Reactions of (h5-C9H7)Rh(h2-C2H4)2 withquinones: molecular structure of[(h5-C9H7)Rh(2,3,5,6-C6O2(CH3)4)]

Stephen A. Westcott, Nicholas J. Taylor, and Todd B. Marder

Abstract: Reactions of (η5-C9H7)Rh(η2-C2H4)2 (1) with quinones were investigated. Substitution of the labile ethyleneligands was observed upon addition of either duroquinone (2,3,5,6-tetramethyl-1,4-benzoquinone) or 1,4-benzoquinoneto complex1. The molecular structure of neutral (η5-C9H7)Rh(2,3,5,6-C6O2(CH3)4) (3), determined by X-ray diffraction,shows that the duroquinone ligand lies in a plane nearly parallel to the indenyl group. The carbonyl moieties ofduroquinone lie in a plane incorporating Rh, C2, and the midpoint between C3a and C7a of the indenyl ring. The slipparameter (∆ = d(average Rh-C3a,7a) –d(average Rh-C1,3)) was calculated to be 0.112(2) Å; whereas a value of ca.0.05 Å had been obtained previously from film data. Values for the hinge angle (HA = angle between normals to theleast-squares planes defined by C1, C2, C3 and C1, C7a, C3a, C3) and fold angle (FA = angle between normals to theleast-squares planes defined by C1, C2, C3 and C3a, C4, C5, C7, C7a) are 7.2° and 4.0°, respectively.

Key words: indenyl, rhodium, quinones, ring-slippage, ground-state distortion.

Résumé: On a étudié les réactions du composé (η5-C9H7)Rh(η2-C2H4)2 (1) avec les quinones. On a observé lasubstitution des ligands éthylène labiles lors de l’addition de l’éther duroquinone (2,3,5,6-tétraméthyl-1,4-benzoquinone)ou de la 1,4-benzoquinone sur le complexe1. La structure moléculaire du composé neutre(η5-C9H7)Rh(2,3,5,6-C6O2(CH3)4) déterminée par diffraction de rayons X, révèle que le ligand duroquinone est lié dansun plan quasi parallèle au groupe indényle. Les unités carbonyle de la duroquinone sont liées dans un plan incorporantRh, C2 et le point milieu entre C3a et C7a du cycle indényle. On a trouvé un paramètre de glissement (∆ = (moyenneRh-C3a, 7a) – (moyenne Rh-C1,3)) de 0,112 (2) Å; alors qu’on a obtenu antérieurement une valeur d’environ 0,05 Å àpartir des données de film. Les valeurs de l’angle d’articulation (AA = angle entre le plan normal et celui desmoindres carrés défini par C1, C2, C3 et C1, C7a, C3a) et l’angle de pliement (AP = angle entre le plan normal etcelui des moindres carrés défini par (C1, C2, C3 et C3a C4, C5, C7, C7a) sont respectivement de 7,2° et de 4,0°.

Mots clés: indényle, rhodium, quinones, glissement de cycle, distorsion de l’état fondamental.

[Traduit par la Rédaction] Westcott et al.: 1 204

Indenylcobalt and rhodium complexes are known to dis-play enhanced catalytic activities compared to their cyclo-pentadienyl analogues for a number of important organicreactions (1). This rate enhancement, or indenyl effect, (2)occurs in both associative (SN2) and dissociative (SN1) reac-tion pathways and, for the associative process, has been at-tributed to a low-energy barrier associated with anη5→η3

indenyl ring slippage, where the metal centre avoids an un-favourable 20-electron transition state. The relative stabilityof theη3-intermediate arises from resonance stabilization ofthe benzene ring in the indenyl ligand (Fig. 1). Indeed, themolecular structures of severalη3-indenyl metal complexeshave been reported (2c, 3).

Interestingly,η5-indenyl d8-ML2 complexes all display acertain degree of ground-state distortion towards this allylicbonding mode within the five-membered ring of the indenylligand (4, 5). We have been examining the degree of thisdistortion as a function of the electronic nature of the ancil-lary ligands (4). While a significant amount of distortionwas found for (η5-C9H7)Rh(CO)2 (4d), an early report usingfilm data suggested that the slip-fold distortion in(η5-C9H7)Rh(2,3,5,6-C6O2(CH3)4) was unusually small (6),making this compound the least distorted of alld8-ML2-indenyl complexes. As the range of values was im-portant to us, this “limiting” value represented a criticalnumber, and it was thus decided to re-examine the structureof this compound. In this article, however, we report that thedegree of distortion in this latter quinone compound is con-

Can. J. Chem.77: 199–204 (1999) © 1999 NRC Canada

199

Received September 15, 1998.

S.A. Westcott,1,3 N.J. Taylor, and T.B. Marder. 2,3 Department of Chemistry, University of Waterloo, ON N2L 3G1, Canada.

1Current address: Department of Chemistry, Mount Allison University, Sackville, NB E4L 1G8, Canada.2Current address: Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, England.3Authors to whom correspondence may be addressed. SAW: Telephone (506) 364–2372. Fax: (506) 364–2313.e-mail: swestcott@mta.ca; TBM: Telephone (44) 191–374–3137. Fax: (44) 191–386–1127. e-mail: todd.marder@durham.ac.uk

siderably more pronounced than initially reported, thus re-ducing the range of values known to date.

General proceduresNMR spectra were recorded on Bruker WM250 (1H at

250 MHz, 13C{1H} at 63 MHz, 31P{1H} at 101 MHz), andNicolet NMC-300 (1H at 250 MHz,13C{1H} at 75.4 MHz,31P{1H} at 121 MHz) spectrometers.1H NMR chemical shiftsare reported in ppm relative to external TMS and were refer-enced to residual protons in the solvent; coupling constantsare in Hz. Multiplicities are reported as (s) singlet, (d) dou-blet, (t) triplet, (q) quartet, (m) multiplet, (br) broad, and(ov) overlapping. THF and hexane were freshly distilledfrom sodium benzophenone ketyl. Benzoquinone, duroqui-none, and TCNQ (Aldrich Chemical Co.) were used as re-ceived. (C9H7)Rh(η2-C2H4)2 (1) was prepared by establishedmethods (7).

Preparation of (η5-C9H7)Rh(C6O2H4 ) (2)(η5-C9H7)Rh(C6O2H4) was prepared by allowing 1,4-ben-

zoquinone (C6O2H4) (119 mg, 1.1 mmol) to react with(η5-C9H7)Rh(C2H4)2 (274 mg, 1 mmol) in THF at room tem-perature. After stirring for 12 h, the solvent was removed invacuo. The residue was washed with cold hexane (2 × 5 mL)and dissolved in THF/hexane (5/1). An orange–yellow prod-uct (271 mg, 83% yield) was obtained after cooling to–10°C. Selected spectroscopic data: IR (CH2Cl2, cm–1):1631 (s sh), 1614 (s), 1578 (m);1H NMR (CD2Cl2) δ:7.36–7.30 (m), 7.29–7.24 (m), 6.06 (d,J = 3 Hz), 5.74(q, JH–Rh5H–H = 3 Hz), 5.00 (s);13C{1H} NMR (CD2Cl2):160.6 (C5O), 128.9, 121.3, 107.5 (d,JC–Rh = 5 Hz), 90.8 (d,JC–Rh = 7 Hz), 82.1 (d,JC–Rh = 8 Hz), 78.6 (d,JC–Rh = 7 Hz)ppm.

Preparation of (η5-C9H7)Rh(2,3,5,6-C6O2(CH3 )4 ) (3)(η5-C9H7)Rh(2,3,5,6-C6O2(CH3)4) was prepared by allow-

ing duroquinone (2,3,5,6-C6O2(CH3)4) (180 mg, 1.1 mmol)

to react with (η5-C9H7)Rh(C2H4)2 (274 mg, 1 mmol) in THF atroom temperature. After stirring for 12 h, the solvent was re-moved in vacuo. The residue was washed with cold hexane(2 × 5mL) and dissolved in THF/hexane (10/1). Orange–yel-low crystals of the product (138 mg, 36% yield) were ob-tained after cooling to –10°C. Selected spectroscopic data:IR (CH2Cl2, cm–1): 1587 (s), 1426 (m), 1369 (m);1H NMR(CD2Cl2) δ: 7.63–7.59 (m), 7.39–7.35 (m), 5.91 (q,JH–Rh5H–H= 3 Hz), 5.68 (d,J = 3 Hz), 2.07 (s) ppm;13C{1H} NMR(CD2Cl2): 140.5 (C5O), 123.7, 119.5, 112.3, 91.9 (d,JC–Rh= 5 Hz), 78.8 (d,JC–Rh= 4 Hz), 43.8 (d,JC–Rh= 12 Hz), 12.3 ppm.

Molecular structure determinationCrystals of3 suitable for X-ray diffraction studies were

obtained by crystallization from a THF–hexane solution at–10°C. A summary of the crystal data and parameters fordata collection is given in Table 1. Data were collected atroom temperature on a Nicolet R3m/V diffractometer usinggraphite-filtered MoKα radiation (λ = 0.710 73 Å) and theωscan method. The data and refinement were performed on aVAX 3100 computer using Siemens SHELXTL (VAX) soft-ware.4 The structure wassolved using Patterson and

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200 Can. J. Chem. Vol. 77, 1999

Formula C19H19O2Rhfw 382.3Crystal system MonoclinicSpace group P21/ca, Å 7.932(2)b, Å 17.879(5)c, Å 11.074(3)β, deg 96.88(2)V, Å3 1559.1(8)Z 4ρcalcd, g cm–3 1.628Crystal size, mm 0.34 × 0.39 × 0.30Temperature, K 295Radiation MoKα (λ = 0.710 73)µ , cm–1 10.81Max 2θ, deg 60.0Data collection method ωScan speed, deg/min 2.93–29.30Scan width, deg 1.20Total unique reflections

measured4545

Total observed reflectionsa 3651Max./min. transmission factorb 0.550, 0.586No. of variables 276Final max.∆/σ 0.023Max. res. density/hole, e Å–3 0.41, –0.28Rc 0.0224Rw 0.0231GoFd 1.55

aFo > 6σ(Fo).bAbsorption correction byψ-scan semi-empirical method.cR = Σ||Fo| – |Fc||/Σ|Fo|; Rw = [Σ(w(|Fo| – |Fc|)

2/Σ(w|Fo|)2]1/2.

d[Σ(w(|Fo| – |Fc|)2/(NO – NV)]½.

Table 1. Crystallographic data collection parameters for(η5-C9H7)Rh(2,3,5,6-C6O2(CH3)4) (3).

η5

η3

M

L L

M

L L

Fig. 1. Two of the possible bonding modes in (η5-C9H7)ML 2

complexes.

4G.M. Sheldrick. SHELXTL-Plus. Version 4.21/v. Siemens Analytical X-Ray Instruments, Inc. Madison, Wisconsin, U.S.A. 1990.

Fourier techniques, andrefinement was performed usingfull-matrix least squares onF, with anisotropic thermal pa-rameters for all non-hydrogen atoms. The hydrogen atomswere located on the Fourier difference electron-density mapsand included in the structure model. The maximum residueand hole are proximate to the rhodium atom. Selected bond

distances and angles are given in Table 2, and final atomiccoordinates are given in Table 3. Complete tables of bonddistances and angles, anisotropic thermal parameters for thenon-hydrogen atoms, hydrogen atom positions, atomic coor-dinates, isotropic thermal parameters, crystal data, refinementparameters, and alternative views of3 have been deposited.5

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Westcott et al.: 1 201

Rh(1)—C(1) 2.164(4) C(4)—C(5) 1.329(7)Rh(1)—C(2) 2.207(4) C(5)—C(6) 1.408(7)Rh(1)—C(3) 2.191(4) C(6)—C(7) 1.347(8)Rh(1)—C(3A) 2.297(3) C(7)—C(7A) 1.422(5)Rh(1)—C(7A) 2.281(3) C(8)—C(9) 1.474(4)Rh(1)—C(8) 2.447(3) C(8)—C(13) 1.468(4)Rh(1)—C(9) 2.198(3) C(8)—O(14) 1.237(4)Rh(1)—C(10) 2.181(3) C(9)—C(10) 1.422(4)Rh(1)—C(11) 2.490(3) C(9)—C(15) 1.494(5)Rh(1)—C(12) 2.195(3) C(10)—C(11) 1.469(4)Rh(1)—C(13) 2.197(3) C(10)—C(16) 1.505(5)C(1)—C(2) 1.399(7) C(11)—C(12) 1.480(4)C(1)—C(7A) 1.443(5) C(11)—O(17) 1.230(4)C(2)—C(3) 1.420(5) C(12)—C(13) 1.412(4)C(3)—C(3A) 1.435(5) C(12)—C(18) 1.507(4)C(3A)—C(4) 1.412(5) C(13)—C(19) 1.501(4)C(3A)—C(7A) 1.432(4)

C(1)-Rh(1)-C(2) 37.3(2) C(1)-Rh(1)-C(11) 126.8(1)C(1)-Rh(1)-C(3) 63.2(1) C(2)-Rh(1)-C(11) 113.7(1)C(2)-Rh(1)-C(3) 37.7(1) C(3)-Rh(1)-C(11) 127.3(1)C(1)-Rh(1)-C(3A) 62.5(1) C(3A)-Rh(1)-C(11) 160.8(1)C(2)-Rh(1)-C(3A) 61.9(1) C(7A)-Rh(1)-C(11) 160.9(1)C(3)-Rh(1)-C(3A) 37.2(1) C(8)-Rh(1)-C(11) 72.0(1)C(1)-Rh(1)-C(7A) 37.8(1) C(9)-Rh(1)-C(11) 64.3(1)C(2)-Rh(1)-C(7A) 61.9(1) C(10)-Rh(1)-C(11) 35.9(1)C(3)-Rh(1)-C(7A) 62.0(1) C(1)-Rh(1)-C(12) 154.8(1)C(3A)-Rh(1)-C(7A) 36.4(1) C(2)-Rh(1)-C(12) 121.9(1)C(1)-Rh(1)-C(8) 136.2(1) C(3)-Rh(1)-C(12) 109.7(1)C(2)-Rh(1)-C(8) 173.1(1) C(3A)-Rh(1)-C(12) 127.9(1)C(3)-Rh(1)-C(8) 142.4(1) C(7A)-Rh(1)-C(12) 162.9(1)C(3A)-Rh(1)-C(8) 114.2(1) C(8)-Rh(1)-C(12) 65.0(1)C(7A)-Rh(1)-C(8) 111.5(1) C(9)-Rh(1)-C(12) 80.7(1)C(1)-Rh(1)-C(9) 109.7(1) C(10)-Rh(1)-C(12) 68.1(1)C(2)-Rh(1)-C(9) 141.1(1) C(11)-Rh(1)-C(12) 36.1(1)C(3)-Rh(1)-C(9) 168.3(1) C(1)-Rh(1)-C(13) 167.6(1)C(3A)-Rh(1)-C(9) 131.9(1) C(2)-Rh(1)-C(13) 149.0(1)C(7A)-Rh(1)-C(9) 106.5(1) C(3)-Rh(1)-C(13) 116.4(1)C(8)-Rh(1)-C(9) 36.5(1) C(3A)-Rh(1)-C(13) 109.1(1)C(1)-Rh(1)-C(10) 105.5(1) C(7A)-Rh(1)-C(13) 130.1(1)C(2)-Rh(1)-C(10) 116.6(1) C(8)-Rh(1)-C(13) 36.4(1)C(3)-Rh(1)-C(10) 150.4(1) C(9)-Rh(1)-C(13) 68.3(1)C(3A)-Rh(1)-C(10) 163.2(1) C(10)-Rh(1)-C(13) 80.6(1)C(7A)-Rh(1)-C(10) 126.9(1) C(11)-Rh(1)-C(13) 64.2(1)C(8)-Rh(1)-C(10) 65.3(1) C(12)-Rh(1)-C(13) 37.5(1)C(9)-Rh(1)-C(10) 37.9(1)

Table 2. Bond distances (Å) and selected bond angles (deg) for (η5-C9H7)Rh(2,3,5,6-C6O2(CH3)4) (3).

5A complete set of data may be purchased from: The Depository of Unpublished Data, Document Delivery, CISTI, National Research Coun-cil Canada, Ottawa, Canada, K1A 0S2. With the exception of thermal parameters, these have also been deposited with the Cambridge Crys-tallographic Data Centre, and can be obtained on request from: The Director, Cambridge Crystallographic Data Centre, University ChemicalLaboratory, 12 Union Road, Cambridge, CB2 1EZ, U.K.

The addition of 1,4-benzoquinone (C6O2H4) and duroqui-none (2,3,5,6-tetramethyl-1,4-benzoquinone, C6O2(CH3)4) to(η5-C9H7)Rh(C2H4)2 (1) gave the corresponding metal com-plexes (η5-C9H7)Rh(C6O2H4) (2) and (η5-C9H7)Rh(2,3,5,6-C6O2(CH3)4) (3) (Scheme 1), respectively, where the quinoneacts as a four-electron diene ligand.

An early report (6) described only a minimum amount ofdistortion within the indenyl ring in complex3, which is notconsistent with data obtained with other (η5-C9H7)RhL2complexes where significant slip-fold distortions are ob-served (1–5). These distortions arise as the ML2 fragmentslips from a pureη5-bonding mode towards the allylicη3-mode, whereby the indenyl ring loses planarity, and as re-sult, the six-membered ring folds away slightly from thebulky metal fragment. One spectroscopic tool that hasproven useful in determining the degree of slip-fold distor-tion in solution is13C{1H} NMR spectroscopy (4f, 5b, 8).The slip distortion has been correlated with the13C{1H}chemical shifts of the ring-junction carbons C3a,7a wherelarger distortions result in significant downfield shifts. Inter-estingly, the chemical shifts for the quaternary carbons inbenzoquinone complex2 at δ 107.5 ppm are significantlyupfield from starting complex1 at δ 112.2 ppm (7). This re-sult is significant in that it suggests that the amount of dis-tortion in this quinone complex is much less than thatobserved in the bis(ethylene) complex (η5-C9H7)Rh(C2H4)2.

13C{1H} NMR data have also been used to describe theamount of distortion observed in quinones when these lig-ands coordinate to metal centres (9). Upon complexation, a

marked deviation in the quinone from theD2h symmetry ofthe free ligand is observed as the carbonyl moieties bendaway from the metal. This distortion has been attributed to arepulsive interaction of the metald orbitals with the filled2b1u and empty 2b2g orbitals of the quinone (10). As a resultof this distortion, the13C{1H} NMR chemical shifts for thiscarbonyl fragments shift significantly upfield. For instance,the carbonyl groups in (η5-C5H5)Rh(C6O2H4) are observed atδ 161.6 ppm, compared to free 1,4-benzoquinone atδ187.0 ppm. A chemical shift atδ 160.6 ppm for2 suggeststhat, like (η5-C5H5)Rh(C6O2H4), only a minor distortionfrom planarity exists within the quinone ligand for this com-plex (9).

Although a minimal amount of distortion was reportedearlier for (η5-C9H7)Rh(2,3,5,6-C6O2(CH3)4) (3) (6), the13C{1H} NMR data for both the indenyl ring and thequinone ligand in this complex suggest otherwise. Indeed,chemical shifts for the indenyl ring-junction carbons atδ112.3 ppm implies that the degree of slip-fold distortion in3, at least in solution, is comparable to complex1. Likewise,the carbonyl groups of the quinone ligand have shifted by36.8 ppm in complex3 (δ 140.5 ppm) compared to freeduroquinone (δ 187.3 ppm). To understand the nature of thisdiscrepancy, we decided to reinvestigate the solid-statestructure of3.

Figure 2 presents a view of the molecular structure withthe atomic numbering scheme. Compound3 consists of a(η5-C9H7)Rh(2,3,5,6-C6O2(CH3)4) unit with the duroquinoneligand lying in a plane nearly parallel to the indenyl group.The carbonyl moieties of duroquinone lie on a plane incor-porating Rh, C2, and the midpoint of the C3a–C7a vector ofthe indenyl ring. Duroquinone behaves as a 1,4-diolefinwhere the rhodium–carbon bond distances to the olefiniccarbons C9, C10, C12, C13 of 2.194(2), 2.100(2), 2.201(2),and 2.195(2) Å (Rh—Cave = 2.193(2) Å) are typical forthose found in otherd8-π-quinone complexes (11). As ex-pected from13C{1H} NMR data, a significant distortion ofthe quinone ligand away from the plane is observed with thecarbonyl groups bending away from the indenylrhodiumfragment. The distances Rh—C8 of 2.448 (2) Å and Rh—C11of 2.450(2) Å are approximately 0.276 Å longer than thoseto the olefinic carbons. In addition to this distortion fromplanarity, the unsaturated C5C bond distances within theduroquinone ligand (mean value 1.417 Å) are also elongatedconsiderably compared to free quinone (1.341 Å) (12).

In contrast to previous work, we find that a modest amountof distortion in the indenyl ring is present in the ground-statestructure of (η5-C9H7)Rh(2,3,5,6-C6O2(CH3)4), which con-tains three “short” and two “long” rhodium–carbon dis-tances. The value of the slip-distortion is∆ = 0.112(3) Å,where ∆ = d(average Rh—C3a,7a) –d(average Rh—C1,3).While a hinge angle of 7.2° (HA = angle between planes de-fined by C1, C2, C3 and C1, C7a, C3a, C3) and a fold angleof FA = 4.0° (FA = angle between planes defined by C1, C2,C3 and C3a, C4, C5, C6, C7, C7a) also suggest a slight dis-tortion towards an allylic bonding mode, there is clearlybonding between the metal atom and ring junction carbonsC3a and C7a, as required to attain an 18-electron configura-tion at the metal. This level of distortion still makes the titlecompound the least distortedd8-RhL2 indenyl complex tohave been structurally characterized to date.

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202 Can. J. Chem. Vol. 77, 1999

x y z U (eq)

Rh(1) 7280.9(2) 998.2(1) 2207.6(2) 24.42(8)C(1) 8307(5) 1491(3) 667(3) 46(1)C(2) 8758(5) 734(3) 695(3) 50(1)C(3) 9766(4) 592(2) 1820(4) 42(1)C(3A) 10122(4) 1290(2) 2436(3) 33.2(8)C(4) 11055(5) 1496(3) 3553(3) 48(1)C(5) 11076(7) 2202(3) 3927(4) 66(2)C(6) 10184(7) 2763(3) 3223(6) 71(2)C(7) 9275(6) 2605(2) 2148(6) 59(1)C(7A) 9212(4) 1856(2) 1712(3) 36.4(9)C(8) 5754(4) 1447(2) 3860(3) 32.2(8)C(9) 5037(4) 1632(2) 2605(3) 31.2(8)C(10) 4529(4) 1049(2) 1766(3) 31.4(8)C(11) 4605(4) 265(2) 2164(3) 32.6(8)C(12) 5998(3) 126(2) 3152(3) 29.7(7)C(13) 6521(4) 700(2) 3990(3) 29.0(7)O(14) 5977(4) 1924(1) 4673(2) 51.6(8)C(15) 4724(6) 2439(2) 2312(4) 46(1)C(16) 3644(5) 1203(3) 511(3) 46(1)O(17) 3790(4) –243(2) 1618(3) 53.4(9)C(18) 6578(6) –674(2) 3321(4) 47(1)C(19) 7725(5) 568(2) 5122(3) 45(1)

Table 3. Atomic coordinates (× 104) and equivalent isotropicdisplacement coefficients for (η5-C9H7)Rh(2,3,5,6-C6O2(CH3)4)(3).

It is interesting to note that the degree of ground-state dis-tortion in 3 does not correlate well with solution data and isstill somewhat less pronounced than in the bis(ethylene)complex1 (∆ = 0.161 Å, HA = 8.08°, FA = 7.40°) (4b, 13).It is plausible that this anomaly arises from theπ-acceptingability of the quinone ligand, wherebyπ-backbonding re-moves electron density from the Rh(I) centre (towards a for-mal Rh(III) configuration) and thus strengthening themetal–ligand interactions and affecting the shift correlation.This type of interaction would also result in an increase incontribution from an “aromatic” resonance structure of thequinone ligand, as shown by the pronounced lengthening ofthe C5C bonds in3. Reactions with related 7,7′,8,8′-tetra-cyanoquinodimethane (TCNQ) gave highly coloured insolu-ble powders, where the quinone is now so strongly oxidizingthat it appears to gives products arising from justcharge-transfer chemistry (146). Unfortunately, attempts togrow single crystals of less sterically hindered2 suitable forX-ray diffraction studies, in order to compare solution andsolid-state distortions, proved unsuccessful. In conclusion,the results described herein narrow significantly the range ofthese distortions, making correlations between ligand elec-tronic and steric parameters and the degree of ground state

distortion ind8-ML2 indenyl complexes much more difficultto assess.

T.B.M. acknowledges support from the Petroleum Re-search Fund, administered by the American Chemical Soci-ety. T.B.M. and S.A.W. also thank the Natural Sciences andEngineering Research Council of Canada and JohnsonMatthey Ltd. for a loan of RhCl3·3H2O. We thank the refer-ees for helpful suggestions and Jacquelyn Burke for assis-tance with the literature search.

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© 1999 NRC Canada

Westcott et al.: 1 203

Rh

OO

Rhquinone

-2C2H4

R R

RR

1 2, R = H3, R = CH3

Scheme 1.

Fig. 2. Molecular structure of (η5-C9H7)Rh(2,3,5,6-C6O2(CH3)4) (3).

6S.A. Westcott and T.B. Marder. Unpublished results. For leading references on the radical anion chemistry of TCNQ, see ref. 14.

© 1999 NRC Canada

204 Can. J. Chem. Vol. 77, 1999

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