J. CHEM. SOC. DALTON TRANS. 1985 1989New Carbonyl Derivatives of Niobium([) and Tantalum([) tFausto Calderazzo," Manola Castellani, and Guido PampaloniDipartimento Chimica e Chimica lndustriale, Sezione Chimica lnorganica, University of Pisa, Via Risorgimento35, 56 100 Pisa, ItalyPier Francesco ZanazziDipartimento Scienze della Terra, Sezione Cristallogra fia, University of Perugia, Piazza Universita, 06 100Perugia, ItalyThe hexacarbonylmetalates( 1 -) of niobium and tantalum are oxidised by H + and halogens to givethe [M,X,(CO),] - anions (X = CI, Br, or I). By exchange reactions, the p-acetato- and p-methoxo-complexes (X = 0,CMe or OMe) were also obtained. The crystal and molecular structure of[H (thf),] [ Nb,CI,(CO),] was investigated by X-ray diffraction methods.Crystals are triclinic, spacegroup P i , with a = 16.283(3), b = 9.293(2), c = 9.050(2) A, 01 = 11 3.01 (2), p = 96.56(2),y = 98.39(2)", U = 1 224.7 A3, D, = 1.792 g CM-~ (Z = 2), p( Mo-K,) = 12.2 cm-'. The two niobiumatoms of the dimeric anion are bridged by three chlorides located at the vertices of anapproximately equilateral triangle perpendicular to the niobium-niobium vector [Nb Nb 3.631 (1 )A]. The seven-co-ordination of niobium is completed by four carbonyl groups. The dimericchloride- bridged complexes of niobium ( I) and tantalum( I) undergo the following reactions: ( a )reduction by sodium to the hexacarbonylmetalate( 1 -); ( 6 ) chloride substitution by C,H,- to thecyclopentadienyl derivatives [M(q5-C5H5) (CO),] in good yields and by arenes in the presence ofaluminium bromide to the new cationic complexes [ M(qs-arene) (CO),] + as theirhexa (bromo)chlorodialuminate derivatives.We have recently developed new preparative procedures forthe hexacarbonylmetalates( 1 - ) of vanadium,' niobium,, andtantalum., The availability of these complexes in relativelylarge quantities has allowed the study of their chemistry, whichis still largely unexplored especially for niobium and tantalum,to be made.The oxidation state zero is unknown for binarycarbonyl derivatives of niobium and tantalum and theoxidation state 1 +was until recently exemplified by the cyclo-pentadienyl derivatives [Nb(q5-C5H5)(CO)4]3 and [Ta(q5-C,H,)(CO),]* only and by some of their substitution products.We have recently shown5 that the hydrido-complex ofvanadium [VH(CO),] is not stable and that stoicheiometricamounts of water promote its ionic dissociation to [H30]+-[v(Co),]-.In attempts to isolate hydrido-complexes ofniobium and/or tantalum, acidification of the correspondinghexacarbonylmetalates was carried out. However, no hydrido-species were observed and the halide-bridged species ofniobium(]) and tantalum(l), [M2X3(co),] -, were unexpectedlyformed,, by an unusual two-electron transfer process frommetal( 1 -) to protons.This paper reports some further extension of the work onthe halide+arbonyl complexes, the full details of the X-rayinvestigation of the niobium derivative with chloride bridges,and the use of the halide<arbonyl complexes as intermediatesfor the synthesis of the new q6-arene-tetracarbonyl complexesof niobium(1) and tantalum(i), [M(q6-arene)(C0),] +, and ofthe already known cyclopentadienyl derivatives.ExperimentalUnless otherwise stated, all of the operations involving thepreparation and manipulation of the metal complexes weret Supplementury dutu uvuiluhk (No.SUP 56231, 5 pp.): thermalparameters, full bond distances and angles, H-atom co-ordinates. SeeInstructions for Authors, J. Chem. Soc., DuIton Truns., 1985, Issue 1, pp.xvii-xix. Structure factors are available from the editorial office.Non-S.I. unit employed: mmHg z 133 Pa.carried out under an atmosphere of prepurified argon instrictly anhydrous solvents, the latter dried by conventionalmethods prior to use.Infrared spectra were measured with aPerkin-Elmer 283 instrument equipped with a suitable grating.The hexacarbonylmetalates of niobium( 1 -) and tantalum( 1 -)were prepared as previously described and products withvariable contents of tetrahydrofuran (thf) were obtained,depending on the drying procedure used.Oxidation of[M(CO),]- (M = Nb or Ta).-(a) By protons:preparation of [H(thf),][M,Cl,(CO),]. The hexacarbonylnio-bate(1 -) anion, as Na[Nb(Co),]*thf (2.036 g, 5.72 mmol), wassuspended in n-heptane (100 cm3) and treated with dry HCl(19.0 mmol) introduced into the cold (ca. -70 "C) suspensionwith a syringe through a rubber stopper. The colour of thesuspension turned immediately to red and the temperature ofthe mixture was allowed to rise slowly; at c a -4O"C, avigorous evolution of gas (dihydrogen and CO) took place.After ca.1 h, when the temperature was substantially atambient, the solvent was evaporated under reduced pressureand the solid residue was treated with dichloromethane (100cm3). The red-brown solution was concentrated to ca. 10 cm3and n-heptane was added, which caused the separation of thebrown compound (1.1 g, 58% yield). Analytical and spectro-scopic data are in Tables 1 and 2, respectively.The corresponding tantalum derivative was preparedsimilarly in 50% yield.Preparation of "a( t hf ),] [ Nb,C13( CO),]. The hexacar-bonylniobate( 1 -) anion, as Na[Nb(CO),]-0.25thf (1.903 g, 6.3mmol), was dissolved in thf (80 cm3) and treated with dry HCl(12.4 mmol) at dry-ice temperature.By slowly raising thetemperature, at -40°C the colour turned to red-brown andevolution of gas (dihydrogen and CO) was observed. Thesolution, maintained at room temperature for ca. 30 min, wasfiltered. After partial evaporation of the solvent (to ca. 5 cm3)under reduced pressure, n-heptane (100 cm3) was added and theochre solid so obtained was collected by filtration and dried inuucuo (58% yield).The similar treatment of Na[Ta(CO),]-thf in thf did not leadto the isolation of a pure product by operating with either HC1990 J. CHEM. soc. DALTON TRANS. 1985Taw 1. Analytical data for carbonyl derivatives of niobium(1) and tantalum(1)(a) Halide complexesAnalysis a (%)fC H28.6 (29.0) 2.5 (2.6)27.2 (28.1) 2.4 (2.4)50.0 (50.1) 2.8 (2.9)27.6 (27.5) 2.0 (1.5)22.7 (22.9) 1.9 (2.0)42.1 (42.9) 2.5 (2.5)(6) Arene complexesco Halogen Other33.2 (33.9) 16.2 (16.1)32.1 (32.1) 14.5 (15.6)33.4 (33.3) 35.4 (35.6)27.1 (27.5) 46.1 (46.8)10.1 (11.1) N: 1.2 (1.2)42.3 (42.6)26.2 (26.7) 8.4 (8.5)N: 1.2 (1.3)IAnalysis a (%)1 co Halogen13.0 (12.7) 56.8 (58.5)12.6 (12.6) 56.9 (57.6)12.6 (12.3) 55.3 (56.7)11.8 (12.0) 54.2 (55.0)1 1.4 (1 1.7) 54.0 (54.0)11.5 (11.6) 51.7 (53.2)11.3 (11.4) 50.9 (52.4)11.1 (11.2) 50.9 (51.7)10.9 (10.9) 50.2 (50.3)blw x Equivalents of halogen ( C x 79.9 + 35.45)a Calculated values are given in parentheses.7 x g of sampleor HI. The i.r. spectrum in the carbonyl stretching region isreported in Table 2.Attempts to isolate the compound bypartial evaporation of the solvent under reduced pressurefailed, due to interaction of the octacarbonyl complex with thf,presumably uia CO dissociation.(6) Oxidation of [Nb(CO),] - by Halogens: preparation ofNa[Nb,X,(CO),] (X = C1, Br, or I). A solution of PhI-CI,'(0.210 g, 0.76 mmol) in thf (30 cm3) was treated at ca. -60 "Cwith Na[Nb(CO),].1.8thf (0.302 g, 0.74 mmol). By warmingup to room temperature, a brown-orange suspension wasobtained, which was stirred for ca. 10 min and then filtered.Partial evaporation of the solvent under reduced pressure andaddition of n-heptane (30 cm3) gave an orange-brown solid,which was collected by filtration and dried in vacuo (23% yield),and identified by its i.r.spectrum and elemental analysis asNa[N b ,C1 3(Co)8] 00.4 thf. The similar treatment of Na [Ta-(CO),]-1.7thf (0.438 g, 0.88 mmol), suspended in n-pentane (50cm3) at -60 "C, with PhI-Cl, (0.250 g, 0.95 mmol) gave abrown suspension after stirring for 20 h at room temperature.The solid was filtered off and its i.r. spectrum in dichloro-methane solution showed the bands (2 013m and 1 891vs cm-')typical of the [Ta2C1,(CO),] - anion (see Table 2).The hexacarbonylniobate( 1 - ) anion, as Na[Nb(CO),].1.9thf (0.555 g, 1.32 mmol), was suspended in n-pentane (50cm3) and treated dropwise with bromine (1.41 mmol) dissolvedin the same solvent (25 cm3) at about - 78 "C. The colour of thesuspension changed rapidly from yellow to brown.The solventwas removed under reduced pressure and the brown residuewas treated with dichloromethane to dissolve the dimericNa[Nb,Br,(CO),]. After filtration and reduction to smallvolume, the dimeric complex was precipitated by addition ofn-heptane, and dried in uacuo (35% yield). The compound wasidentified spectroscopically and analytically (Br, CO). Byoperating in the same manner with I, in n-pentane, the dimerNa[Nb,I,(CO),] was obtained in 39% yield and identifiedspectroscopically and analytically (I, CO).Preparation of Bis( tripheny1phosphine)iminium Derivatives ofthe [M,Cl,(CO),]- (M = Nb or Ta) Anions.-The sodiumderivative [Na(thf),][Nb2C1,(CO),] (0.40 g, 0.58 mmol)dissolved in dichloromethane (10 cm3) was treated with 0.401 g(0.70 mmol) of [N(PPh,),]CI.The brown-orange solution wasstirred for ca. 30 min and then, after partial evaporation of thesolvent under reduced pressure, diethyl ether was added toprecipitate the orange-brown [N(PPh,),] + derivative, whichwas collected by filtration and dried in uacuo (0.43 g, 70% yield).The tantalum derivative was similarly obtained by treatmentof [H(thf),][Ta,CI,(Co),] with the stoicheiometric amountof [N(PPh,),]CI in dichloromethane (76% yield).Collection and Reduction of X-Ray Data for the Complex[H(thf),][Nb,CI,(CO),].-A crystal of approximate dimen-sions 0.31 x 0.23 x 0.15 mm was sealed in a glass capillaryunder argon and mounted on a Philips PW 1 100 four-cycleautomatic diffractometer. The crystals are triclinic, Cl,H $13-Nb,O,, M = 661.5; lattice parameters, as resulting from theleast-squares method applied to the angular setting values of 25reflections, are a = 16.283(3), b = 9.293(2), c = 9.050(2) A,F(OO0) = 652.The space group P f , assumed on the basis of theintensity statistics, was later confirmed by the structuralanalysis. A cell content of two formula units yielded a calculateddensity of 1.792 g cm-,; experimental measurement of thedensity was not possible due to the low stability of thecompound towards air and moisture.The intensity data were measured with graphite-mono-chromated Mo-K, radiation (h = 0.710 69 A). Scans of 1.6"(0)were made in the m-28 mode, at a speed of 0.05" s-l. A periodica = 113.01(2), f3 = 96.56(2), y = 98.39(2)", U = 1224.7 A3J. CHEM.SOC. DALTON TRANS. 1985 1991Table 2. Infrared spectroscopic data for metal(1) complexes of Group 5A in the carbonyl stretching region(a) M'[M,X,(CO),] complexesvco/cm-'I AM'HHHHHHNaNaNaNaNaNaNaNaNaNaNaNaNaN(PPh3)2cN(PPh 3) 2N(PPh3)2N(PPh3)2NaNaNaNaNaXC1c1C1C1C1CICIC1C1C1C1BrBrBrBrIIIIC1c1c1C10,CMe0,CMe0,CMeOMeOMeMediumTolueneE t 2 0thfCH,CI,AcetoneTolueneE t 2 0t hfCH,Cl,TolueneEt,Ot hfCH,CI,TolueneEt,OthfCH,Cl,t hfCH,CI,Acetonet hfCH2CI,t hfAcetoneH2OH2OEt2O(C2C1F3)~M = Nb2023, 19102007,19072 014, 19042015, 1910--2 023, 19102 008,19072007,19062017, 19092 022, 1 9022 022, 1 9132 019, 1 9122019m, 1924w, 1888vs2 023, 1 9092020, 19132018, 19132 014m, 1 930w, 1 895vs2016, 19072012, 19102007,19042012, 19122008, 18792 015, 1 8922010, 18731 987, 1 8621988, 1 863-M = V2 027, 1933(c) [ M(q 6-arene)(CO)4][AI,Br6Cl]Arene M = VC6H5Me -- C6H4Me2-pC,H,Me3-1,3,5 2 065, 1 986fC,H,Me4-1,2,4,5 2 063, 1 987/C6Me6 2 062, 1 9891M = Nb2031, 1935M = Nb2 073, 1 992'2071, 19902 068, 19842 065, 19822057, 1973M = Ta2010, 18952 005, 1892b2014, 18952017, 18902 OOO, 1 870--b.----------2005, 1895b2010, 18932 OOO, 1 874-----M = Ta2 025, 1 922M = Ta2 069, 1 9812 069, 1 9802063, 19742061, 19722055, 1964a The relative intensities of the two bands are medium and very strong respectively.The complex reacts with the medium. In n-heptane. Indichloromethane, in CaF, cells. Not isolated. As [V(CO),] - derivative.check of three standard reflections showed no significant inten-sity variation. A total of 2 271 independent reflections weremeasured in the range 2 6 8 6 20"; 506 reflections havingI d 3a(I) were rejected.* 1 765 Reflections were corrected forLorentz-polarisation and absorption effects and used insubsequent calculations. The absorption correction (p = 12.2cm-' for Mo-K,) was applied according to a semiempiricalmethod* based on the w scans of some reflections; correctionfactors on the intensity were in the range 1 .00-1.35.The structure was solved by Patterson and Fourier methodsand refined by the full-matrix least-squares program of theSHELX-76 package.' The C-0 carbonyl distance was con-strained to 1.16 k 0.03 A.Anisotropic thermal parameters wererefined for Nb, C1, and carbonyl oxygens. The H atoms of thf* Standard deviations on intensities were computed as ~ ( l ) = [ P +0.25(T,/T,J2(B, + B,) + (0.02/)']4, where P is the total peak countin a scan of time Tp, B, and B, are the background counts each in atime TB, and I is the intensity, equal to P - O.S(Tp/TB)(B, + B,).were introduced at calculated positions and were refined main-taining the geometry of the methylene groups (C-H 1.08 A).Their common isotropic thermal parameter refined to U =0.16(1) A'. In the final stages of the refinement (when R =0.05), a difference Fourier map revealed a residual electronicdensity maximum of 1.5 e ik3, located at cu.1 A from theoxygen atom of a thf unit. This position was assigned to theresidual H atom of the chemical formula, and its contributionwas included in the calculation of the last cycle, by constrainingthe O-H distance to 1.00 f 0.05 A. The final R value [R =E(Fo - Fc)/EFoJ decreased to 0.046 for 1765 reflections and191 parameters fR' = 0.059; the quantity minimised in theleast-squares refinement was Zw(F, - FJ2, where u' =l/[o(Fo) + aF2]; a assumed the value 0.0308 in the last cycle}.The maximum residual density on the difference map was 0.7 eA -3. Atomic scattering factors corrected for real and imaginaryparts of anomalous dispersion were taken from the library ofthe SHELX-76 system of programs for C1, 0, C, and H, andfrom ref.10 for Nb. Table 3 gives the fractional atomic co1992 J. CHEM. SOC. DALTON TRANS. 1985Table 3. Fractional atomic co-ordinates for non-hydrogen atoms of [H(thf)2][Nb2CI,(CO)8] with estimated standard deviations in parenthesesXi00.2262( 1 )0.2714(1)0.2681( 1)0.1385(1)0.3368( 1)0.2087( 1 )0.0423( 1)0.3607( 1)0.2004( 1 )0.2951(1)0.1359(1)0.2946( 1)0.4560( 1)0.2139( 1)0.1070( 1 )0.31 30( 1)Ylh1.2441(1)0.9093( 1 )0.9758( 1)1.0365( 1 )1.21 37( 1)1.2543( 1 1 )1.2970(11)1.538 1 ( 10)1.5731(11)0.5763( 11)0.6170(11)0.8969( 13)0.8539( 14)1.25O4( 13)1.2767( 13)1.4298( 15)z j c0.2046( I )-0.1294( 1 )0.1775(1)- 0.0800( 1 I0.0160( 1)0.5560( 10)0.2747( 1 1)0.4862( 12)0.1922( 1 3)-0.1210(11)-0.4120(11)-0.4833( 10)-0.1902( 12)0.4275( 15)0.2498( 14)0.3853( 14)x / u0.2067( 1 )0.2849( 1 )0.1 8 17( 1)0.2845( 1 )0.392 1 (1)0.0490( 1 )0.0589( 11)-0.0188(1)- O.O096( 1 3)- 0.0545( 10)0.4893( 10)0.5505( 10)0.5347( 1 1)0.4737( 1 )0.4442( 1 1 )Yjh1.45 18( 15)0.698 1 (1 5)0.7220( 15)0.9043( 14)0.8759( 15)0.624 1 ( 10)0.7658( 14)0.8533(20)0.7724(24)0.6272(20)0.8667( 17)0.771 l(18)0.6368(21)0.65O0( 18)0.7964(22)Zlc0.1905( 14)-0.125 1 (14)- 0.3094( 15)-0.3559(15)- 0.1635( I 5)0.0732( 1 1)0.1218(14)0.2993( 20)0.3527(25)0.2143(19)0.4297( 18)0.4498( 18)0.2844(21)0.1 724( 18)0.2539(2 1 )Table 4.Relevant boRd distances (A) and angles (") in [H(thf),]-[Nb,Cl,(CO),] with estimated standard deviations in parenthesesNb(1)-Cl(1)Nb( 1)-C1(2)Nb( 1)-Cl(3)Nb(2)-CI( 1)Nb(2)-C1(2)Nb(2)-CI( 3)Nb( 1)-C( 1)C1( 1)-Nb( 1)-C1(2)CI( 1)-Nb( 1)-C1(3)C1(2)-Nb( 1)-Cl(3)C( 1 )-N b( 1 )-C1( 3)C( 1 )-Nb( 1 jCl(2)C( 1 )-Nb( 1 jCl(3)C( 1 f N b ( 1 )-C(2)C(ltNb(l)-C(3)C( l)-Nb(l)-C(4)C( 2)-N b( 1 )-Cl( 1)C(2)-Nb( 1 jCl(2)C(2)-Nb( 1)-Cl(3)C(a-Nb( lFC(3)C(2)-Nb( 1 K ( 4 )C( 3)-N b( l)-Cl( 1)C(3)-Nb( 1)-C1(2)C(3)-Nb( ljCl(3)C(3FNW 1 W ( 4 )C(4)-Nb( 1 )-Cl( 1)C(4)-Nb( 1 )-Cl(2)Nb(l)-CI(l)-Nb(2)Nb( l)-C1(2)-Nb(2)Nb(l)-C1(3)-Nb(2)C(4)-N b( 1 ) X I ( 3)2.608(3) Nb( 1)-C(2) 2.069( 11)2.620(3) Nb( 1)-C(3) 2.051(12)2.592(3) Nb( 1)-C(4) 2.050( 12)2.607(3) Nb(2)-C(5) 2.028( 12)2.614(3) Nb(2)-C(6) 2.077(12)2.600(3) Nb(2)-C(7) 2.069( 12)2.021(21) Nb(2)-C(8) 2.073(12)76.4( 1)76.9( 1 )77.2( 1)79.6(3)127.3(3)140.1(3)69.6(4)69.3(4)110.7(4)I18.5(3)82.0( 3)150.3(3)1 07.8 (4)70.7(4)108.8(3)163.4(3)88.5(3)72.1(4)168.7(3)99.6( 3)91.9(3)88.2( 1)87.8( 1)88.8( 1)76.5(1)76.8( 1)77.2( 1)80.1(3)1 26.1 (3)141.7(3)69.2(4)109.6( 5)70.9(5)119.5(3)8 1.5(3)149.1 (3)70.6( 4)110.2(5)168.6(3)101.1(3)91.8(3)72.3(4)106.7( 3)162.7(3)86.9(3)179(1)179(1)178( 1)175(1)178(1)176(1)177(1)176(1)partial decomposition (CO evolution) was observed.Theaqueous solution was extracted with diethyl ether and the etherlayer was evaporated to dryness. The solid residue was dis-solved in thf (3 cm3) and the acetato-complex, which was thenprecipitated by addition of n-heptane (50 cm3), was collected byfiltration and dried in vacuo (30% yield). Carboxylic vibrationswere observed in (C2C1F3), at 1 556 and 1 466 cm-'.Preparation of the p-Methoxo Derivative, Na[Nb,(OMe),-(CO),] .-The p-C1 derivative [ H(t hf )2] [Nb,CI ,(CO),] (0.4 10g, 0.62 mmol) was treated at ca. - 50 "C with sodium methoxide(0.171 g, 3.17 mmol) in thf (50 cm3). The colour changed frombrown-yellow to red and after ca. 30 min the reaction was over.The reaction mixture was filtered at room temperature, thesolution was concentrated to ca.10 cm3 by evaporation underreduced pressure, and n-heptane (30 cm3) was added to pre-cipitate the p-methoxo-derivative, which was collected by fil-tration and dried in vacuo (0.232 g, 71% yield). Analytical andspectroscopic data are in Tables 1 and 2, respectively.Reduction of [M,CI,(CO),] - with Sodium.-The sodiumderivative [Na(thf),][Nb,CI,(CO),] (0.308 g, 0.45 mmol)dissolved in thf (50 cm3) was treated with sodium sand (0.052 g,2.2 mmol) under an argon atmosphere. Within a few minutes ayellow solution was formed, together with some black solid.The latter was filtered off and from the solution sodiumhexacarbonylniobate( 1 -) was obtained in 69% yield, afterconcentration and cooling to dry-ice temperature, followed byfiltration and drying in vacuo.In the case of the corresponding tantalum derivative,[H(thf),][Ta,Cl,(CO),], the reaction occurred similarly andthe hexacarbonyltantalate( 1 - ) was isolated in 80",, yield, byoperating under an atmosphere of carbon monoxide.Thereduction in the case of niobium was shown to operate undercarbon monoxide, with increased yields with respect to thoseobtained under argon.ordinates with estimated standard deviations. Relevant inter-atomic bond distances and bond angles are in Table 4. TheFigure shows the [Nb,CI,(CO),] - anion with the atomnumbering scheme used.Preparation of the p- A cetato Derivative, Na[N b2( 0 ,CM e) ,-(CO),] .-The sodi um derivative [ Na( t h f )J [ N b2C1 ,( CO), J(0.158 g, 0.23 mmol) was treated with an aqueous H0,CMe-0,CMe- buffer solution (2 mol dm-3, 5 cm3).Dissolution withReaction of [M,Cl,(CO),] - with Lithium Cyclopenta-dienide.-The sodium derivative [Na(thf)2][Nb2C13(CO),1(0.832 g, 1.22 mmol) in thf (50 cm3) was treated at 0 "C withlithium cyclopentadienide (0.342 g, 4.75 mmol) under an argonatmosphere and the resulting solution was stirred for ca. 1 h,while the temperature was slowly raised. After evaporation ofthe solvent under reduced pressure, the residue was sublimed at105 "C (ca. mmHg) obtaining 0.60 g (92% yield) ofThe corresponding tantalum derivative [Ta(q5-C,H5)-W" '-C5H5)(CO)4IJ. CHEM. SOC. DALTON TRANS. 1985 1993(CO),] was similarly obtained in 80% yield by sublimation ofthe solid residue from the treatment of [H(thf),][Ta,Cl,-(CO),] with Li(C,H,) in a 20% excess with respect to therequired stoicheiometry.The i.r. data in the carbonyl stretchingregion of the [M(q'-C,H,)(CO),] complexes are in Table 2.Preparation of the [M(CO),(q6-arene)] + (M = Nb or Ta)Cations.-The cations were prepared from the [M,Cl,(CO),] -anions by reaction with AlBr, in the neat aromatic hydro-carbon, or in dichloromethane as solvent in the case of solidaromatic hydrocarbons, at room temperature.(a) Neat aromatic hydrocarbon. The octacarbonyl derivativeof niobium(I), [H(thf)2][Nb,Cl,(CO)8] (0.618 g, 0.93 mmol)was reacted with 1,3,5-trimethylbenzene (30 cm3) under carbonmonoxide at atmospheric pressure in the presence of AlBr,(1.534 g, 5.75 mmol). The colour of the suspension becamerapidly dark upon mixing the reagents and an oily brown layerseparated out.The mixture was stirred overnight at roomtemperature, then filtered, and the solid product on the filterwas washed with the aromatic hydrocarbon. It was thendissolved in dichloromethane (50 cm3); the solution, afterfiltration, was concentrated to small volume (10 cm3) byevaporation under reduced pressure and n-heptane (ca. 50 cm3)was added to complete the precipitation of the red arenecomplex. The latter was recovered by filtration and drying invacuo (32% yield). The preparations of the p-xylene derivative(M = Nb) and of the toluene, p-xylene, and 1,3,5-trimethyl-benzene (M = Ta) complexes (see Table 1) were carried outsimilarly.(b) In dichloromethane solution. The niobium derivative[H(thf)2][Nb2C1,(CO),] (0.680 g, 1.03 mmol) dissolved indichloromethane (50 cm3) was treated at 0 "C with AIBr, (1.49g, 5.58 mmol) and 1,2,4,5-tetramethylbenzene (1.79 g, 13.4mmol).The temperature was slowly allowed to rise, and afterca. 30 min at room temperature the i.r. spectrum of the solutiondid not show any absorption due to the starting niobiumderivative. The red arene complex (only partially soluble indichloromethane) was present as a precipitate; the latter wasfiltered off, washed with dichloromethane, and dried in uacuo(58% yield). By similar procedures were prepared the 1,2,4,5-tetramethyl- (M = Ta) and hexamethyl-benzene derivatives(M = Nb or Ta) reported in Tables 1 and 2.The arene complexes are very sensitive to both air andmoisture.They have little solubility in saturated hydrocarbons,and with aromatic hydrocarbons they usually give a bilayersystem; they are slightly soluble in halogenated solvents but theless methyl-substituted derivatives rapidly react with them.Thermal decomposition occurred more easily as the degree ofmethyl substitution on the ring decreased. The 1,3,5-trimethyl-benzene derivative [Nb(q6-C,H,Me,-l,3,5)(CO),][A12Br,Cl]was heated at ca. 70 "C (ca. lo-, mmHg): rapid decompositiontook place under these conditions, and the aromatic hydro-carbon was recognised as one of the volatile products by i.r.spectroscopy.Arene Exchange Reactions.-Due to the low solubility of thecomplexes in other solvents, the exchange reaction of the arenederivatives was studied in dichloromethane: competitive re-action with the solvent was generally present, except in the caseof the more stable hexamethylbenzene complex.The 1,3,5-trimethylbenzene-niobium complex (0.121 g, 0.135 mmol) wasdissolved in dichloromethane (10 cm3) and hexamethylbenzene(0.220 g, 1.36 mmol) was added to the solution, while thetemperature of the stirred solution was maintained at 20.0 "C inthe dark. After 5 h, the i.r. carbonyl bands of the startingcomplex had completely disappeared and the new bands of thehexamethylbenzene derivative had appeared (see Table 2). Thenewly formed hexamethylbenzene complex was precipitated byaddition of n-heptane and found to analyse correctly for theformulation expected, namely [N b(q 6-C6 Me,)(CO),] -[A1 2 Br6Cl].Results and DiscussionThis paper deals with the treatment of the hexacarbonyl-metalates(1 -) of niobium and tantalum with protons and withthe discovery that they undergo a two-electron oxidation tocarbonyl derivatives of niobium(1) and tantalum(I), equation(1).Carbonylmetalates may perform, at least in principle,several modes of reactivity with protons, as follows. (a) Attackat the metal with formation of metal carbonyl hydrido-complexes. This is normally the case of complexes containingstrong metal-hydrogen bonds with weak acidity in water,'such as [MnH(CO),] being prepared by protonation" of[Mn(CO),]-. (b) Base-catalysed proton transfer, such as in thecase of the acidification of [Co(CO),] - and [v(Co),] -, in thepresence of nitrogen bases" or water,' to give [NHR,]+-[co(Co),] - and [H,O] +[V(CO),] -, respectively.This istypical of protonated carbonylmetalates behaving as strongacids in water. In this connection, it is interesting to note that[MnH(CO) '1 in hydrocarbon solution is unaffected by tertiaryamines,' contrary to [CoH(CO),]. l 2 (c) One-electron transferto proton, such as in the protonation of [v(Co),]- with dryHCl in anhydrous ethers or hydrocarbon solvents, under whichconditions H, and [v(Co),] are the products.' ( d ) Proton-ation of the carbonyl oxygen, especially in the case of triplybridging or doubly bridging ' ' carbonyl groups. (e) Proton-ation of the carbonyl carbon to give a formyl group, M-C(O)H,a case still unobserved.Reaction (1) was carried out both under anhydrous con-ditions and in aqueous solution, making sure that the pH wasnot too low.An appropriate medium is a 1 mol dm-3 solutionof pyridine and HCl (1 : 1) (hydrolytic pH corresponding to 2.7pH units).Strictly connected with the problem of the two-electrontransfer process is the lability of the intermediate arising fromthe primary attack of the proton to [M(CO),]-. It has alreadybeen pointed out in earlier publications ' that acidification ofanhydrous and solvent-free Na[V(Co),] leads to the formationof a thermally unstable precursor to [V(CO),], (C,HO,V),which rapidly evolves dihydrogen, according to equation (2).In the case of niobium and tantalum, the second electrontransfer to proton is presumably taking place from a hydrido-complex (not isolatec! or even observed) reacting with HCl,equations (3) and (4).The main difference between the{C,HO,M) + HCl-H, + {MC1(CO)6) --+ Products (4)vanadium system and niobium and tantalum is that, while withvanadium the intermediate arising from the proton attack to[v(co)6] - is not hydridic in n a t ~ r e , ' ~ the correspondingproducts of niobium and tantalum may have a certain hydridiccharacter, such that they undergo fast attack by excess HCl.Consistent with this interpretation is the general qualitativeobservation 5b that the stability of metal carbonyl hydrido-complexes increases by descending a vertical sequence o1994 J.CHEM. SOC. DALTON TRANS. 198501CI 12) 16)Figure. View of the [Nb,CI,(CO),] - anion along a direction approxi-mately perpendicular to the Nb-Nb vectorelements, although quantitative data concerning this point arestill lacking.*A second method of preparation of the dinuclear anions ofniobium(1) and tantalum(1) is the oxidation of the hexacar-bonylmetalates with halogens, according to the stoicheiometryof equation ( 5 ) (X = C1, Br, or I). The chloro-carbonyl of2[M(CO)6]- + 2x2 - [M,X,(CO),]- + X- + 4CO (5)niobium(I), [H(thf)2][Nb2C1,(Co)8], has been studied byX-ray diffraction methods. The Figure shows the molecularstructure of the anion. The two niobium atoms of the dimer arebridged through three chlorides located at the vertices of anapproximately equilateral triangle, on a plane perpendicular tothe Nb-Nb vector.This plane is a nearly perfect mirror planerelating the two halves of the complex. Another pseudo-mirrorplane perpendicular to this one passes through C1( l), two Nbatoms and four carbonyl groups [C( l), C(4), C(5), and C(7) andthe oxygen atoms linked to them]. An approximate two-foldaxis passing through Cl(l), along the intersection of theseplanes completes the idealised C2, symmetry assumed by theanion. The Nb Nb non-bonding separation is 3.631(1) A.The niobium-chloride distances average 2.607 A, in the range2.592(3)-2.620(3) A. The C1-Nb-Cl angles average 76.8", inthe range 76.4( 1 )-77.2( 1)'. The mean Nb-C1-Nb angle is 88.3'.Each Nb atom is bonded to four carbonyl groups. The meanniobium-carbon distance is 2.05 A, in the range 2.02(1)-2.08(1) A, close to the values found2*I6 in other carbonylderivatives of niobium.The packing may be described as astacking of alternate layers of dimers and tetrahydrofuranmolecules parallel to the (100) plane.The i.r. spectra in the carbonyl stretching region (see Table 2)are in agreement with the C,, symmetry of the tetracarbonylmoiety of the dimeric anion, for which two i.r.-active bands(A, + E) are expected. As mentioned above, the dimeric anionhas approximately C,, symmetry, but this should require fivebands (3A, + B , + B2), thus suggesting that coupling of the* Pertinent to this point are the quantitative data concerningdeprotonation of metal carbonyl hydrides as a function of the metal invertical sequences of the Periodic Table: B.V. Lokshin and A. A.Pasinskj, J . Organomet. Chem., 1973, 55, 315; D. C. Harris and H. B.Gray, Inorg. Chem., 1974,14,1215; A. Nakamura and S. Otsuka, J. Mol.Catal., 1976, 1, 285; H. W. Walker, C. T. Kresge, P. C. Ford, and R. G.Pearson, J. Am. Chem. Soc., 1979, 101, 7428; R. F. Jordan and J. R.Norton, J . Am. Chem. Soc., 1982, 104, 1255; 'Mechanistic Aspects ofInorganic Reactions,' A.C.S. Symposium Series No. 198, 1982, pp.4 0 3 4 2 3 ; R. G. Pearson and P. C. Ford, Comments Inorg. Chem., 1982,1,279; H. W. Walker, R. G. Pearson, and P. C. Ford, J. Am. Chem. Soc.,1983, 105, 1179.vibrations along the chloride bridges is virtually non-existent.The halide bridge of the niobium dimer can be substituted byother bridging ligands (Y), such as 0,CMe and OMe, equation(6). Based on the i.r.spectra in solution (see Table 2), the same[Nb,X,(CO),]- + Y- __+ X- + [Nb,Y,(CO)J- (6)overall geometry as that of the halide complexes is assigned tothese compounds.The niobium(1) dinuclear compounds appear to be morestable than the corresponding tantalum derivatives. Yields ofthe latter are usually lower and carbonyl substitution wasnoted in solvents such as tetrahydrofuran. Spectral changes inthe carbonyl stretching region were observed under argon for[Ta,Cl,(CO),] - in thf, which could be reversed, althoughincompletely, under carbon monoxide.The halide complexes of niobium(1) and tantalum(1) arereadily reduced by sodium in thf to give the hexacarbonyl-metalate, equation (7).The tendency to form the hexacarbonyl-metalate under these conditions is so high that even in theabsence of CO, ligand redistribution occurred with partialdecomposition to unidentified low-valent metal complexes,[NbLJ-, equation (8).3[Nb2X3(CO)8] - + 12Na4Na[Nb(CO),] + 8NaX + X - + 2[NbL,]- (8)The niobium(1) and tantalum(1) halide derivatives are thefirst halide carbonyl complexes of Group 5A elements to bereported in the literature and they are useful for the preparationof other complexes of these metals. The halide ligands can bedisplaced, for example, by the cyclopentadienide anion and thecorresponding q5-C,H, complexes were obtained, equation (9).The q '-cyclopentadienyl derivatives are usually prepared 3,4 bythe reductive carbonylation at high temperature and pressure ofhalidesyclopentadienyl complexes in good (Nb) or moderate(Ta) yields.Since the hexacarbonylniobate(1 -) anion can beprepared at atmospheric pressure of CO and room temperature,reaction (9) can be suggested as an alternative route to thecyclopentadienyl derivative. The i.r. spectra of the [M(q5-C,H,)(CO),] complexes (Table 2) show that the three q5-cyclopentadienyl derivatives are isostructural with a C40 localsymmetry (A, + E) of the four carbonyl groups. The crystaland molecular structure of [V(q 5-C,Hs)(CO),] has appeared.' 'Moreover, the wavenumber values of the carbonyl stretchingvibrations decrease in the sequence Nb > V > Ta.This istypical of families of compounds in a vertical sequence ofmetals, in which the 4d element is believed to have the lowestdegree of II back donation.Halide displacement by aluminium bromide was found topromote the formation of the q4-arene-tetracarbonyl com-plexes, equation (10). Comparison of the i.r. spectra of the arene[M2x,(co),] - + 4AlBr, + 2(arene) - 2[M(q 6-arene)(CO),] CAI, Br,X] + X - (1 0)complexes of the three metals (V, Nb, Ta) shows again that thecompounds are isostructural. The crystal and molecularstructure of one of the vanadium(1) derivatives was recentlJ. CHEM. SOC. DALTON TRANS. 1985 1995solved l 8 and shown to contain the planar six-membered ringsymmetrically bonded to vanadium together with the fourcarbonyl groups in an arrangement of local C,, symmetry. Thei.r.spectra in solution of the niobium and tantalum derivativesare in agreement with this geometry. Again, also in this case,similar to the cyclopentadienyl complexes, the 4d element hasthe highest wavenumber value of the CO stretching vibrations.While the q6-arene cationic complexes of vanadium(1) areproduced ’ 8*19 by disproportionation of [v(co)6], the unavail-ability of the zero-valent niobium and tantalum carbonylcomplexes made the A1Br3-[M2X3(CO)8] - system the onlypossible route to the q6-arene complexes of these metals. Thelatter complexes show a rather low stability, both thermal andchemical. For example, heating the compounds at ca. 70 “C inuacuo led to complete decomposition with evolution of CO andarene.The arene ligands are also readily displaced by oxy-genated solvents and halide ions: e.g. as in equation (11).2[M(q6-arene)(C0),]+ + 3X- ---[M2X3(CO)8] - + 2(arene) (1 1)The formation of the dinuclear halide complexes wasobserved spectroscopically. The q 6-arene ligands are so labilewith respect to their displacement by halides that reaction (1 1)occurred even during spectral measurements upon contact withthe KBr windows of the i.r. cells. The decomposition by theoxygenated solvents is also presumably induced by the X-anions resulting from the interaction of the solvent with the[Al,Br,Cl] - counteranion of the complexes. Attempts tosubstitute the [Al,Br,CI]- anions with others, such as[BPh,] -, [PF,] -, or [v(co)6] - failed, because of unfavour-able solubility properties of the reagents.The arene complexes reported in this paper are the firstarene-arbonyl complexes of niobium and tantalum to bereported in the literature. However, some q6-arene derivativesof these metals, both mononuclear and polynuclear have beenreported previously, containing one arene per metal atom 2oand two arenes per metal atom.21 The i.r.data of Table 2 showa decrease of the carbonyl wavenumber values with increasingmethyl substitution on the ring. This is a well known”phenomenon for q6-arene carbonyl derivatives of Group 6Ametals, which is usually attributed to a higher basicity of theligand and to a corresponding higher degree of 7c back don-ation. Thus, increasing methyl substitution is expected toincrease the stability of the complex, which is substantiated bythermochemical measurements on [M(q6-arene)(C0),] com-plexes of Group 6A metals.23 Consistent with these data, ourarene complexes underwent arene exchange, the more methyl-substituted product being favoured, equation (1 2).Because of[M(q‘-arene)(CO),] + + arene’ ---+[M(q6-arene’)(CO),] + + arene (12)solubility problems, dichloromethane had to be used as solventof reaction (12) and some decomposition due to the solvent wasobserved with the less methyl-substituted products. In the caseof the more stable trimethyl- and hexamethyl-benzene com-plexes of niobium, the exchange reaction was exclusivelyobserved and C6Me6 was found to displace the trimethyl-benzene ligand completely.AcknowledgementsWe wish to thank the National Research Council (C.N.R.,Roma) through the Progetto Finalizzato di Chimica Fine eSecondaria for financial support of this work and Dr.R.Zamboni for elemental analyses.References1 F. Calderazzo and G. Pampaloni, J. Organomet. Chem., 1983, 250,c33.2 F. Calderazzo, U. Englert, G. Pampaloni, G. Pelizzi, and R.Zamboni, Inorg. Chem., 1983.22, 1865.3 R. B. King, Z. Naturforsch., Teil B, 1963, 18, 157; W. A. Herrmannand H. Biersack, Chem. Ber., 1979, 112,3942; J. Organornet. Chem.,1980,191,397; R. B. King and C. D. Hoff, J. Organomet. Chem., 1982,225, 245.4 R. P. M. Werner, A. H. Filbey, and S. A. Manastyrskyj, Inorg. Chem.,1964, 3, 298; E. Guggolz, M.L. Ziegler, H. Biersack, and W. A.Herrmann, J. Organomet. Chem., 1980, 194,317.5 (a) F. 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