$ - see front matter � 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.ica.2005.09.029

* Corresponding author.E-mail address: joachim.wachter@chemie.uni-regensburg.de (J. Wach-

ter).

Ru3ðCOÞ12 þ BH3 � THF ������!toluene; 75 �C

ðaÞHRu6BðCOÞ17

10%

Co2ðCOÞ8 þ BH3 � SMe2

�������!toluene; 75 �C

ðbÞCo5BðBHÞðCOÞ14

<1%

þCo4ðCOÞ12

ð1Þ

Still more difficult is the incorporation of boron into cobaltcarbonyl clusters. The reaction of Co2(CO)8 withBH3 Æ SMe2 gives Co4(CO)12, along with Co5B(BH)(CO)14in less than 1% yield (Eq. 1(b)) [6]. (CO)4CoBH2, whichis accessible from BH3 Æ THF and Co2(CO)8, is not suitableas a cluster building block [7]. The formation of ‘‘Co6B-(CO)18’’ from Co2(CO)8 and BBr3 at 60 �C or fromCo2(CO)8 and B2H6 under pressure has been reported in1975 by Schmid et al. [8], but there is no structuralconfirmation.

In this work, we report on the reaction of Cp*2NbBH4

(Cp* = C5Me5) with Ru3(CO)12 or Co2(CO)8. The result-ing metal carbonyl clusters contain interstitial main groupatoms, but it was difficult to determine the true nature ofthe incorporated main group atoms.

A. Lange et al. / Inorganica Chimica Acta 359 (2006) 1006–1011 1007

2. Experimental

2.1. Materials and methods

All procedures were carried out under N2 using dry sol-vents with Schlenk tube techniques. Elemental analyses (C,H) were performed at the Mikroanalytisches Laborato-rium, Universitat Regensburg. IR spectra were obtainedwith a Bio Rad FT IR FTS 155 spectrometer, NMR spec-tra were measured on a Bruker Avance 400 instrument. ESImass spectra were obtained on a ThermoQuestFinniganTSQ 7000 spectrometer. Cp*2NbBH4 was prepared fromNbCl5, LiCp* and NaBH4 [9].

2.2. Reaction of Cp*2NbBH4 with Ru3(CO)12

A solution of 280 mg (0.74 mmol) of Cp*2NbBH4 and390 mg (0.61 mmol) of Ru3(CO)12 in 100 ml of toluenewas stirred for 3 h under reflux. After cooling, the dark-brown precipitate was separated by decantation of the li-quid phase. Then, the residue was extracted with 100 mlof THF. The remaining residue was dissolved in acetone.The resulting solution was filtered and concentrated to dry-ness giving a red product (73 mg, 0.04 mmol, 13%) mainlyconsisting of [Cp*2Nb(CO)2]2[Ru6(CO)16C] (1). The THFextract was concentrated to dryness and the residue was dis-solved in toluene/acetone at a ratio of 2:1. Chromatographyon SiO2 (column 7 · 3 cm) gave upon elution with toluene/acetone 2:1 an intensive red band containing 217 mg of 2.The product still contains small amounts of 1. Further redand brown bands could not be identified. Crystallizationof 1 from CH2Cl2 or a 2:1 THF:Et2O mixture yielded brightred rectangular plates. Crystallization of 2 from CH2Cl2first gave some crystals of 1 and then dark red-brown prismsof 2. These were not suitable for X-ray diffraction.

2.2.1. Complex 1Anal. Calc. for C61H60Nb2O20Ru6 (1905.35): C, 38.45;

H, 3.17. Found: C, 38.48; H, 3.47%. IR m/cm�1 (CH2Cl2):2021s, 1977vs, 1959sh, 1918w, 1781m. 13C NMR (100MHz, CD2Cl2): d 234.78 (Nb–C–O), 214.10 (Ru–C–O),108.63 (C–CH3), 10.97 (C–CH3). MS (PI-ESI) (CH3CN):m/z 419.0 ([Cp*2Nb(CO)2]

+); NI-ESI (CH3CN): m/z533.8 ([Ru6C(CO)16]

2�) (sim. 534).

2.2.2. Complex 2IR m/cm�1 (CH2Cl2): 2069w, 2035sh, 2015vs, 1960m,

1816br, m. 1H NMR (400 MHz, CD2Cl2): d 1.94 (s). 11BNMR (128 MHz, CD2Cl2): d 198.16 (s). MS (PI-ESI)(CH2Cl2/CH3CN): m/z 419.0 ([Cp*2Nb(CO)2]

+); NI-ESI(CH2Cl2/CH3CN): m/z 744.6 ([Ru4H3(CO)12]

�) (sim. 745)(rel. intensity 100%), 1094.5 ([Ru6B(CO)17]

�) (sim. 1095)(rel. intensity 7%).

2.3. Reaction of Cp*2NbBH4 with Co2(CO)8

A mixture of 230 mg (0.61 mmol) of Cp*2NbBH4,250 mg (0.73 mmol) of Co2(CO)8 and 100 ml of toluene

was stirred for 3 h under reflux. After cooling, the black-brown precipitate was separated and washed three timeswith 20 ml each of toluene. Then, the residue was extractedwith THF giving a soluble fraction and a dark residue.After evaporation of the solvent from the THF fraction,the remaining residue was dissolved in 7 ml of CH2Cl2.Chromatography on SiO2 (column 6 · 3 cm) gave after elu-tion with CH2Cl2 a narrow brown band, which wasdiscarded, and with 50:1 CH2Cl2:acetone mixture a brownband containing 47 mg (0.03 mmol, 12%) of [Cp*2Nb-(CO)2]2[Co6(CO)15C] (4). Dark-red rods of 4 were obtainedfrom 3:5 acetone:ether mixture at �25 �C. The dark residuefrom the THF extraction was dissolved in CH2Cl2 andchromatographed on SiO2 (column 6 · 3 cm). A first weakbrown band was eluted with CH2Cl2, followed by a brownband with 10:1 CH2Cl2:acetone mixture, containing 97 mg(0.04 mmol, 32%) of [Cp*2Nb(CO)2]3[Co13(CO)24C2] (5).Crystallization of 5 from 3:9 acetone:ether mixture gaveblack shiny cubes of composition 5 Æ 3.25C3H6O. Afterincreasing the ether volume, fine needles of composition5 Æ 2C3H6O were obtained.

2.3.1. Complex 4Anal. Calc. for C60H60Co6Nb2O19 (1624.52): C, 44.36;

H, 3.72. Found: C, 39.31; H, 3.79%. IR m/cm�1 (KBr):2034m, 2020s, 1971vs, 1963vs, 1890w, 1863m, 1838m,1815vs, 1780w, 1764m, 1746w.

2.3.2. Complex 5Anal. Calc. for C92H90Co13Nb3O30 (2720.53): C, 40.61;

H, 3.33. Found: C, 40.12; H, 4.02%. IR m/cm�1 (CH2Cl2):2019m, 1987vs, 1961s, 1813m, 1785w.

2.4. Crystallography

Structure solution of 5: The data were collected on aSTOE imaging plate diffraction system (IPDS) at173 K. Due to the great lattice constants, two measure-ments with different distances between crystal and detec-tor were carried out to fulfill on one hand the demand ofmeasuring reflections with 2HP 50� and on the otherhand to resolve neighboring reflections. The two datasets were scaled to each other and the resulting dataset was used for all calculations. The structure wassolved by direct methods (SIR97) and refined by full-ma-trix least-squares procedures on F2, using all data, withthe SHELX 97 program. During the structure refinement,it turned out that the ligands of two of the Nb cationsshow positional disorder. Around Nb(5), one CO liganddirects in two positions with a probability of 50% and inthe Nb(6)-cation one CO group together with one Cp*group must also be described in two positions. In addi-tion, 4.5 of the 6.5 acetone solvent molecules are so dis-ordered that they may be only described as a cloud ofelectron density. The sum of electrons in these solventaccessible voids clearly proves that only acetone ispresent.

Table 1Crystallographic data for [Cp*2Nb(CO)2]2[Ru6(CO)16C] (1), [Cp*2Nb(CO)2]2[Co6(CO)15C] (4) and [Cp*2Nb (CO)2]3[Co13(CO)24C2] Æ 3.25C3H6O (5)

1 4 5

Empirical formula C61H60Nb2O20Ru6 C60H60Co6Nb2O19 C203.5H219Co26Nb6O66.5

Formula weight (g/mol) 1905.33 1624.4 5818.4Crystal system triclinic orthorhombic triclinicCrystal size (mm) 0.26 · 0.23 · 0.05 0.50 · 0.26 · 0.20 0.52 · 0.38 · 0.22Space group P�1 Pnma P�1Unit cell dimensions

a (A) 10.8221(8) 25.4716(17) 17.6706(16)b (A) 10.8552(8) 16.6689(8) 18.3091(19)c (A) 28.043(2) 14.4585(7) 34.280(3)a (�) 89.278(9) 90 86.391(12)b (�) 88.423(9) 90 89.759(11)c (�) 89.120(8) 90 86.410(11)

V (A3) 3292.5(4) 6138.8(6) 11047.0(18)Z 2 8 2Dcalc (g/cm

3) 1.922 1.758 1.749F(000) 1864 3264 5840Diffractometer STOE-IPDS STOE-IPDS STOE-IPDST (K) 173 173 173h Range for data collection (�) 1.88–25.81 1.86–25.78 1.78–25.89Reflections collected 45959 42026 121572Reflections unique 11833 6065 40088Rint 0.0317 0.0326 0.0409Reflections observed 10373 (I > 2) 5239 (I > 2) 31471 (I > 2)l (mm�1) 1.743 2.011 2.27Parameters number 822 448 2831Goodness-of-fit on F2 1.169 0.992 1.055Absorption correction numerical numerical analyticalTmax, Tmin 0.9805, 0.9077 0.9301, 0.8716 0.6019, 0.4166R1, wR2 (I > 2r) 0.0293, 0.0787 0.0208, 0.0508 0.0453, 0.1226R1, wR2 (all data) 0.0349, 0.0806 0.0259, 0.0521 0.0578, 0.1285

1008 A. Lange et al. / Inorganica Chimica Acta 359 (2006) 1006–1011

The structures of 1 and 4 were solved in an analogousmanner, but without any specific problem. All crystallo-graphic data are summarized in Table 1.

3. Results and discussion

3.1. Reaction of Cp*2NbBH4 with Ru3(CO)12

Reaction of Cp*2NbBH4 with Ru3(CO)12 in boilingtoluene gave a red-brown precipitate consisting of a mix-ture of different salts. Their extraction with THF gave asa less soluble product [Cp*2Nb(CO)2]2[Ru6(CO)16C] (1)and in the more soluble fraction after chromatographicwork-up complex 2 along with two other products of stillunknown nature. Whereas 2 did not crystallize well, crys-tals suitable for X-ray diffraction have been obtained forcomplex 1. These were accompanied in one case by crys-tals of 3 which were identified as [Cp*2Nb(CO)2][H3Ru4(CO)12]. The [H3Ru4(CO)12]

� anion has already beendescribed in [PPN][H3Ru4(CO)12] ([PPN]+ = [PPh3 =N = PPh3]

+) [10]. From spectroscopic data (see below),there is evidence that 3 very often appears as impurityin samples of 1 and 2.

Analysis of 1 by means of ESI mass spectrometry gavein the cationic spectrum a peak at m/z = 419.0, which cor-responds to the [Cp*2Nb(CO)2]

+ cation. The NI-ESI-MS re-veals a peak with center of gravity at m/z = 533.8. This peakmay be assigned to the doubly charged [Ru6(CO)16-C]

2�

or [HRu6(CO)16B]2� dianions. Elemental analysis of 1 is

in agreement with both formulas. The 1H NMR spectrumof 1 reveals one signal at d = 1.95 ppm, characteristic of theCp* methyl groups. A 11B NMR spectrum did not showany boron resonance. The 13C NMR spectrum containsresonances, which may be attributed to the CO ligandscoordinated at Ru and Nb and to the Cp* carbon atoms.A resonance signal for the interstitial carbon could notbe observed. The IR spectrum in CH2Cl2 is composed ofstrong terminal and bridging CO absorptions, which areidentical with those described for [PPN]2[Ru6(CO)16C] inCH2Cl2 [11]. Additional m(CO) frequencies at 2021 and1890 cm�1 may be assigned to the [Cp*2Nb (CO)2]

+ cation[12]. Overall, analytical and spectroscopic data of 1 are infavor of a [Cp*2Nb(CO)2]2[Ru6(CO)16C] composition.

The negative ESI mass spectrum of 2 reveals among oth-ers a peak at m/z 1094.5, the pattern of which correspondsto the simulated spectrum for [Ru6(CO)17B]

�. However,the most intensive peak is that at m/z = 744.6, which corre-sponds to [H3Ru4(CO)12]

�. This means that 2 is contami-nated with 3 and some other cluster compounds. As itis impossible to completely remove these impurities byrepeated recrystallization, correct elemental analyses couldnot be obtained. The IR spectrum of 2 and NMR spectraalso indicate the presence of a mixture of several com-pounds. The 1H NMR spectrum of 2 in CD2Cl2 reveals,apart from the resonance for the CH3 groups of the cation,singlets at d �17.06 and �19.05 ppm. These signals agree

A. Lange et al. / Inorganica Chimica Acta 359 (2006) 1006–1011 1009

with the hydride resonances reported in the literaturefor [H3Ru4(CO)12]

� and [H2Ru4(CO)12]2� [10,13]. The fit-

ting cation in 2 may be [Cp*2Nb(CO)2]+. The 11B NMR

spectrum in CD2Cl2 solution contains a singlet at d =198.16 ppm and another one of lower intensity at d =196.14 ppm. Comparable boron shifts have been proposedfor [PPN][Ru6(CO)17B] (d 198.8 in CD2Cl2 [5,14]). Otherproposals for ruthenium boride clusters [4] are [PPN][Ru7(CO)20B] (d = 195.7 ppm in CD2Cl2 [14]) or [Ru7(CO)19B]

�

[15], but the exact structures of these clusters are not yetknown.

The crystal structure of 1 contains two molecular unitsand each unit consists of one [Ru6(CO)16C]

2� dianionand two [Cp*2Nb(CO)2]

+ cations. The structure of the lat-ter, which is a bent peralkylated niobocene bearing two ter-minal CO ligands, is identical with that in salts composedof [Cp*2Nb(CO)2]

+ and metal telluride cluster anions[12]. The structure of the anion is defined by a Ru6 octahe-dron with an interstitial main group atom in the center. Asa consequence of the spectroscopic investigations, we de-fine this atom as C(73). Each of the Ru atoms bears twoterminal CO ligands, whereas the remaining four COgroups bridge the Ru–Ru edges. The geometric parametersof the anionic cluster core of 1 are close to those in othercompounds containing the same dianion [16]. This meansthat the Ru–Ru distances vary between 2.8387(5) and2.8579(5) A for the CO-bridged edges, whereas the otheredges are within 2.8873(5) and 3.0343(5) A.

3.2. Reaction of Cp*2NbBH4 with Co2(CO)8

The reaction of Cp*2NbBH4 with Co2(CO)8 in boilingtoluene gave a mixture of black-brown products. This

Fig. 1. Molecular structure of [Cp*2Nb(CO)2]2[Ru6(CO)16C] (1): cation (left)Ru(1)–Ru(2) 2.918(1), Ru(1)–Ru(4) 2.887(1), Ru(1)–Ru(5) 2.851(1), Ru(1)–R2.839(1), Ru(3)–Ru(4) 2.897(1), Ru(4)–Ru(5) 2.926(1), Ru(4)–Ru(6) 2.850(1), RC(73) 2.045(3), Ru(5)–C(73) 2.067(3), Ru(6)–C(73) 2.070(3).

was separated by means of extraction with THF and bytroublesome chromatography into better soluble [Cp*2Nb-(CO)2]2[Co6(CO)15C] (4) and less soluble [Cp*2Nb(CO)2]3-[Co13(CO)24C2] (5). Other products were also observed butcould not be characterized. Elemental analyses confirmcomposition of 5, but did not give a reasonable result inthe case of 4. PI-ESI mass spectra allowed to identify thecations, but NI-ESI mass spectra did not give any concreteresult. Presumably, the anionic clusters decompose duringmeasurement for neither molecular peaks nor fragmentswere observed.

The IR spectra of 4 and 5 exhibit terminal m(CO) fre-quencies at 2020 and 1890 cm�1, typical of the [Cp*2Nb-(CO)2]

+ cation. Additionally, very strong absorptions at1963 and 1815 cm�1 appear for 4. Weak absorptions,e.g., at 1890 and 1780 cm�1, may arise from impurities.The spectrum of 5 contains intensive absorptions with cen-ter of gravity at 1976 cm�1 for the terminal CO groups andtwo absorptions at 1809 and 1785 cm�1 for CO bridges.

1H NMR spectra of 4 and 5, respectively, reveal only theCH3 resonance at d = 1.92 ppm (CD2Cl2) characteristic ofthe cation. 11B NMR spectra in CD2Cl2 did not show any11B resonance. These investigations were handicapped bythe high molecular weights of 4 and 5 and their low solubil-ity in organic solvents.

The cyclic voltammogram of 5 in CH3CN exhibits tworeversible waves A1=A

01 and B1=B

01 at �0.50 and �1.03 V.

Both one-electron reduction steps agree well with thoseobserved for the cluster anions [Co13(CO)24C2]

m� (m = 3–5) [17]. A third wave reported in the literature at �1.68 Vmay be superposed by the first reduction wave of the[Cp*2Nb(CO)2]

+ cation at �1.56 V [12]. From this result,it is evident that the anion in 5 may be assigned to the

and anion (right). The interstitial atom is C(73). Important distances (A):u(6) 2.997(1), Ru(2)–Ru(3) 2.901(1), Ru(2)–Ru(5) 3.034(1), Ru(2)–Ru(6)u(1)–C(73) 2.039(4), Ru(2)–C(73) 2.059(3), Ru(3)–C(73) 2.055(4), Ru(4)–

1010 A. Lange et al. / Inorganica Chimica Acta 359 (2006) 1006–1011

cobalt carbonyl cluster anion [Co13(CO)24C2]3� containing

interstitial carbon instead of the expected boron. Complex4 was not subjected to electrochemical studies.

Crystal structure determinations have been carried outfor 4 and 5. The crystal structure of 4 contains eight molec-ular units and each unit contains two [Cp*2Nb(CO)2]

+ cat-ions and one [Co6(CO)15C]

2� dianion. The structure of thecation is analogous to that described in Fig. 1. The dianion

Fig. 2. Structure of the [Co6(CO)15C]2� anion in 4. Important distances

(A): Co(1)–Co(3) 2.535(1), Co(1)–Co(2) 2.560(1), Co(1)–Co(1a) 2.584(1),Co(2)–Co(3) 2.557(1), Co(2)–Co(2a) 2.586(1), Co(1)–C(3) 1.978(2), Co(1)–C(4) 1.919(2), Co(1)–C(5) 1.775(2), Co(1)–C(1) 1.965(2), Co(1)–C(34)1.961(2), Co(2)–C(34) 1.966(2), Co(3)–C(34) 1.955(2).

Fig. 3. Structure of the [Co13(CO)24C2]3� anion in 5 (left) and view of the C

Co(1)–Co(2) 2.5413(8), Co(1)–Co(8) 2.575(1), Co(1)–Co(5) 2.660(1), Co(1)–C2.587(1), Co(3)–Co(4) 2.500(1), Co(3)–Co(6) 2.559(1), Co(3)–Co(7) 2.586(1), CCo(12) 2.573(1), Co(5)–Co(6) 2.591(1), Co(5)–Co(13) 2.643(1), Co(5)–Co(82.658(1), C(301)–Co(mean) 1.955(6), C(302)–Co(mean) 1.976(6).

[Co6(CO)15C]2� consists of a trigonal prism of cobalt

atoms, in the center of which lies an interstitial main groupatom (Fig. 2). As X-ray crystallography is not able to dis-tinguish between carbon and boron, we assign this atom tocarbon as a result of spectroscopic data. The resulting clus-ter is structurally nearly identical with the dianions in[(PhCH2)(CH3)3N]2[Co6(CO)15C] [18] and [Et4N]2[Co6-(CO)15C] [19].

The crystal structure of 5 contains two molecular units.Each unit consists of two anions and six cations and 6.5molecules of acetone. The structures of the [Cp*2Nb-(CO)2]

+ cations are analogous to those in 1 and 4. Thestructure of the trianion in 5 (Fig. 3) may be describedby the parallel arrangement of two planar Co4 squaresabove and below an approximately planar Co5 ring. Theinterstitial main group atoms are surrounded by trigonal-prismatic Co6 polyhedra which are connected by Co(8).Each of the outer Co atoms bears one terminal CO ligand,the remaining 12 CO groups are in bridging positionsbetween eight Co–Co edges of the two outer squares andfour Co–Co edges of the middle Co5 ring. The distancesbetween the interstitial atoms and the trigonal-prismaticcobalt vertices are between 1.931(4) and 2.010(4) A.

Overall, the structure of the trianion in 5 is very close tothat of the [Co13(CO)24C2]

3� anion in [N(CH2Ph)Me3]3-[Co13(CO)24C2] [20]. A comparison of the structuralparameters in the anions [Co13(CO)24C2]

3�, [Co13(CO)24-C2]

4� and [Co13(CO)24N2]3� shows only a small influence

of charge and nature of the interstitial atom [17]. For thisreason, one may expect similar bond lengths for the hypo-thetical cobalt boride cluster [Co13(CO)24B2]

3�. It alsoshows that X-ray diffraction alone is not a suitable methodto determine the identity of the interstitial main groupatoms in our complexes.

o13C2 cluster core (right). Important distances (A): Co(1)–Co(4) 2.502(1),o(9) 2.668(1), Co(2)–Co(3) 2.473(1), Co(2)–Co(7) 2.473(1), Co(2)–Co(8)o(4)–Co(5) 2.584(1), Co(4)–Co(6) 2.640(1), Co(5)–Co(9) 2.410(1), Co(5)–) 2.705(1), Co(6)–Co(7) 2.414(1), Co(6)–Co(13) 2.580(1), Co(6)–Co(11)

A. Lange et al. / Inorganica Chimica Acta 359 (2006) 1006–1011 1011

In conclusion, Cp*2NbBH4 is a new reagent for the syn-thesis of ruthenium and cobalt carbonyl clusters with inter-stitial main group atoms, but unfortunately the selectivityis rather low. Whereas the incorporation of boron seemsto be limited to the Ru series, the spectroscopic data indi-cate preferential formation of carbide clusters in bothcases. For this kind of reaction, one may propose thereduction of one or two CO ligands by Cp*2NbBH4 withinthe coordination sphere of the metals. This also means thatin the reverse way two CO groups migrate from the metalcarbonyl to the niobocene fragment, thus forming the verystable [Cp*2Nb(CO)2]

+ cation. The versatility of this cationin stabilizing large metal cluster anions has already beendemonstrated [12]. In this context, it is interesting to notethat Cp*2NbBH4 does not react with Na[Co(CO)4]. How-ever, if Co2(CO)8 is added to the reaction mixture, one ob-serves formation of [Cp*2Nb(CO)2]2[Co6(CO)15] [21],which contains the carbide free [Co6(CO)15]

2� anion [22].

4. Supplementary materials

CIF tables for the structural analyses of compounds 1, 4and 5 have been deposited with the Cambridge Crystallo-graphic Data Center. They can be obtained free of chargefrom the Cambridge Crystallographic Data Center, 12Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223336033, or deposit@ccdc.cam.ac.uk. The deposit numbersare CCD Nos. 278311 (1), 278310 (4) and 278312 (5).

Acknowledgments

We thank Prof. Dr. H. Brunner for continuous supportover many years and Dr. H. Cattey and Prof. Y. Mugnier,Laboratoire de Synthese et d�Electrosynthese Organome-talliques, Universite de Bourgogne, Dijon, for the electro-chemical studies of 5.

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