Edge-sharing Bioctahedral Research Articles

Edge-sharing bi-octahedral diruthenium(IV,IV) compounds containing Ru-Ru bonds chelated and bridged by two carbonate and two oxo groups.

From paddle-wheel starting material Na3Ru2(CO3)4·6H2O, a family of edge-sharing bi-octahedral (ESBO) diruthenium(IV,IV) compounds formulated as Ru2O2(CO3)2(H2O)2L2·nH2O [L = piperazine (1) or 2-methylpiperazine (2), n = 4, and L = 2,2-dimethylpiperazine (3), n = 12] and Ru2O2(CO3)2(OH)4{M(H2O)4}2·nH2O [M = Mg (4), n = 4, and Ni (5), n = 2] were prepared and structurally characterized. The Ru28+ dimer is chelated and bridged by two CO32- and two μ-O in a trans manner, and the Ru-Ru distances fall in the range 2.3808(6)-2.4001(4) Å. Compound 2 shows the shortest Ru-Ru distance for all known ESBO Ru2 compounds reported thus far. Increasing -CH3 groups of terminal piperazine ligands coordinated to the Ru(μ-O)2(μ-O3C)2Ru core, and according to Raman spectra experiments combined with theoretical calculations, the intense bands of compounds 1-3 appearing at ∼360 cm-1 can be assigned to the stretching of Ru-Ru bonds.

Reaction of the dirhenium(III,II) complex Re2(µ-O2CCH3)Cl4(µ-dppm)2 with dithioterephthalic acids: A combined experimental and theoretical study

The reaction between the multiply-bonded dirhenium(III,II) complex [Re2(µ-O2CCH3)Cl4(µ-dppm)2] (dppm = Ph2PCH2PPh2) and dithioterephthalic acid or 2,5-dimethyldithioterephthalic acid in refluxing ethanol affords the paramagnetic Re25+ complexes [Re2(μ-Cl)2(μ-dppm)2(η2-OSCC6H2R2CO2CH2CH3)2]Cl (R = H, 2; R = Me, 3). In the course of the reaction, one of the thio-acid (COS) coordination sites of the ligand chelated with dirhenium unit whereas another site esterifies with the ethanol solvent. These are the first example of Re2 complexes containing thio-terephthalate ligands. The spectroscopic (UV–vis-NIR, EPR) and electrochemical (CV, DPV) properties of the complexes are reported. The edge-sharing bioctahedral (ESBO) structure of complex 2 has been established by single-crystal X-ray diffraction study. The Re-Re bond length 2.6504(4) Å is compatible with the electronic configuration σ2π2δ*2δ2π*1 obtained from the density functional theory (DFT) calculation. Time dependent DFT (TDDFT) calculation has been included to rationalize the absorption spectra of the complexes.

A family of edge-sharing bi-octahedral diruthenium(III,III) compounds containing Ru-Ru single bonds.

From paddlewheel starting reactants Ru2(R'CO2)4+, a family of edge-sharing bi-octahedral (ESBO) diruthenium(III,III) compounds has been prepared, formulated as Ru2(μ-O2CR')2(μ-OR)2(η-L)2 (1-10) [R' = CH3, R = CH3, L = acac (1), tfac (2); R' = CH3, R = CH2CH3, L = hfac (3); R' = CH2CH3, R = CH3, L = acac (4), tfac (5); R' = CH2CH3, R = CH2CH3, L = hfac (6); R' = CH2Cl, R = CH3, L = tfac (7); R' = CH2Cl, R = CH2CH3, L = hfac (8); R' = C6H5, R = CH3, L = tfac (9); and R' = H, R = CH3, L = acac (10); here, acac, tfac and hfac represent acetylacetone, trifluoroacetylacetone and hexafluoroacetylacetone, respectively]. Compounds 1-10 have a similar ESBO coordination geometry of the Ru(μ-O2CR')2(μ-OR)2Ru core with a Ru-Ru center chelated and bridged by two μ-O2CR' and two μ-OR in a trans manner, and each Ru center is also coordinated with a η2-L bidentate ligand. The Ru-Ru distances fall in the range of 2.4560(9)-2.4771(4) Å. The investigation of the electronic spectra and vibrational frequencies as well as theoretical studies with density functional theory (DFT) reveal that compounds 1-10 are ESBO bimetallic species of d5-d5 valence electron counts showing a σ2π2δ2δ*2π*2 electronic configuration. Varying -CH3 to -CF3 groups on the η2-L bidentate ligands coordinating to the Ru(μ-O2CR')2(μ-OR)2Ru core, and according to Raman spectrum measurements combined with theoretical calculations, the intense bands of compounds 1-10 appearing at ∼345 cm-1 in the small-wavenumber region can be assigned to the stretching of the Ru-Ru single bond.

Facile synthesis of “WCl3(DME)” and structural and electronic characterizations

AbstractIn contrast to the readily accessed mid‐valent molybdenum precursors, trivalent tungsten compounds as good synthons are rare. The most commonly used trivalent ditungsten species, NaW2Cl7(THF)5, for the synthesis of the triply W − W bonded compounds of the type W2X6(X = bulky anionic ligand) can no longer be prepared because it is obtained from Na/Hg reduction of WCl4, and the use of mercury is banned. In this work, we report the characterization of a useful ditungsten(III) complex W2(μ‐Cl)2Cl4(κ2‐DME)2(2) (DME = 1,2‐dimethoxyethane) to replace NaW2Cl7(THF)5. The efficient reduction of the monomeric W(IV) compound WCl4(DME) (1) with granular Sn in DME gives2in a 97% isolated yield.2is a diamagnetic species based on SQUID magnetometry, and its formulation was established by single‐crystal X‐ray crystallography. The structure of2shows an edge‐sharing bioctahedral (ESBO) conformation with overallCisymmetry and the W − W bond length is 2.6443 (6) Å. Density functional theory (DFT) calculations suggest a σ2π2δ*2ground state configuration corresponding to the W − W single bond.

Exploring the Intricacies of Weak Interactions in Metal-Metal Bonds Using an Unsymmetrical Carbonyl Precursor and a Triple-Bonded W2(6+) Paddlewheel.

Stepwise reaction of W(CO)6 with tetramethylated bicyclic guanidinate ligands, characterized by a central C(N)3 unit joining two fused six-membered rings with CH2CMe2CH2 units spanning two of the nitrogen atoms, allowed isolation of W2(μ-CO)2(μ-TMhpp)2(η(2)-TMhpp)2, 1, a precursor of W2(TMhpp)4Cl2 ( J. Am. Chem. Soc. 2013 , 135 , 17889 ; TMhpp = [(CH2CMe2CH2)2(C(N)3)]). Subsequent heating of 1 followed by reaction with TlPF6 generates [W2(TMhpp)4](PF6)2, 2. Compound 1 has an edge-sharing bioctahedral (ESBO) arrangement with a W2(μ-CO)2(4+) core having semibridging carbonyl groups, while 2 has a paddlewheel structure with a W2(6+) core spanned by four tetramethyl-substituted bicyclic guanidinate ligands. This compound also has hexafluorophosphate anions along the metal-metal bond that are nestled within methylene groups with the aid of a network of weak C-H···F interactions that prevent a close approach of the fluorine atoms to the dimetal unit. Theoretical computations were carried out on ditungsten model complexes supported by three ligand sets: bicyclic guanidinate, guanidinate, and formamidinate. The computations show that the π-accepting ability of the carbonyl groups significantly lowers the energy of the σ* orbital, and thus, the energy falls below that of the δ orbital. This information along with the diamagnetism of both 1 and 2-as shown by the sharp signals in the (1)H NMR spectra that support a lack of unpaired electrons (S = 0)-is consistent with the electronic configuration of σ(2)π(2)σ*(2)δ(2) (π(2)δ(2)) and thus a formal bond order of 2 for 1 and σ(2)π(4) for the triple-bonded W2(6+) core in 2. A comparison of the W-W bond lengths in 2, its chloro precursor W2(TMhpp)4Cl2, and the corresponding analogue W2(hpp)4Cl2 shows a substantial effect from the axially coordinated ligand, distal lone pair in determining the length of the metal-metal bond for these paddlewheel species. The importance of the ligands in tuning the energy level of the metal-metal bonds that may lead to dramatic changes in physical properties is also discussed. It is noteworthy that bicyclic guanidinates with the strongest π-donating ability push upward the energy level of the δ orbital, thus allowing the compounds to be easily oxidized.

A Strong Metal-to-Metal Interaction in an Edge-Sharing Bioctahedral Compound that Leads to a Very Short Tungsten–Tungsten Double Bond

An ionic edge-sharing bioctahedral (ESBO) species has been prepared having a tetramethylated bicyclic guanidinate with two fused six-membered rings characterized by a fairly flat N-C(N)-N skeleton and abbreviated as TMhpp. The anion has two W(IV) atoms bridged by two oxo groups; the metal atoms are also spanned by two bridging guanidinate ligands, and each has two monodentate chlorine atoms. The complex formulated as (H2TMhpp)2[W(μ-O)(μ-TMhpp)Cl2]2 has the shortest W-W distance (2.3318(8) Å) of any species with a σ(2)π(2) electronic configuration. The anion and cations are connected by hydrogen bonds. To unambiguously ascertain the existence of the double-bonded W2(μ-O)2 entity, density functional theory calculations and natural bond orbital analyses were done on an analogous but hypothetical species with a W2(μ-OH)2 core having trivalent tungsten atoms and a possible σ(2)π(2)δ(2) electronic configuration. The calculations decidedly support the presence of tungsten-oxo instead of tungsten-hydroxo groups and thus the existence of the double-bonded W2(μ-O)2 core. The strong bonding interaction between metal atoms is a clear indication that under certain circumstances the two octahedra in ESBO species do not behave as the sum of two mononuclear compounds.

Synthesis and characterization of a new enantiopure hydroxylated phosphine, its rhodium(I) and (III) complexes and their performance in catalytic carbonylation

The enantiomerically pure hydroxylated phosphine (1 S,2 S,5 R)-1-diphenylphosphinomethyl-2-isopropyl-5-methyl-cyclohexanol 3 was prepared in a two-step stereoselective process from the commercially available and low cost (−)-menthone. Reaction of this new ligand with the appropriate rhodium precursor gave the respective Rh(I) and Rh(III) complexes. For rhodium(I), the binuclear complex trans-P,P-bis-{μ-chlorocarbonyl[(1 S,2 S,5 R)-1-diphenylphosphinomethyl-2-isopropyl-5-methyl-cyclohexanol- P]rhodium(I)} 6 was obtained by reaction with [Rh(μ-Cl)(CO) 2] 2. For Rh(III), two isomeric edge-sharing bioctahedral (ESBO) complexes, trans-O,O- trans-P,P-bis{(OC-6-32)-μ-chlorodichloro[(1 S,2 S,5 R)-1-diphenylphosphinomethyl-2-isopropyl-5-methyl-cyclohexanol- O, P]rhodium(III)} 7a and trans-O,O- cis-P,P-bis{(OC-6-32)-μ-chlorodichloro[(1 S,2 S,5 R)-1-diphenylphosphinomethyl-2-isopropyl-5-methyl-cyclohexanol- O, P]rhodium(III)} 7b, and the partially hydrolyzed product, trans-O,O- cis-P,P-(OC-6-32)-{μ-chloro-μ-hydroxo-tetrachlorobis[(1 S,2 S,5 R)-1-diphenylphosphinomethyl-2-isopropyl-5-methyl-cyclohexanol- O, P]dirhodium(III)} 8 were characterized in solid state by single-crystal X-ray diffraction analysis. The O-axial- P-equatorial trans-O, O arrangements observed in compounds 7a, b and 8 are preferred over the more common O-equatorial- P-equatorial structure, presumably due to the stereo-electronic coordination preferences of the chiral hydroxylated phosphine. To the best of our knowledge, the ligand arrangement observed in 7b and 8 is unique among ESBO complexes. The rhodium(I) complex 6 is active as a catalyst precursor in the hydroformylation of styrene, yielding initial turnover frequencies up to 440 h −1 and selectivities up to 74% in the branched aldehyde. Unfortunately, no enantiomeric excesses have been observed within experimental error.

Mixed-metal assemblies containing multiply bonded dirhenium species linked through thiocyanate- and cyanide-containing bridging units.

The lability of the terminal Re-Cl bond that is cis to the bridging CO ligand in the edge-sharing bioctahedral complexes Re(2)(mu-Cl)(mu-CO)(mu-PP)(2)Cl(3)(L), where PP = Ph(2)PC(=CH(2))PPh(2) (dppE) when L = CO (1) and PP = Ph(2)PCH(2)PPh(2) (dppm) when L = CO (2) or XyINC (3), has been exploited in the preparation of mixed-metal Re(4)Pd(2), Re(2)Ag, Re(2)W, Re(2)Pt, and Re(2)Rh assemblies, in which the dirhenium units are bound to the other metals through NCS or CN bridges. These complexes, which retain the Re=Re bonds of the parent dirhenium complexes, comprise the novel centrosymmetric complex [Re(2)Cl(3)(mu-dppE)(2)(CO)(2)(mu-NCS)](2)Pd(2)(mu-SCN)(mu-NCS)Cl(2) (9), and the trimetallic complexes Re(2)Cl(3)(mu-dppE)(2)(CO)(2)[(mu-NC)Ag(CN)] (10), Re(2)Cl(3)(mu-dppE)(2)(CO)(2)[(mu-NC)W(CO)(5)] (11), [Re(2)Cl(3)(mu-dppE)(2)(CO)(2)[(mu-NC)Pt(CN)(CN-t-Bu)(2)]]PF(6) (12), [Re(2)Cl(3)(mu-dppE)(2)(CO)(2)[(mu-N(CN)(2))Rh(CO)(PPh(3))(2)]]O(3)SCF(3) (13), and Re(2)Cl(3)(mu-dppm)(2)(CO)(2)[(mu-NC)W(CO)(5)] (16). The identities of 9 and 16 have been established by X-ray crystallography, and all complexes characterized by IR and NMR spectroscopy and cyclic voltammetry. The reactions of the dicarbonyl complex 1, and the isomeric pair of complexes Re(2)Cl(4)(mu-dppm)(2)(CO)(CNXyl), which have edge-sharing bioctahedral (ESBO) (3) and open bioctahedral (OBO) (4) geometries, with Na[N(CN)(2)] and K[C(CN)(3)] have been used to prepare complexes in which the uncoordinated CN groups have the potential to coordinate other mono- or dimetal units to form extended arrays. The complexes which have been prepared and characterized are the monosubstituted species Re(2)Cl(3)(X)(mu-dppE)(2)(CO)(2) (X = N(CN)(2) (14) or C(CN)(3) (15)) and Re(2)Cl(3)(X)(mu-dppm)(2)(CO)(CNXyl) (X = N(CN)(2) (17) or C(CN)(3) (18) with ESBO structures; X = N(CN)(2) (19) or C(CN)(3) (20) with OBO structures), of which 15, 18, and 20 have been characterized by single-crystal X-ray structure determinations. The substitutional labilities of the Re-Cl bonds in the complexes Re(2)Cl(4)(mu-dppm)(2)(CO) (5), Re(2)Cl(4)(mu-dppm)(2)(CNXyl) (6), and Re(2)Cl(4)(mu-dppm)(2) (7) toward Na[N(CN)(2)] and K[C(CN)(3)] have also been explored and the complexes Re(2)Cl(3)(X)(mu-dppm)(2)(CO) (X = N(CN)(2) (21) or C(CN)(3) (22)), Re(2)Cl(3)(X)(mu-dppm)(2)(CNXyl) (X = N(CN)(2) (23) or C(CN)(3) (24)), Re(2)Cl(2)(X)(2)(mu-dppm)(2)(CNXyl) (X = N(CN)(2) (25) or C(CN)(3) (26)), Re(2)[N(CN)(2)](4)(mu-dppm)(2) (27), and Re(2)[C(CN)(3)](4)(mu-dppm)(2) (28) isolated in good yield. Single-crystal X-ray structure determinations of 24, 26, and 27 have shown that the Re-Re triple bonds present in the starting materials 5-7 are retained in these products.

The structure of the dichromium compound Cr 2(μ-OH) 2(μ-3,5Cl 2-form) 2(η 2-3,5Cl 2-form) 2

With exposure to trace amounts of air and moisture, the Cr 2(II, II) complex Cr 2(μ-3,5Cl 2-form) 4, where 3,5Cl 2-form is [(3,5-Cl 2C 6H 3)NC(H)N(3,5-Cl 2C 6H 3) −], undergoes an oxidative addition reaction. Structural information from the X-ray crystal structure of the edge-sharing bioctahedral (ESBO) Cr 2(III, III) product Cr 2(μ-OH) 2(μ-3,5Cl 2-form) 2(η 2-3,5Cl 2-form) 2 ( 1) indicates 1 has a significantly longer Cr–Cr distance [2.732(2) Å] than Cr 2(μ-3,5Cl 2-form) 4 [1.9162(10) Å], but the shortest Cr–Cr distance in an ESBO Cr 2(III, III) complex recorded to date.

Preparation and Structure of Two Ditungsten Compounds Synthesized with the 3,5-Dichlorophenylformamidinate Ligand.

The structure, preparation, and spectroscopy of a W(2)(II, II) paddlewheel complex with four formamidinate ligands is reported. Upon exposure to air and moisture in solution, the W(2)(II, II) core undergoes an oxidative addition reaction to form a W(2)(III, III) edge-sharing bioctahedral (ESBO) complex with two bridging hydroxides. The products, W(2)(&mgr;-DCPF)(4) (1) and W(2)(&mgr;-OH)(2)(&mgr;-DCPF)(2)(eta(2)-DCPF)(2) (2) where DCPF is [(3,5-Cl(2)C(6)H(3))NC(H)N(3,5-Cl(2)C(6)H(3))(-)], were characterized by UV-vis and (1)H NMR spectroscopy. Structural data are presented for W(2)(&mgr;-DCPF)(4) with four bridging formamidinate ligands and W(2)(&mgr;-OH)(2)(&mgr;-DCPF)(2)(eta(2)-DCPF)(2) with the formamidinate ligands in both chelating and bridging positions. Crystallographic data for W(2)(&mgr;-DCPF)(4) (1) and W(2)(&mgr;-OH)(2)(&mgr;-DCPF)(2)(eta(2)-DCPF)(2) (2) are as follows: (1), C(68)H(60)Cl(16)N(8)O(4)W(2), a = 12.6906(3) Å, b = 12.7818(4) Å, c = 13.0450(3) Å, alpha = 109.173(1) degrees, beta = 93.865(1) degrees, gamma = 103.746(1) degrees, triclinic, P&onemacr; (No. 2), and Z = 1 and (2), C(59)H(35)Cl(16)N(8)O(2)W(2), a = 17.1524(1) Å, b = 20.6568(3) Å, c = 19.3959(3) Å, beta = 102.791(1) degrees, monoclinic, C2/c (No. 15), and Z = 4.

Synthesis of a dinuclear tungsten complex, W 2(μ-H)(μ-OC 2H 5)(μ-O 2CC 6H 5)(η 1-OC 2H 5) 2Cl 2(η 2- P, P′-dppp) where dppp is 1,3 bis(diphenylphosphino) propane

During the exploration of new synthetic routes to multiply-bonded ditungsten compounds, a distorted edge-sharing bioctahedral product W 2(μ-H)(μ-OC 2H 5)(μ-O 2CC 6H 5)(η 1-OC 2H 5) 2Cl 2(η 2- P, P′-dppp) where dppp is 1,3 bis(diphenylphosphino) propane was synthesized. X-ray crystallographic data were obtained for the W 2 7+ complex which contains a W–W bond distance of 2.4791(7) Å, similar to the W–W bond distances observed in W 2 6+ edge-sharing bioctahedral (ESBO) compounds (∼2.5 Å).

Two-electron photoelimination reactions from edge-sharing bioctahedral dimolybdenum(III) complexes

Dimolybdenum(III) edge-sharing bioctahedral (ESBO) complexes, Mo2X6(dtbppm)2 [dtbppm = bis(di(4-tert-butylphenyl)phosphinomethane), X= Cl, Br, I], photoreact when irradiated with near-UV light, cleanly producing the corresponding quadruple metal–metal bonded complexes, Mo2X4(dtbppm)2.

W 2(μ-H) 2Cl 4(μ-dppm) 2: the bulk synthesis of a bridging hydride

The synthesis of the ditungsten species W 2(μ-H) 2Cl 4(μ-dppm) 2 marks the first bulk preparation of a bridging hydride W(III–III) species. The compound retains the characteristic edge-sharing bioctahedral (ESBO) geometry with a W W bond distance of 2.3918(7)Å. Although several Group VI ESBO complesxes have been prepared, only two with bridging hydride groups, W 2(μ-H) 2Cl 4(μ-dppm) 2 and W 2(μ-H) 2(μ-O 2CC 6H 5) 2 Cl 2(P(C 6H 5) 3) 2, have been reported to date. The complex W 2(μ-H) 2Cl 4(μ-dppm) 2 has been characterized with 31P 1H, 1H NMR, UV-vis, and IR spectroscopic techniques. A singlet is observed in the 31P 1H NMR spectra of W 2(μ-H) 2Cl 4(μ-dppm) 2 at δ = 20.464 ppm with J P−W = 126Hz and the IR spectra includes the tungsten-hydride symmetric stretch at 1602 cm −1.

The evaluation of singlet-triplet separations for [W 2(μ-H)(μ-Cl)Cl 4(μ-dppm) 2] and [W 2(μ-H) 2 (μ-O 2CC 6H 5) 2(P(C 6H 5) 3) 2] based on 31P NMR and crystallographic data

The singlet-triplet separations for the edge-sharing bioctahedral (ESBO) complex W 2(μ-H)(μ-Cl)(Cl 4(μ-dppm) 2 · (THF) 3 ( II) has been studied by 31P NMR spectroscopy. The structural characterization of [W 2(μ-H) 2(μ-O 2CC 6H 5) 2Cl 2(P(C 6H 5) 3) 2] ( I) by single-crystal X-ray crystallography has allowed the comparison of the energy of the HOMOLUMO separation determined using the Fenske-Hall method for a series of ESBO complexes with two hydride bridging atoms, two chloride bridging atoms and the mixed case with a chloride and hydride bridging atom. The complex representing the mixed case, [W 2(μ-H)(μ-Cl)Cl 4(μ-dppm) 2 · (THF) 3] ( II), has been synthesized and the value of −2 J determined from variable-temperature 31P NMR spectroscopy.

Edge-Sharing Bioctahedral Diruthenium(III) Complexes Containing Methoxy and Carboxylate Bridges: X-ray Structures, Redox Behavior, and Core Stability

Edge-sharing bioctahedral (ESBO) complexes [Ru-2(OMe)(O2CC6H4-p-X)3(1-MeIm)(4)](ClO4)2 (X = OMe (1a), Me (1b)) and [Ru-2(O2CC6H4-P-X)(4)(1-MeIm)(4)](ClO4)(2) (X = OMe (2a), Me (2b)) are prepared by reacting Ru2Cl(O(2)CR)(4) with 1-methylimidazole (1-MeIm) in methanol followed by treatment with NaClO4. Complex 2a and the PF6- salt (1a') of 1a have been structurally characterized. Crystal data for 1a.1.5MeCN. 0.5Et(2)O: triclinic, P (1) over bar, a = 13.125(2) Angstrom, b = 15.529(3) Angstrom, c 17.314(5) Angstrom, a; 67.03(2)degrees, beta 68.05(2)degrees, gamma = 81.38(1)degrees, V 3014(1) Angstrom(3), Z = 2. Crystal data for 2a: triclinic, P (1) over bar, a 8.950(1) Angstrom, b = 12.089(3) Angstrom, c = 13.735(3) Angstrom, alpha 81.09(2)degrees, beta = 72.27(1)degrees, gamma = 83.15(2)degrees, V = 1394(1) Angstrom(3), Z = 1. The complexes consist of a diruthenium(III) unit held by two monoatomic and two three-atom bridging ligands. The 1-MeIm ligands are at the terminal sites of the [Ru-2(mu-L)(eta(1):mu-O(2)CR)(eta(1):eta(1):mu-O(2)CR)(2)](2+) core having a Ru-Ru single bond (L = OMe or eta(1)-O(2)CR). The Ru-Ru distance and the Ru-O-Ru angle in the core of 1a' and 2a are 2.49 Angstrom and similar to 76 degrees. The complexes undergo one-electron oxidation and reduction processes in MeCN-0.1 M TBAP to form mixed-valence diruthenium species with Ru-Ru bonds of orders 1.5 and 0.5, respectively.

Further considerations on the structure and bonding in edge-sharing bioctahedral complexes

Extended Huckel MO calculations have been carried out on model compounds that mimic transition metal edge-sharing bioctahedral (ESBO) complexes of stoichiometry M 2X 6L 4 (X=anionic ligand with lone pairs for bridge-bonding and for π bonding, L=neutral 2-electron donor ligand), and for the corresponding mononuclear cis-MX 4L 2 system. The widening of the cis-L eq-M-L eq angle (eq=equatorial) is shown to cause the narrowing of the opposite X-M-X angle which, for the ESBO complexes, disfavors the formation of a strong metal-metal interaction. The calculations also address the importance of the M-X π interactions in metal-metal bonded dimers. The X ax (ax=axial) ligands are shown to be better π donors than the X eq ligands for the configurations d 1-d 1 through d 5-d 5, the differential between the π donating abilities in the two different positions being maximal for the d 1-d 1 configuration. This effect is proposed to be responsible for the preference of d 1-d 1 systems for the ESBO structure having all L ligands in equatorial positions, whereas the metal-metal bonded ESBO compounds of all other electronic configurations as well as all non-bonded ESBO complexes prefer the structure with two L eq on one metal and two L ax, on the other one on steric grounds. The MO model presented here is also in excellent agreement with the observed trends of M-Cl ax, M-Cl eq and M-Cl br (br=bridging) bond distances as a function of d nd- n configuration.

Some new dinuclear rhodium compounds: preparations and structures

The following seven dinuclear rhodium compounds have been prepared, characterized spectroscopically and their crystal and molecular structures determined: 1,5,6-Rh 2Cl 6(PEt 3) 3 ( 1), 1,3,6,8-Rh 2Cl 6(PEt 3) 4 ( 2), 1,5,6-Rh 2Cl 6(PPr n 3) 3 ( 3), 1,5,6-Rh 2Br 6(PEt 3) 3 ( 4), 1,5,6-Rh 2Br 6(PPr n 3) 3 ( 5), 1,5,6-Rh 2Br 6(PBu n 3) 3 ( 6), 1,3,6,8-Rh 2Br 6(PBu n 3) 4 ( 7). In each case there are marked structural trans effects such that RhX br bonds trans to PR 3 ligands are 0.12–0.17 Å (av. 0.15 Å) longer than those trans to an RhX bond. The fact that for the face-sharing bioctahedral (FSBO) molecules only one of the two possible isomers was formed and for the edge-sharing bioctahedral (ESBO) molecules only one of the nine possible isomers was formed is discussed and some principles that largely account for these observations are invoked. These principles are (i) that no μ-X ligand will have both of its RhX bonds trans to PR 3 and (ii) that in ESBO species phosphine ligands on two adjacent axial positions are sterically disfavored.