The preparation, characterisation and low temperature X-ray structure of Ru6(η-μ4-CO)2(CO)13(η6-C6Me6)

The new organometallic cluster (η²-μ₄-CO)₂(CO)₁₃(η⁶-C₆Me₆) has been prepared by the thermolysis of Ru₃(CO)₁₂ with hexamethylbenzene in octane and characterised by a single crystal X-ray diffraction study. It is isostructural with the known cluster Ru₆(η²-μ₄-CO)₂(CO)₁₃(η⁶-C₆H₃Me₃) and the metal core constitutnts the same tetrahedral Ru₄ unit with two edge-bridging Ru atoms. The mesitylene derivative has been shown to undergo rearrangement to afford the octahedral carbido cluster Ru₆C(CO)₁₄(η⁶-C₆H₃Me₃), but this conversion is not observed for the new hexamethylbenzene derivative.     

Chiroptical properties of heterotrimetallic ‘wing-tip’ butterflies; synthesis and crystal structure of (μ-H)Ru2Fe2PdC(CO)12 (η-ß-C10H15)

HRu₂Fe₂PdC(CO)₁₂ (η³-ß-C₁₀H₁₅) cluster was prepared in the reaction of (Et₄N) [HFe₂Ru₂C(CO)₁₂] with [Pd(η³-ß-C₁₀H₁₅)Cl]₂. X-ray structural study of HRu₂Fe₂PdC(CO)₁₂ (η³-ß-C₁₀H₁₅) (where ß-C₁₀H₁₅ is ß-pinenyl) revealed a wing-tip butterfly geometry of the metal core and (1R, 2S, 3S, 5R) absolute configuration for both crystallography independent molecules in the crystal. Chiroptical properties of this cluster are compared with other clusters containing a Pd(η³-ß-C₁₀H₁₅) fragment and discussed.     

Mixed-metal butterfly acetamidediato clusters derived from a highly reactive ruthenium oxo cluster: synthesis and characterization of [(PPh3)2N][MRu3(CO)12(η-μ3-NC(μ-O)CH3)], (M = Mn or Re)

Analogy with the isolable oxo cluster [Fe₃(CO)₉(μ₃-O)]²⁻, which is structurally interesting and synthetically useful, prompted the present attempt to synthesize its ruthenium analog. Although the high reactivity of [Ru₃(CO)₉(μ₃-O)]²⁻ (I) prevented its isolation, the reaction of this species with [M(CO)₃(NCCH₃)]⁺, where M = Mn or Re, yields [PPN][MRu₃(CO)₁₂(η²-μ₃-NC(μ-O)CH₃]⁻. The high nucleophilicity of the oxo ligand in [Ru₃(CO)₉(μ₃-O)]²⁻ (I) appears to be responsible for the conversion of acetonitrile to an acetamidediato ligand and for the instability of I. The crystal structure of [PPN][MnRu₃(CO)₁₂(η²-μ₃-NC(μ-O)CH₃)]] reveals a hinged butterfly array of metal atoms in which the acetamidediato ligand bridges the two wings with μ₃-N bonding to an Mn and two Ru atoms, and μ-O bonding to an Ru atom.     

Photocatalyzed isomerization of 1-pentene by Ru3(CO)12 adsorbed onto porous Vycor glass

UV photolysis of Ru₃(CO)₁₂ physisorbed onto porous Vycor glass leads to the oxidative addition product (μ-H)Ru₃(CO)₁₀(μ-OSi). The latter reacts thermally with 1-pentene to form a stable adduct, HRu₃(CO)₁₀(OSi)(1-C₅H₁₀), and photolysis of the adduct results in isomerization of the alkene. HRu₃(CO)₁₀(OSi)(1-C₅H₁₀) + hv → (μ-H)Ru₃(CO)₁₀(μ-OSi) + 2-pentene As with other photoactivated hybrid systems, the cis-/trans-2-pentene product ratio changes during photolysis. Unlike the other systems, where light generates a thermal catalyst, the data gathered here indicate a photoassisted catalytic process in which photoactivation of HRu₃(CO)₁₀(OSi)(1-C₅H₁₀) leads to an excited state similar to a π-allyl complex.     

Synthesis, characterization and reactivity of platinum-substituted ketenes [PtX{η-C(PPh3)CO}L2]BF4, (X=Me, Cl; L2=1,5-cyclooctadiene, 1,2-bis(diphenylphosphino)ethane, cis-1,2-bis(diphenylphosphino)ethene). Steric and electronic effects of the ancillary ligands

Ketenylidenetriphenylphosphorane, Ph₃PC=C=O, 1, has been used to synthesize platinum-substituted ketenes [PtMe{η¹-C(PPh₃)CO}L₂]BF₄, 2a, b (L₂=1,5-cyclooctadiene, cod (a), 1,2-bis(diphenylphosphino)ethane, diphos (b)). Parent compound [PtCl{η¹-C(PPh₃)CO}L₂]BF₄, 3, with L₂=cis-1,2-bis(diphenylphosphino)ethene, diphoe, was also synthesized, which is stable only at low temperature. The stability of 2 and 3 and the reactivity of the C=C=O moiety have been examined and discussed in terms of the electronic and steric characteristics of the ancillary ligands, also taking into account the reactivity of the ‘PtXL₂’ fragment with other carbonyl stabilized phosphorus ylides, Ph₃PCHCOR (R=Me, Ph, OMe, OEt).     

Synthesis and properties of the dichloro [tetramethyl (trifluoromethyl)cyclopentadienyl]ruthenium dimer: X-ray structure of [M2(η-C5Me4CF3)2Cl2(μ-Cl)2] (M=Ru, Rh)

The reaction of RuCl₃(H₂O), with C₅Me₄CF₃J in refluxing EtOH gives [Ru₂(η⁵-C₅Me₁CF₂)₂ (μ-Cl₂] (20 in 44% yield. Dimer 2 antiferromagnetic (−2J=200 cm¹). The crystal structures of 2 (rhombohedral system, R3 space group, Z=9, R=0.0589) and [Rh₂(η⁵-C₅Me₄CF₃(₂Cl₂(μ-Cl)₂] (3) (rhombohedral system. space group, Z = 9, R = 0.0641) were solved; both complexes have dimeric structures with a trans arrangement of the η⁵-C₅Me₄CF₄ rings. Comparison of the geometry of 2 and 3 with those of the corresponding η⁵-C₅Me₅ complexes shows that lowering the ring symmetry causes significant distortion of the M₂(μ-Cl)₂ moiety. The analysis of the MCl₃ fragment conformations in 2 and 3 and in the η⁵-C₅ME₅ analogues shows that they are correlated with the M---M distances. The Cl atoms are displaced by Br on reaction of 2 with KBr in MeOH to give the diamagnetic dimer [Ru₂(η⁵-C₅Me₄CF₃)₂Br₂ (μ-Br₂] (4). Complex 2 reacts with O₂ in CH₂Cl₂ solution at ambient temperature to form a mixture of isomeric η⁶-fulvene dimers [Ru₂(η⁶-C₅Me₃CF₃ = CH₂)₂Cl₂(μ-Cl)₂] (5). Reactions of 5 with CO and allyl chloride give Ru(η⁵-C₅Me₃CF₃CH₂Cl)(CO)₂Cl (6) and Ru(η⁵-C₅Me₃CF₃CF₃CH₂Cl)(η³-C₃H₅)Cl₂ (7) respectively.     

Reactivity of {(Ph3P)Pt[μ-η-H---SiH(Ar)]}2 (Ar=2-isopropyl-6-methylphenyl) with phosphines. X-ray crystal structure of trans-{(dppe)Pt[μ-SiH(Ar)]}2

The dinuclear Pt---Si complex {(Ph₃P)Pt{μ-η²-H---SiH(IMP)]}₂ (trans-1a–cis-1b=3:1; IMP=2-isopropyl-6-methylphenyl) reacted with basic phosphines such as 1,2-bis(diphenylphosphino)ethane (dppe) and dimethylphenylphosphine (PMe₂Ph) to afford different dinuclear Pt---Si complexes with loss of H₂, {(P)₂Pt[μ-SiH(IMP)]}₂ [P=dppe, trans-2a (major), cis-2b (trace); PMe₂Ph, 3 (trans only)]. Complexes 2 and 3 were characterized by multinuclear NMR spectroscopy and X-ray crystallography (2a). In contrast, the reaction of 1a,b with the sterically demanding tricyclohexylphosphine (PCy₃) afforded {(Cy₃P)Pt{μ-η²-H---SiH(IMP)]}₂ (trans-4a–cis-4b 2:1) analogous to 1a,b where the central Pt₂Si₂(μ-H)₂ core remains intact but the PPh₃ ligands have been replaced by PCy₃. Complexes 4a and 4b was characterized by multinuclear NMR and IR spectroscopies.     

Reaction of [Pt{Fe(CO)3(NO)}2(PhCN)2] with diphenyl(2-pyridyl)phosphine selenide. Crystal structure of [(CO)3Fe(μ3-Se){Pt(CO)P(2-C5H4N)Ph2}2] and its theoretical study

The reaction between the linear trinuclear complex [Pt{Fe(CO)₃(NO)}₂(PhCN)₂] and Ph₂(2-C₅H₄N)PSe led to the isolation and characterization of the 46-electron cluster [(CO)₃Fe(μ₃-Se){Pt(CO)P(2-C₅H₄N)Ph₂}₂] (1), whose structure has been determined by X-ray diffraction methods. The cluster typology, which consists of an open triangle Pt---Fe---Pt capped by a μ₃-Se atom, is rather rare. The chemical bonding in 1 and in similar systems has been analyzed through density functional theory (DFT) and qualitative MO approaches. A strict analogy with the well understood L₂M(μ-acetylene)ML₂ systems is invoked by considering 1 as formed by the (CO)₃FeSe tetrahedral unit stabilized by sidewise interactions of the triple bond with two d¹⁰-L₂M fragments. Otherwise, the 18-electron (CO)₃FeSe monomer is unstable as an isolate molecule. This is confirmed by our DFT calculations that indicate how the well characterized dimer (CO)₃Fe(μ-Se₂)Fe(CO)₃ lies as much as, approximately, 58 kcal mol⁻¹ deeper in energy. Finally, by considering an analogy with [L₂M(μ-dichalcogen)ML₂]^{0, +2} redox systems (M=Pd, Pt), reduction of 1 to a dianion has been hypothesized and the structure of the latter has been tentatively explored by DFT calculations.     

Hetero-polynuclear complexes containing bridging diphenylphosphino-alkenes and -alkynes. The crystal structure of an Rh2{μ−ν:ν−P(Ph2) (CH2)3C≡CR}Co2 complex

The phosphinoalkenes Ph₂P(CH₂)_nCH=CH₂ (n= 1, 2, 3) and phosphinoalkynes Ph₂P(CH₂)_n C≡CR (R = H, N = 2, 3; R = CH₃, N = 1) have been prepared and reacted with the dirhodium complex (η−C₅H₅)₂Rh₂(μ−CO) (μ−η²−CF₃C₂CF₃). Six new complexes of the type (ν−C₅H₅)₂(Rh₂(CO) (μ−η¹:η¹−CF₃C₂CF₃)L, where L is a P-coordinated phosphinoalkene, or phosphinoalkyne have been isolated and fully characterized; the carbonyl and phosphine ligands are predominantly trans on the Rh---Rh bond, but there is spectroscopic evidence that a small amount of the cis-isomer is formed also. Treatment of the dirhodium-phosphinoalkene complexes with (η−CH₃C₅H₄)Mn(CO)₂thf resulted in coordination of the manganese to the alkene function. The Rh₂---Mn complex [(η−C₅H₅)₂Rh₂(CO) (μ−η¹:η¹−CF₃C₂CF₃) {Ph₂P(CH₂)₃CH=CH₂} (η−CH₃C₅H₄)Mn(CO)₂] was fully characterized. Simi treatment of the dirhodium-phosphinoalkyne complexes with Co₂(CO)₈ resulted in the coordination of Co₂(CO)₆ to the alkyne function. The Rh₂---Co₂ complex [(η−C₅H₅)₂Rh₂(CO) (μ−η¹:η¹−CF₃C₂CF₃) {Ph₂PCH₂C≡CCH₃}Co₂(CO)₂], C₃₇H₂₅Co₂F₆O₇PRh₂, was fully characteriz spectroscopically, and the molecular structure of this complex was determined by a single crystal X-ray diffraction study. It is triclinic, space group (C_i¹, No. 2) with a = 18.454(6), B = 11.418(3), C = 10.124(3) Å, = 112.16(2), β = 102.34(3), γ = 91.62(3)°, Z = 2. Conventional R on |F| was 0.052 fo observed (I > 3σ(I)) reflections. The Rh₂ and Co₂ parts of the molecule are distinct, the carbonyl and phosphine are mutually trans on the Rh---Rh bond, and the orientations of the alkynes are parallel for Rh₂ and perpendicular for Co₂. Attempts to induce Rh₂Co₂ cluster formation were unsuccessful.     

The structure of (η-C6H5Cl)Cr(CO)2PPh3

In a synthetic route that varies from the standard procedure requiring irradiation, the (η⁶-C₆H₅Cl)Cr(CO)₂PPh₃ complex is obtained upon reacting (η⁶-C₆H₅Cl)Cr(CO)₃ with tetrakis(triphenylphosphine)palladium(0), CuI, and trimethylsilylphenylacetylene in triethylamine. The X-ray crystal structure of the yellow–orange crystals of (η⁶-C₆H₅Cl)Cr(CO)₂PPh₃ allows structural comparisons to related (arene)Cr(CO)₂PR₃ complexes.     

Olefin versus sulfur coordination of benzo[b]thiophene (BT) in Cp(CO)2Re(η:η-μ2-BT)Cr(CO)3

Reactions of Cr(CO)₃(η⁶-BT), in which the Cr is π-coordinated to the benzene ring of benzo[b]thiophene (BT), with Cp′(CO)₂Re(THF), where Cp′ = η⁵-C₅H₅ or η⁵-C₅Me₅, give the products Cp′(CO)₂Re(η²:η⁶-μ₂-BT)Cr(CO)₃ in which the Cr remains coordinated to the benzene ring and Re is bound to the C(2)=C(3) double bond. An X-ray diffraction study of Cp(CO)₂Re(η²:η⁶-μ₂-BT)Cr(CO)₃ (3) provides details of the geometry. This structure contrasts with that of the Cp′(CO)₂Re(BT) complexes that exist as mixtures of isomers in which the BT is coordinated to the Re through either the double bond (2,3-η²) or the sulfur (η¹(S)). Thus, the electron-withdrawing Cr(CO)₃ group in 3 stabilizes the 2,3-η² mode of BT coordination to the Cp′(CO)₂Re fragment. Implications of these results for catalytic hydrodesulfurization of BT are discussed. Crystal data for 3: triclinic, space group

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Chelated alkyl halide manganese complexes (η:η-C5H4CH2CH2X)Mn(CO)2 from photolysis of (η-C5H4CH2CH2X)Mn(CO)3

Manganese tricarbonyl complexes (η⁵-C₅H₄CH₂CH₂Br)Mn(CO)₃ (3) and (η⁵-C₅H₄CH₂CH₂I)Mn(CO)₃ (4), with an alkyl halide side chain attached to the cyclopentadienyl ligand, were synthesized as possible precursors to chelated alkyl halide manganese complexes. Photolysis of 3 or 4 in toluene, hexane or acetone-d₆ resulted in CO dissociation and intramolecular coordination of the alkyl halide to manganese to produce (η⁵:η¹-C₅H₄CH₂CH₂Br)Mn(CO)₂ (5) and (η⁵:η¹-C₅H₄CH₂CH₂I)Mn(CO)₂ (6). Low temperature NMR and IR spectroscopy established the structures of 5 and 6. Photolysis of 3 in a glass matrix at 91 K demonstrated CO release from manganese. Low temperature NMR spectroscopy established that the coordinated alkyl halide complexes are stable to approximately −20°C.     

Comparison in halide binding ability between the unsaturated clusters [Pd3(μ-dppm)3(CO)] and [PdPtCo(μ-dppm)2(CO)3(CNBu)] (dppm = Ph2PCH2PPh2)

The trinuclear clusters [Pd₃(μ-dppm)₃(CO)]²⁺ and [PtPdCo(μ-dppm)₂(CO)₃(CN^tBu)]⁺ exhibit a large and a small cavity, respectively, formed by the phenyl rings of the bridging diphosphine ligands. Their binding constants (K₁₁) with halide ions (X⁻) were obtained by UV-Vis spectroscopy. The binding ability varies as I⁻ > Br⁻ > Cl⁻, and [Pd₃(μ-dppm)₃(CO)]²⁺ > [ptPdCo(μ-dppm)₂-(CO)₃(CN^tBu)]⁺. The MO diagram for the related cluster [Pd₂Co(μ-dppm)₂(CO)₄]⁺ has been addressed theoretically in order to predict the nature of the lowest energy electronic bands. For this class of compounds, the lowest energy bands are assigned to charge transfers from the Co center to the Pd₂ centers.     

Cis- and trans-titanium complexes with doubly silyl-bridged dicyclopentadienyl ligands: molecular structure of [(TiCl)2(μ-O){(SiMe2)2(η-C5H3)2}]2(μ-O)2

The reaction of TiCl₄ with Li₂[(SiMe₂)₂(η⁵-C₅H₃)₂] in toluene at room temperature afforded a mixture of cis- and trans-[(TiCl₃)₂{(SiMe₂)₂(η⁵-C₅H₃)₂}] in a molar ratio of 1/2 after recrystallization. The complex trans-[(TiCl₃)₂{(SiMe₂)₂(η⁵-C₅H₃)₂}] was hydrolyzed immediately by the addition of water to THF solutions to give trans-[(TiCl₂)₂(μ-O){(SiMe₂)₂(η⁵-C₅H₃)₂}] as a solid insoluble in all organic solvents, whereas hydrolysis of cis-[(TiCl₃)₂{(SiMe₂)₂(η⁵-C₅H₃)₂}] under different conditions led to the dinuclear μ-oxo complex cis-[(TiCl₂)₂)(μ-O){(SiMe₂)₂(η⁵-C₅H₃)₂}] and two oxo complexes of the same stoichiometry [(TiCl)₂(μ-O){(SiMe₂)₂(η⁵-C₅H₃)₂}]₂(μ-O)₂ as crystalline solids. Alkylation of cis- and trans-[(TiCl₃)₂{(SiMe₂)₂(η⁵-C₅H₃)₂}] with MgCIMe led respectively to the partially alkylated cis-[(TiMe₂Cl)₂{(SiMe₂)₂(η⁵-C₅H₃)₂}] and the totally alkylated trans-[(TiMe₃)₂{(SiMe₂)₂(η⁵-C₅H₃)₂}] compounds. The crystal and molecular structure of the tetranuclear oxo complex [(TiCl)₂(μ-O){(SiMe₂)₂(η⁵-C₅H₃)₂}]₂(μ-O)₂ was determined by X-ray diffraction.     

From the {Cu(μ2-S)N}4 butterfly architecture to the {Cu(μ3-S)N}12 double wheel

Copper(I) complexes with {Cu(μ₂-S)N}₄ and {Cu(μ₃-S)N}₁₂ core portions of butterfly-shaped or double wheel architectures have been isolated in the reaction of Cu(I) with the Schiff base ligand C₆H₄(CHNC₆H₄S)₂, “iso-abt”, under different conditions. containing the tetranuclear electroneutral complex is formed by the reaction of CuI in acetonitrilic solution and recrystallization from DMF, whereas containing dodecanuclear wheels is accessible starting from CuBF₄. Complexes 2 and 4 represent the first examples of cyclic complexes with the same overall stoichiometry but different ring sizes. The ligand induces two different coordination environments around copper(I) by switching between μ₂- and μ₃-sulfur bridging modes.     

The synthesis, fluctionality and structural characterization of ReMo3H4(CO)12

The reaction of ReH₉²⁻ with Mo(diglyme)(CO)₃ leads to the formation of the mixed metal cluster trianion, ReMo₃H₄(CO)₁₂³⁻. This species has been characterized analytically, spectroscopically and through X-ray diffraction analysis. A pseudo-tetrahedral arrangement of M(CO)₃ fragments is adopted, such that each set of three carbonyl ligands eclipses the adjacent three tetrahedral edges, an apparent result of the location of the hydride ligands on the tetrahedral faces. Variable temperature NMR studies revealed a fluctional process for some of the carbonyl ligands, but not for the hydrides. Crystal data for [Me₄N]₃[ReMo₃H₄(CO)₁₂]·THF; space group P2₁/n, a = 12.157(2), B = 21.480(4), C = 15.964(3) Å, β = 98.26(1)°, Z = 4, R = 0.067 and R_w = 0.076.     

The coordination chemistry of cationic main group clusters: syntheses and structures of [Fe3(Se2)2(CO)10] and [Fe4(Se2)3(CO)12]

The reactions of the polysulfur and selenium cationic clusters S₈²⁺ and Se₈²⁺ with various iron carbonyls were investigated. Several new chalcogen containing iron carbonyl cluster cations were isolated, depending on the nature of the counteranion. In the presence of SbF₆⁻ as a counterion, the cluster [Fe₃(E₂)₂(CO)₁₀] [SbF₆]₂·SO₂ (E = S, Se) could be isolated from the reaction of E₈²⁺ and excess iron carbonyl. The cluster is a picnic-basket shaped molecule of two iron centers linked by two Se₂ groups, with the whole fragment capped by an Fe(CO)₄ group. Crystallographic data for C₁₀O₁₂Fe₃Se₄Sb₂F₁₂S (I): space group monoclinic P2₁/c, A = 11.810(9), b = 24.023(6), c = 10.853(7) Å, β = 107.15(5)°, V = 2942(3) ų, Z = 4, R = 0.0426, R_w = 0.0503. When Sb₂F₁₁⁻ is present as the counterion, or Se₄[Sb₂F₁₁]₂ is used as the cluster cation source, a different cluster can be isolated, which has the formula [Fe₄(Se₂)₃(CO)₁₂] [SbF₆]₂·3SO₂. The dication contains two Fe₂Se₂ fragments bridged by an Se₂ group. Crystallographic data for C₁₂O₁₈Fe₄Se₆Sb₂F₁₂S₃ (III): space group triclinic , b = 18.400(9), C = 10.253(4) Å, = 93.10(4), β = 103.74(3), γ = 93.98(3)°, V = 1995(1) ų, Z = 2, R = 0.0328, R_w = 0.0325. The CO stretches in the IR spectrum all show a large shift to higher wavenumbers, suggesting almost no τ backbonding from the metals. This also correlates with the observed bond distances. All the compounds are extremely sensitive to air and water, and readily lose SO₂ when removed from the solvent. Thus all the crystals were handled at −100°C. The clusters seem to be either insoluble or unstable in all solvents investigated.     

Ligand induced P---H bond formation and a comparison of the reactivity of two isomers of the carbonyl hydride [Pt2(μ-PBu2)2(H)(CO)(PBu2H)]CF3SO3

The Pt₂ ^(II) isomeric terminal hydrides [(CO)(H)Pt(μ-PBu^t ₂)₂Pt(PBu^t ₂H)]CF₃SO₃ (1a), and [(CO)Pt(μ-PBu^t ₂)₂Pt(PBu^t ₂H)(H)]CF₃SO₃ (1b), react rapidly with 1 atm of carbon monoxide to give the same mixture of two isomers of the Pt₂ ^(I) dicarbonyl [Pt₂(μ-PBu^t ₂)(CO)₂(PBu^t ₂H)₂]CF₃SO₃ (3-Pt); the solid state structure of the isomer bearing the carbonyl ligands pseudo-trans to the bridging phosphide was solved by X-ray diffraction. A remarkable difference was instead found between the reactivity of 1a and 1b towards carbon disulfide or isoprene. In both cases 1b reacts slowly to afford [Pt₂(μ-PBu^t ₂)(μ,η²,η²-CS₂)(PBu^t ₂H)₂]CF₃SO₃ (4-Pt), and [Pt₂(μ-PBu^t ₂)(μ,η²,η²-isoprene) (PBu^t ₂H)₂]CF₃SO₃ (6-Pt), respectively. In the same experimental conditions, 1a is totally inert. A common mechanism, proceeding through the preassociation of the incoming ligand followed by the P---H bond formation between one of the bridging P atoms and the hydride ligand, has been suggested for these reactions.     

The ReH6[P(C6H5)3]2 ion as a ligand: complexes with M(CO)3 fragments (M = Cr, Mo, W)

The reactions of [(H₅C₆)₃P]₂ReH₆⁻ with (CH₃CN)₃Cr(CO)₃, (diglyme)Mo(CO)₃ or (C₃H₇CN)₃W(CO)₃ led to the formation of [(H₅C₆)₃P]₂ReH₆M(CO)₃⁻ (M = Cr, Mo, W) complexes. These have been characterized by IR and NMR spectroscopies, as well as elemental analyses. A single crystal X-ray diffraction study has also been carried out for the M = Cr complex as a K(18-crown-6)⁺ salt. The complex crystallizes as a THF monosolvate in the monoclinic space group P2₁/n with a = 22.323(6), B = 9.523(2), C = 27.502(5) Å, β = 104.98(2)⁰ and V = 5648 ų for Z = 4. The Re---Cr separation is 2.5745(12) Å, and the two phosphine ligands are oriented unsymmetrically. Although the hydride ligands were not found, the presence of three bridging hydrides and a dodecahedral coordination geometry about rhenium could be inferred. Low temperature ¹H and ³¹P NMR spectroscopic studies did not reveal the low symmetry of the solid state structure.     

Reactivity, variable temperature solution H NMR studies and MO calculations of dimolybdenum allenylidene complexes of the type [(η-C5H5)2Mo2(CO)4(μ,η(4e)-C=C=CR1R2)]

The reactivity, towards nucleophiles and electrophiles, of dimolybdenum allenylidene complexes of the type [Cp₂Mo₂(CO)₄(μ,η²(4e)-C=C=CR1R2)] (Cp=η⁵-C₅H₅) has been investigated. The nucleophilic attacks occur at the C_γ carbon atom, while electrophiles affec the C atom. Variable temperature solution ¹H NMR studies show a dynamic behavior of these complexes consisting of an equilibrium between two enantiomers with a symmetrical [Cp₂Mo₂(CO)₄(μ-σ,σ(2e)-C=C=CR1R2)] transition state. Extended Hückel MO calculations have been carried out on the model [Cp₂Mo₂(CO)₄(μ,η²-C=C=CH₂]. The calculated charges of the allenylidene carbon atoms suggest that the electrophilic attacks are under charge control, while the nucleophilic attacks are rather under orbital control.