Die metall-kohlenstoff-bindung in carben- und carbin-komplexen. aussagen der 183W13C-kopplungen

The nuclear spin coupling constants¹J(¹⁸³W¹³C) and in some cases ²J(¹⁸³W¹³C) and ³J(¹⁸³W¹³C) are determined for 10 tungsten carbene and 9 tungsten carbyne complexes. ¹J is of analytical importance, being characteristically greater for WC than for WC bonds. This is due to different hybridisation at the carbon atom, and provides information about bond angles and polarities of WC and WCR units.Substituents R and R' in (CO)₅WCRR' and X(CO)₄WCR as well as the halogens X lead to minor changes in ¹J. These changes are comparable to those of ¹J(¹³C¹H) in correspondingly substituted methanes. Unexpectedly ¹J in_ creases with X = Cl, Br, I. ²J(¹⁸³W¹³C) though being much smaller than ¹J reflects different hydridisation at the β carbon atom.     

13C, 15N and 29Si nuclear magnetic resonance studies of some aminosilanes and aminodisilanes

¹³C, ¹⁵N (at natural abundance) and ²⁹Si NMR data (chemical shifts and coupling constants) are reported for aminosilanes R₂R′SiNHR¹ (1), bis(silyl)amines Me₂R′SiNHSiMe₃ (2), 1,2-bis(amino)-ethanes (3), bis(amino)silanes RR′Si(NHR¹)₂ (4), 1,2-bis(amino)tetramethyldisilanes (5) and 1,1,2,2-tetrakis(amino)dimethyldisilanes (6). The δ¹⁵N values depend more on the nature of the substituents R¹(H, alkyl, aryl) at the nitrogen atom (in the same way as for other amines) than on different substituents at the silicon atom. A linear correlation between ¹J(²⁹Si¹⁵N) and ¹J(²⁹Si¹³C) is proposed for silanes in which the SiN unit is replaced by the SiCH unit. This correlation comprises all ¹J(²⁹Si¹⁵N) values for aminosilanes R_4-nSi(N)n (n = 1–4) and—most likely—also for aminodisilanes, and it predicts ¹J(²⁹Si¹⁵N)>0 if the corresponding value |¹J(²⁹Si¹³C)|>25 Hz. For the first time a two-bond coupling across Si, ²J(²⁹Si ¹⁵N) = 6.9 Hz, has been observed for 6a. In the case of 6b (R¹ = ^sBu) all resonances for the diastereomers are resolved in the ¹⁵N and ²⁹Si NMR spectra in contrast to the ¹H and ¹³C NMR spectra.     

195Pt, 119Sn and 31P NMR studies of alkyl,aryl and acyl trichlorostannate complexes of platinum(II). The crystal structure of trans-[Pt(SnCl3)(COC6H5)(PEt3)2]

¹⁹⁵Pt, ¹¹⁹Sn and ³¹P NMR characteristics of the complexes trans-[Pt(SnCl₃)(carbon ligand)(PEt₃)₂] (1a-1e) are reported, (carbon ligand = CH₃ (1a), CH₂Ph (1b), COPh (1c), C₆Cl₅ (1d), C₆Cl₄Y (e); Y = meta- and para-NO₂, CF₃, Br, H, CH₃, OCH₃, or Pt(SnCl₃)(PEt₃)₂. The values of ¹J(¹⁹⁵Pt, ¹¹⁹Sn) vary from 2376 to 11895 Hz with the COPh ligand having the smallest and the C₆Cl₅ ligand the largest value, making a total range for this coupling constant, when the dimer syn-trans-[PtCl(SnCl₃)(PEt₃)]₂ is included, of ca. 33000 Hz. In the meta- and para-substituted phenyl complexes ¹J(¹⁹⁵Pt, ¹¹⁹Sn) (a) is greater for electron-withdrawing substituents, (b) varies more for the meta-substituted derivatives (5634 to 7906 Hz) than for the para analogues (6088 to 7644 Hz) and (c) has the lowest values when the Pt(SnCl₃)(PEt₃)₂ group is the meta- or para-substituent. The direction of the change in ¹J(¹⁹⁵Pt, ¹¹⁹Sn) is opposite to that found for ¹J(¹⁹⁵Pt, ¹¹⁹P). For the aryl complexes linear correlations are observed between δ(¹¹⁹Sn), ¹J(¹⁹⁵Pt, ¹¹⁹Sn), ¹J(¹⁹⁵Pt, ³¹P), ¹J(¹¹⁹Sn, ³¹P) and the Hammett substituent constant σⁿ. δ(¹¹⁹Sn) and ¹J(¹⁹⁵Pt, ¹¹⁹Sn) are related linearly to v(Pt-H) in the complexes trans-[PtH(C₆H₄Y)(PEt₃)₂]; δ(¹¹⁹Sn) and δ(¹H) (hydride) are also linearly related. Based on ¹J(¹⁹⁵Pt, ¹¹⁹Sn), the acyl ligand is suggested to have a very large NMR trans influence. The differences in the NMR parameters for (1a-e) are rationalized in terms of differing σ- and π-bonding abilities of the carbon ligands.The structure of 1c has been determined by crystallographic methods. The complex has a slightly distorted square planar geometry with trans-PEt₃ ligands. Relevant bond lengths (Å) and bond angles (°) are: PtSn, 2.634(1), PtP, 2.324(4) and 2.329(4), PtC, 2.05(1); PPtP, 170.7(6), SnPtC, 173.0(3), SnPtP, 92.1(1), 91.7(1), PPtC, 88.8(4) and 88.3(4). The PtSn bond separation is the longest yet observed for square-planar platinum trichlorostannate complexes, and would be consistent with a large crystallographic trans influence of the benzoyl ligand. The PtSn bond separation is shown to correlate with ¹J(¹⁹⁵Pt, ¹¹⁹Sn).     

Triferrocenyl-komplexe von wolfram(VI): Molekülstruktur im festkörper und dynamisches verhalten in Lösung von triferrocenyl-ferrocenoxy-oxo-wolfram,WO(OFc)Fc3

Triferrocenyltungsten complexes of the type WO(X)Fc₃ (X = Cl, (1), OMe (2), OFc (3) and OⁿBu (4)) were obtained by treating WOCl₄ with ferrocenyllithium, FcLi, in tetrahydrofuran solution¹. Reaction of WOCl₄ with a threefold excess of FcLi gives 1, which may be converted into 2 using KOCH₃. Reaction of WOCl₄ with a sixfold excess of FcLi gives a mixture containing 3 und 4 in addition to ferrocene and biferrocene. According to the X-ray crystallographic analysis, WO(OFc)Fc₃ (3) has a trigonal-bipyramidal structure with three ferrocenyl ligands occupying the equatorial positions and an axial ferrocenoxy group coordinated trans to the oxo ligand. The three WC(ferrocenyl) (average 2.092 Å) and the OC(ferrocenyl) (1.33(1) Å) bond distances are remarkably short. The axial tungsten—oxygen distances correspond to a WO double and a WO single bond (1.705(5) and 1.945(5) Å), respectively. The ¹H and ¹³C NMR spectra of WO(OFc)Fc₃ (3) are temperature-dependent. This is ascribed to a hindered rotation of the ferrocenyl ligands around the WC(ferrocenyl) bonds; the free activation enthalpy ΔG^≠(T_c) of this intramolecular dynamic process is 62.5 ± 0.5 kJ mol⁻¹.     

Cumulated double bond systems as ligands : IV. 1H and 13C NMR studies of the intra- and inter-molecular exchange processes of cationic dialkylsulfurdiimine compounds of RhI,IrI and PtII

The preparation and properties are described of trans-[(Ph₃P)₂(CO)M(RNSNR)] [ClO₄] (M  Rh^I, Ir^I; R  Me, Et, i-Pr, t-Bu) and of cis- or trans-[L₂Pt(RNSNR)X] [ClO₄] (X  Cl^?, L  Et₂S, PhMe₂As, PhMe₂P, R  Me, t-Bu; X  CH₃, L  PhMe₂P, R  Me).¹H and ¹³C NMR data show the existence of various isomers in solution which may interconvert via intra- and inter-molecular exchange processes. A general reaction scheme for the intramolecular exchange processes is discussed.     

The syntheses and coordination properties of M(CO)3X(DAB) (M = Mn,Re; X = Cl,Br, I; DAB = 1,4-diazabutadiene)

M(CO)₅X (M = Mn, Re; X = Cl, Br, I) reacts with DAB (1,4-diazabutadiene = R₁N=C(R₂)C(R₂)′=NR′₁) to give M(CO)₃X(DAB). The ¹H, ¹³C NMR and IR spectra indicate that the facial isomer is formed exclusively. A comparison of the ¹³C NMR spectra of M(CO)₃X(DAB) (M = Mn, Re; X = Cl, Br, I; DAB = glyoxalbis-t-butylimine, glyoxyalbisisopropylimine) and the related M(CO)₄DAB complexes (M = Cr, Mo, W) with Fe(CO)₃DAB complexes shows that the charge density on the ligands is comparable in both types of d⁶ metal complexes but is slightly different in the Fe-d⁸ complexes. The effect of the DAB substituents on the carbonyl stretching frequencies is in agreement with the A′(cis) > A″ (cis) > A′(trans) band ordering.Mn(CO)₃Cl(t-BuNCHCHNt-Bu) reacts with AgBF₄ under a CO atmosphere yielding [Mn(CO)₄(t-BuNCHCHN-t-Bu)]BF₄. The cationic complex is isoelectronic with M(CO)₄(t-BuNCHCHNt-Bu) (M = Cr, Mo, W).     

Zur reaktion von metall-koordiniertem kohlenmonoxid mit yliden : VII. Trimethylphosphoranylidenmethyl(trimethylsiloxy)carben-komplexe von chrom,molybdän und wolfram: aufbau einer metallvinyl-einheit durch c(carbanion)c(carben)-π-wechselwirkung und photochemische e/z-isomerisierung

The treatment of the hexacarbonylmetal compounds M(CO)₆ (M = Cr. Mo, W) with two equivalents Me₃PCH₂ yields the phosphonium acylmetalphosphorus ylides Me₄P[(CO)₅MC(O)CHPMe₃] 1a–1c. Their reaction with Me₃SiOSO₂CF₃ leads via O-silylation to formation of the neutral “siloxy(ylidecarbene) complexes” (CO)₅MC(OSiMe₃)CHPMe₃2a–2c, which are protonated by HX (X = Cl, CF₃SO₃) to give the thermolabile carbene complexes [(CO)₅MC(OSiMe₃)H₂CPMe₃]X, 3a, 3b. ¹H, ¹³C NMR and IR data suggest, that delocalization of the ylidic charge to the carbene carbon generates a metal-coordinated vinyl group in the case of 2a–2c. In addition this fact is proved by the X-ray analysis of 2c, for which a C(ylide)C(carbene) bond distance of 133 pm is found. 2a–2c are obtained as pure E-isomers but can be converted to the Z-isomers 2a′–2c′ upon photolysis.     

Insertionsreaktionen von Bis(η-cyclopentadienyl)hydridorhenium mit monosubstituierten Acetylenen

Bis(η-cyclopentadienyl)hydridorhenium Cp₂ReH undergoes stereospecific trans insertion reactions when treated with monosubstituted acetylenes HCCR (R  CO₂Me, CN, CF₃). The cis alkenyl complexes Cp₂Re[η¹-(Z)-CHCHR] thus formed isomerize thermally or under acid catalysis to produce the trans isomers Cp₂Re[η¹-(E)-CHCHR]. When Cp₂ReH adds to HCCCOMe only the trans isomer is observed. The regiospecific β-addition of Cp₂ReH contrasts with the α-addition of Cp₂MoH₂ and Cp₂WH₂. The insertion of acetylenes HCCR′ into the metalcarbon bond of some alkenyl complexes Cp₂Re[η¹-(E)-CHCHR] affords butadienyl complexes Cp₂Re[η¹-{(1E,3E)-CHCHR′CHCHR&}] (R,R′  COMe, CO₂Me). The (E,E)-configuration of these compounds is deduced from ³J(¹³-C¹H) coupling constants.     

Comparison of free and metal coordinated 1,4-disubstituted-1,4-diaza-1,3-butadienes : Crystal and molecular structures of 1,4-dicyclohexyl-1,4-diaza-1,3-butadiene and trans-[dichloro(triphenylphosphine)(1,4-di-tert-butyl- 1,4-diaza-1,3-butadiene)palladium(II)]

The crystal and molecular structures of c-Hex-DAB (c-hexyl-NC(H)C(H)N-c-hexyl; DAB = 1,4-diaza-1,3-butadiene) and of trans-[PdCl₂(PPh₃)(t-Bu-DAB)] are reported. Crystals of c-Hex-DAB are monoclinic with space group C_2/c and cell constants: a = 24.70(1), b = 4.660(2), c = 12.268(3)Å, β = 107.66(4)°, Z = 4. The molecule has a flat E-s-trans-E structure with bond lengths of 1.258(3)Å for the CN double bond and 1.457(3)Å for the central CC′ bond. These bond lengths and the NC-C′ angle of 120.8(2)° indicate that the C- and N-atoms are purely sp²-hybridized and that there is little or no conjugation within the central DAB skeleton. Crystals of trans-[PdCl₂(PPh₃)(t-Bu-DAB)] are triclinic with space group P-1 and cell constants: a = 17.122(3), b = 18.279(3), c = 10.008(5)Å, α = 96.77(2), β = 95.29(3), γ = 109.79(2). Z = 4. The t-Bu-DAB ligand is coordinated to the metal via one lone pair only. In this 2e; σ-N coordination mode the E-s-trans-E conformation of the free DAB-ligand is still present and the bonding distances within the DAB-ligand are hardly affected. [CN: 1.261(10)Å; CC′: 1.479(10)Å (mean).] The PdN-, NC- and central CC′-bond lengths are compared with those found in other metal -R-DAB complexes.     

29Si NMR Spektroskopie von Cyclopentasilanen

The structure and ²⁹Si chemical shifts of the halodimethylsilylnonamethylcyclopentasilanes Si₅Me₉SiMe₂X (1–4) and the halononamethylcyclopentasilanes Si₅Me₉X (5–8) (X = F, Cl, Br, I) have been assigned using ¹J(SiSi) and ²J(SiSi) coupling constants derived from ²⁹Si-INADEQUATE and ²⁹Si-INEPT—INADEQUATE NMR spectra. The compounds exhibit good correlation between chemical shift, ¹J(SiSi) and Pauling electronegativities.     

1,4-dicyanobutene-bridged binuclear rhodium(I) complexes and their catalytic activities

Reactions of Rh(ClO₄)(CO)(PPh₃)₂ with dicyano olefins, cis-NCCHCH-CH₂CH₂CN (c-DC1B), rans-NCCHCHCH₂CH₂CN (t-DC1B), trans-NCCH₂CHCHCH₂CN (t-DC2B), and NCCH₂CH₂CH₂CN (DCB) produce the binuclear dicationic rhodium(I) complexes, [(CO)(PPh₃)₂RhNCACNRh-(PPh₃)₂(CO)](ClO₄)₂ (NCACN = c-DC1B 1), t-DC1B (2), t-DC2B (3), DCB (4). Complexes 1 and 2 are catalytically active for the hydrognation of c-DC1B and t-DC1B, respectively, to give DCB, while complex 3 catalyze the isomerization of t-DC2B to give c-DC1B and t-DC1B, and the hydrogenation of t-DC2B to DCB at 100°C.     

Molecular mechanics calculations for conformational energies and torional force constants in fluoro propenes and butenes

Experimental data on conformational energies of the molecules FH₂CHCCH₂, FH₂CFCCH₂, FH₂C(CH₃)C&.dbnd;CH₂ trans-FH₂CHCC(CH₃)H have been used to establish parameter values for the nonbonding atom ⋯ atom interaction F ⋯ C(sp²) within the Morse potential formulation. Torsional potentials have been calculated for the four molecules mentioned above and in addition for cis- and trans-FH₂CHCCHF, (FH₂C)₂CCH₂, cis-FH₂CHCCHCH₂F, CH₃FCHHCCH₂ and FH₂CCH₂HCCH₂. Calculated results have been compared with experimental values. Torsional force constants for the molecules have been obtained. A comparison between fluoro, chloro and bromo compounds is presented.     

Insertion d'une phosphine acétylénique dans un complexe μ-alkylidénique du tungstène suivie de la rupture d'une liaison CP

The diphosphinoalkyne Ph₂PCCPPh₂ (2) reacts with the μ-alkylidene complex (CO)₉W₂[CHCHC(CH₃)₂] (1) to give, upon insertion of the alkyne into one of the CW bonds of the bridging carbene followed by rupture of a CP bond, a phosphido complex (CO)₈W₂[C(PPh₂)CCHCHC(CH₃)₂] PPh₂ (3). An unexpected long-range ¹H³¹P coupling, through five bonds, is observed in complex 3.     

A proton and carbon-13 nuclear magnetic resonance study of neopentylmercury compounds

Substituent effects on ¹⁹⁹Hg¹H and ¹⁹⁹Hg¹³C spinspin coupling constants have been studied for neopentylmercury derivatives, (CH₃)₃CCH₂HgR(or X), where R is covalently bonded Me, Et, t-Bu, neopentyl, and vinyl, and X is easily ionizable CN, Br, Cl, OCOCH₃, and ONO₂. Linear relationships exist between the methylene J(¹³CH) and ²J(HgH), ⁴J(HgC) and ²J(HgC) and ³J(HgC); but deviations from linearity occur for the chloride, bromide, acetate, and nitrate in the relationships between ²J(HgH) and ⁴J(HgH), ²J(HGH) and ²J(HGC). These deviations are discussed in terms of hyperconjugative p_πd_π bonding between the methylene CH bonds and mercury.     

Intramolecular metallation of o-carboranylphosphine-palladium(II) and -platinum(II) complexes

Preparation of trans-[P^cb₂MCl₂]-type complexes (P^cb= o-HCB₁₀H₁₀CCH₂PPh₂ M = Pd, Pt), which readily undergo intramolecular metallation through the BH bonds of the carborane cage to form exocyclic compounds involving a $\text{PCCBM}$ bond system, is described. Both monomeric compounds, trans-[MCl(B-P)P^cb], and bridged complexes, such as [Pd₂Cl₂(BP)₂], are formed, where (BP) is intramolecular-metallated carborane phosphine. The bridging bond is readily cleaved under the action of various ligands (pyridine, PEt₃, etc.) to form monomeric compounds.     

Concerning the silicon-germanium bond: mass spectral fragmentations of isomeric SiGe compounds R3EE′R3 (E  Si,E′  Ge,R  Ph,Me,(η5-C5H4)Fe (C5H5), (η5-C5H5)Fe(CO)2 and (η5-C9H7) Fe(CO)2)

Isomeric pairs of silicon-germanium compounds containing a SiGe bond (Me₃SiGePh₃ (I) and Ph₃SiGeMe₃ (II); FpSiMe₂GeMe₃ (III) and FpGeMe₂SiMe₃ (IV) (Fp = (η⁵-C₅H₅)Fe(CO)₂); IFpSiMe₂GeMe₃ (V) and IFpGeMe₂SiMe₃ (VI) (IFp = (η⁵-C₉H₇)Fe(CO)₂); IFpSiMe₂GePh₃ (VII) and IFpGeMe₂SiPh₃ (VIII) and the complex FcSiMe₂GeMe₂Fc (IX) (Fc = ferrocenyl) have been synthesized and examined by mass spectrometry.The R₃SiGeR′₃ compounds I and II exhibit considerable exchange of R groups to produce [R_3-nR′_nSi]⁺ and [R′_3-nR_nGe]⁺ ion in progressively lesser amounts as n = 1 → 2 → 3. For the metal-substituted complexes containing the grouping FeSiGe fragmentation occurs predominantly via SiGe bond cleavage with formation of ions containing the silylene ligand [FeSiR₂]⁺. Complexes with the FeGeSi backbone undergo preferential scission of the FeGe bond, illustrating the general bond strength trend FeSi > SiGe. Upon direct cleavage of the SiGe bond in R₃SiGeR₃ compounds, the percentage of the charge carried by [R₃Si]⁺ ions significantly exceeds that carried by [R₃Ge]⁺ ions, reflecting the greater electronegativity of Ge polarizing the SiGe bond.     

Organolead chemistry : III. 207Pb chemical shifts in some organolead compounds

²⁰⁷Pb chemical shifts are reported for the compounds (CH₃)_4?nPb X_n, where n = 1 · 4, X = 4-FC₆H₄; n = 1, 2, 4, X = CH₃ CC; n = 1, 4, X = CH₂CH; n = 1, X = Cl, CH₃O, CH₃CO₂. A correlation between δ(²⁰⁷Pb) and δ(¹⁹F) for the 4-fluorophenyl derivatives is discussed, and solvent effects on δ(²⁰⁷Pb) for the propynyl derivatives are interpreted in terms of complex formation.     

Pulsed fourier transform NMR of substituted aryltrimethyltin derivatives : III. Proton data at 100 MHz and the derived Hammett σ-constant of the trimethyltin group

Proton NMR data at 100 MHz are reported for thirteen para- and meta-substituted phenyltrimethyltin compounds, XC₆H₄Sn(CH₃)₃, where X = para-N(CH₃)₂, para-OCH₃, para-OC₂H₅, para-CH₃, meta-CH₃, -H, para-F, meta-OCH₃, para-Cl, para-Br, meta-F, meta-Cl and para-Sn(CH₃)₃. Correlation coefficients with Hammett σ-constants of greater than 0.95 are obtained with the methyltin proton chemical shifts and coupling constants to carbon [¹J(¹³C¹H)] and tin [²J(SnC¹H)]. Solvent effects and other extraneous factors invalidate comparisons of ? values in terms of the relative attenuation of the transmission of substituent effects through homologous carbon, silicon, germanium and tin systems, but coupling constant data reflect a diminution of ca. one tenthfold per bond in the order ?[C(1)Sn] > ? [SnC] > ? [CH]. Satisfactory correlations (r > 0.95) are obtained in this series of closely-related compounds among the conventionally recorded two-bond, ²J(SnC¹H) and the constituent, one-bond ¹J (Sn¹³C) and J(¹³C¹H) coupling constants, but the correlation coefficient for the comparison between the two one-bond couplings, ¹J(Sn¹³C) and ¹J(¹³C¹H) is lower (r = 0.872). Changes in the couplings at the methyltin carbon bond tin-119 atoms are interpreted in terms of isovalent hybridization; a model based upon effective nuclear charges is tested with respect to both NMR coupling constants and ¹¹⁹Sn Mössbauer Isomer shifts at tin and is invalidated. Proton and carbon-13 NMR, chemical shift and coupling constant data are used to derive a Hammett σ-constant for the para-trimethyltin group of ?0.14, and the significance of this value is discussed.     

A new route to iron(0) thiocarbonyl complex involving desulphurization of the Fe(η2-CS2R)+ cation with P-n-Bu3. Crystal structure of Fe(CS)(CO)2(PPh3)2

Reaction of [Fe(η²-CS₂R)(CO)₂(PPh₃)₂][X] (R = CH₃, CH₂Ph; X⁻ = PF₆⁻, I⁻) with P-n-Bu₃ or PEt₃ gives Fe(CS)(CO)₂(PPh₃)₂ (3a); (ν(CS) 1235 cm⁻¹; δ(¹³C) 324.28 ppm). The structure of 3a has been determined by X-ray diffraction. Crystal data are: a 18.821(5), b 12.113(3), c 18.149(5) Å, β 117.76(6)°, monoclinic, space group P2₁, Z = 4. The structure is a trigonal-bypyramid with equatorial CS group, trans PPh₃ ligands, a FeC(S) bond distance of 1.768(8) and a CS bond distance of 1.563(8) Å.     

Übergangsmetall—methylen-komplexe: LXI. Übergangsmetall—vinyliden-komplexe: Neuartige synthese aus diazoalken-vorstufen

The diazoolefines of composition N₂CCR₂ (R/R = CH₃/CH₃ and(-CH₂-)₅) are suitable precursors of the corresponding vinylidene ligands CCR₂. Thus, treatment of the RhRh complex [(η⁵-C₅Me₅)Rh(μ-CO)]₂ (1) with the N-nitrosourethanes 2a and 2b, resp., in the presence of lithium t-butoxide yields the otherwise inaccessible μ-vinylidene complexes (μ-CCR₂)[(η⁵-C₅Me₅)Rh(CO)]₂ (R = CH₃ (3a), R,R = (-CH₂-)₅ (3b)). The analogous cobalt compound (μ-CCMe₂)[(η⁵-C₅Me₅)Co(CO)]₂ (5a) is obtained similarly. This procedure extends the well-documented diazoalkane method for the synthesis of μ-alkylidene complexes to the less stable diazoalkenes. A single-crystal X-ray diffraction study of the dimethylvinylidene derivative 3a shows the CMe₂ ligand to adopt an almost symmetrically metal-bridging position (d(RhC) 197.8(1) and 204.3(1) pm), with a rhodium-rhodium single bond completing a three-membered Rh₂C-metallacycle (d(RhRh) 268.4(0) pm) analogous with cyclopropane.