Reactivity of electron-deficient triosmium quinoline cluster [Os3(CO)9(μ3-η-C9H6N)(μ-H)] with alkynes

Reactions of the electron-deficient triosmium cluster [Os₃(CO)₉(μ₃-η²-C₉H₆N)(μ-H)] (1) with various alkynes are described. Cluster 1 readily reacts with the activated alkyne dimethyl acetylenedicarboxylate (dmad) upon mild heating (65-70 °C) to give the adduct [Os₃(CO)₉(μ-C₉H₆N)(μ₃-MeO₂CCCHCO₂Me)] (2). In contrast, a similar reaction of 1 with diphenylacetylene affords previously reported compounds [Os₃(CO)₁₀(μ-η²-C₉H₆N)(μ-H)] (3), [Os₃(CO)₉(μ-C₄Ph₄)] (4) and [Os₃(CO)₈{μ₃-C(C₆H₄)C₃Ph₃}(μ-H)] (5) while with 2-butyne gives only the known compound [Os₃(CO)₇(μ-C₄Me₄)(μ₃-C₂Me₂)] (6). The new cluster 2 has been characterized by a combination of spectroscopic data and single crystal X-ray diffraction analysis.     

SS Bond-activation of diorganyl disulfide by anionic [Mn(CO)5]: crystal structures of [Mn(SC5H4NO)3] and [(CO)3Mn(μ-SR)3Co(μ-SR)3Mn(CO)3] (R=C6H4NHCOPh)

The SS bond-activation of diorganyl disulfide by the anionic metal carbonyl fragment [Mn(CO)₅]⁻ gives rise to an extensive chemistry. Oxidative decarbonylation addition of 2,2′-dithiobis(pyridine-N-oxide) to [Mn(CO)₅]⁻, followed by chelation and metal-center oxidation, led to the formation of [Mn^II(SC₅H₄NO)₃]⁻ (1). The effective magnetic moment in solid state by SQUID magnetometer was 5.88 μ_B for complex 1, which is consistent with the Mn^II having a high-spin d⁵ electronic configuration in an octahedral ligand field. The average Mn(II)S, SC and NO bond lengths of 2.581(1), 1.692(4) and 1.326(4) Å, respectively, indicate that the negative charge of the bidentate 1-oxo-2-thiopyridinato [SC₅H₄NO]⁻ ligand in complex 1 is mainly localized on the oxygen atom. The results are consistent with thiolate-donor [SC₅H₄NO]⁻ stabilization of the lower oxidation state of manganese (Mn(I)), while the O,S-chelating [SC₅H₄NO]⁻ ligand enhances the stability of manganese in the higher oxidation state (Mn(II)). Activation of SS bond as well as OH bond of 2,2′-dithiosalicylic acid by [Mn(CO)₅]⁻ yielded [(CO)₃Mn(μ-SC₆H₄C(O)O)₂Mn(CO)₃]²⁻ (4). Oxidative addition of bis(o-benzamidophenyl) disulfide to [Mn(CO)₅]⁻ resulted in the formation of cis-[Mn(CO)₄(SR)₂]⁻ (R=C₆H₄NHCOPh) which was employed as a chelating metallo ligand to synthesize heterotrinuclear [(CO)₃Mn(μ-SR)₃Co(μ-SR)₃Mn(CO)₃]⁻ (8) possessing a homoleptic hexathiolatocobalt(III) core.     

Synthesis of some pentanuclear platinum clusters [Pt5(CO)6(L)4]. The X-ray structure of [Pt5(μ-CO)5(CO)(PCy3)4]

[Pt₅(μ-CO)₅(CO)L₄] (L = PPh₃1, PPh₂Bz 2, AsPh₃3, PEt₃4, PCy₃5) have been synthesized by reacting [Pt₃(μ-CO)₃(PR₃)₃] with H₂O₂ (1 and 2), by reduction of cis-[PtCl₂(CO)(PEt₃)] with Zn dust (4), and by the Zn reduction of [Pt₃(μ-CO)₃(PCy₃)₃] in the presence of [PtCl₂(CH₃CN)₂] (5). Complex 5 has not been observed previously and has been characterized by X-ray crystallography. Oxidation of the phosphine ligands with H₂O₂ is a new way to synthesize 1 and 2. The first complete NMR characterization of these complexes has also been achieved, and showed that these pentanuclear cluster complexes exhibit similar stereochemistries in solution and in the solid state. The observed ¹J_Pt-Pt values do not have any correlation with the corresponding bond lengths, again pointing out the irregular behaviour of such parameter in Pt complexes.     

Novel Rh-Mn mixed-metal carbonyl cluster containing μ3 and μ-bridging carbyne ligands, [(μ3-CC6H5)(μ-CC6H5)Rh2Mn2Cp2(μ-CO)3(CO)3]: structure, electrochemistry and catalytic properties

The cationic carbyne complex [Cp(CO)₂MnCC₆H₅]BBr₄ (1) reacts with PPN[Rh(CO)₄] (2) to give the title cluster [(μ₃-CC₆H₅)(μ-CC₆H₅) Rh₂Mn₂Cp₂(μ-CO)₃(CO)₃] (3) whose structure has been determined by X-ray diffraction. The electrochemical properties of 3 have been investigated using cyclic voltammetric method. At 60 °C and 2.0 MPa of initial total CO/H₂ (1:1) pressure, the catalytic activity of 3 towards hydroformylation of styrene has also been checked.     

A versatile entry into aqueous niobium chemistry: isolation and structure of the intermediate Nb4(μ2-O)2(μ2-OC2H5)4Cl4(OC2H5)4(HOC2H5)4

The reduction of ethanolic solutions of niobium pentachloride with zinc, followed by treatment with aqueous acids serves as a versatile entry into the aqueous solution chemistry of niobium. From the zinc-reduced solution, the major intermediate, Nb₄(μ₂-O)₂(μ₂-OC₂H₅)₄Cl₄(OC₂H₅)₄(HOC₂H₅)₄, was isolated and the crystal structure determined by X-ray crystallography. The complex crystallizes in the orthorhombic space group Pccn, with Z=4, a=21.0105(9), b=11.0387(5), c=19.1389(8), V=4438.9(3) ų, M_r=1090.19,R₁=0.0327 and wR₂=0.0876. The structure revealed a centrosymmetric tetrameric Nb(IV) complex, consisting of a pair of edge-sharing bi-octahedral Nb₂(μ₂-OC₂H₅)₄Cl₂(OC₂H₅)₂(HOC₂H₅)₂ units that are joined by two axial oxo ligands. The Nb-Nb distance of 2.7458(3) Å is consistent with a single metal-metal bond.     

Reactivity of [PdCl(μ-med)]2 with monodentate or bidentate ligands. Structure of the dinuclear complexes [Pd(μ-med)(PPh3)]2(BF4)2 and [Pd(μ-med)(bpy)]2(BF4)2. [Hmed=N-(2-mercaptoethyl)-3,5-dimethylpyrazole]

Treatment of the ligand N-(2-mercaptoethyl)-3,5-dimethylpyrazole with [Pd(CH₃COO)₂]₃ and reaction of [PdCl(μ-med)]₂ with pyridine (py) or triphenylphosphine (PPh₃) in the presence of AgBF₄ produced the following complexes: [Pd(CH₃COO)(μ-med)]₂, [Pd(μ-med)(py)]₂(BF₄)₂ and [Pd(μ-med)(PPh₃)]₂(BF₄)₂. Similar reactions carried out with 2,2^′-bipyridine (bpy) or 1,3-bis(diphenylphosphino)propane (dppp) produced [Pd(μ-med)(bpy)]_x(BF₄)_x (x=1 or 2) and [Pd(μ-med)(dppp)]_x(BF₄)_x (x=1 or 2). Treatment of [Pd(μ-med)(bpy)]_x(BF₄)_x with [PdCl₂(CH₃CN)₂] produced [Pd₃Cl₂(μ-med)₂(bpy)₂](BF₄)₂. Treatment of [Pd(μ-med)(dppp)]_x(BF₄)_x with [PdCl₂(CH₃CN)₂] produced a mixture of [Pd(μ-Cl)(dppp)]₂(BF₄)₂ and [Pd(μ-med)₂(dppp)]²⁺. X-ray crystal structures of [Pd(μ-med)(PPh₃)]₂(BF₄)₂ · 2CH₃CN and [Pd(μ-med)(bpy)]₂(BF₄)₂ · 0.5CH₃OH are presented.     

Chalcogenolato-bridged cyclometallated binuclear palladium complexes: Synthesis, spectroscopy, structures of [Pd2(μ-Cl)(μ-SMes)(C10H6NMe2-C,N)2] and [Pd2(μ-SePh)2(C10H6NMe2-C,N)2]

Reactions of orthometallated binuclear palladium complexes with NaER, obtained by NaBH₄ reduction of R₂E₂ in methanol, gave complexes, [Pd₂(μ-ER)₂(C^∩Y)₂] (HC^∩Y = N,N-dimethylbenzylamine (C₆H₅CH₂NMe₂), N,N-dimethylnaphthylamine (C₁₀H₇NMe₂), tri-o-tolylphosphine {P(tol-o)₃}; ER=SePh, SeMes, TePh, TeMes (Mes = 2,4,6-Me₃C₆H₂). Similar reactions of [Pd₂(μ-Cl)₂(C₁₀H₆NMe₂-C,N)₂] with Pb(SMes)₂ or MesSH in the presence of NaHCO₃ gave chloro/thiolato-bridged complex [Pd₂(μ-Cl)(μ-SMes)(C₁₀H₆NMe₂-C,N)₂]. The newly synthesized complexes were characterized by elemental analysis, UV-Vis, IR, NMR (¹H, ¹³C, ³¹P, ⁷⁷Se, ¹²⁵Te) spectroscopy. These complexes crystallized out preferentially in sym-cis configuration. A low energy charge transfer transition has been identified from chalcogenolate centers to an emptyπ^∗ orbital of cyclometallated ligand in absorption spectroscopy in these complexes. The structures of [Pd₂(μ-Cl)(μ-SMes)(C₁₀H₆NMe₂-C,N)₂] (1) and [Pd₂(μ-SePh)₂(C₁₀H₆NMe₂-C,N) ₂] (3) have been established by single crystal X-ray diffraction analyses. In the former, the two palladium atoms are held together by chloro and thiolato bridges whereas in the latter, the two phenylselenolato ligands bridge two palladium atoms. The pyrolysis of [Pd(μ-TeMes)(C₁₀H₆NMe₂-C,N)]₂ (10) in a furnace gave Pd₇Te₃ whereas thermolysis in TOPO afforded primarily PdTe₂.     

Reaction of cyanamide and their derivatives with (η-C5H5)Mn(CO)3 and (η-C5H4CH3)Mn(CO)3

The reaction of cyanamide and its derivatives with the (η⁵-C₅H₅)Mn(CO)₂(THF) and (η⁵-C₅H₄CH₃)Mn(CO)₂(THF) complexes affords the cyanamide substituted complexes of types (η⁵-C₅H₅)Mn(CO)₂(NCN(R′)(R″)) (2a-d) and (η⁵-C₅H₄CH₃)Mn(CO)₂(NCN(R′)(R″)) (3a-e). All complexes were characterized by spectroscopy (¹H, ¹³C NMR, IR), elemental and mass spectroscopy analysis. Complex 2b (η⁵-C₅H₅)Mn(CO)₂(NCN(CH₃)₂) was additionally examined by single crystal X-ray structure determination.     

Reaction of [Pt3(μ-CO)3(PBu3)3] with activated olefins and alkynes: the X-ray-structure of [Pt(CO)(PBu3)(CF3CCCF3)]

The reactions of the triangulo-cluster [Pt₃(μ-CO)₃(P^tBu₃)₃] with activated olefins and alkynes have been examined under various conditions. At low temperature, cluster fragmentation occurs yielding the Pt(0) complexes [Pt(CO)(P^tBu₃)(olefin)] (olefin = maleic anhydride and maleimide), while di(tert-butyl)acetylenedicarboxilate reacts quantitatively giving the dinuclear Pt(0) complex [Pt₂(CO)₂(P^tBu₃)₂(μ-η²:η²-^tBuO₂CCCCO₂^tBu)]. At higher temperature and in the presence of alkyne in large excess, the latter dimer converts quantitatively to the monomers [Pt(CO)(P^tBu₃)(alkyne)] (alkyne = CF₃CCCF₃ and ^tBuO₂CCCCO₂^tBu). The stereochemistry of these complexes has been established by NMR and IR measurements. The structure of [Pt(CO)(P^tBu₃)(CF₃CCCF₃)] was confirmed by X-ray diffraction analysis.     

Reduction chemistry of the mixed ligand metallocene [(C5Me5)(C8H8)U]2(μ-C8H8) with bipyridines

The U⁴⁺ cyclooctatetraenyl complex, [(C₅Me₅)(C₈H₈)U]₂(μ-C₈H₈), 1, reacts with two equiv of 4,4′-dimethyl-2,2′-bipyridine (Me₂bipy) and 2 equiv of 2,2′-bipyridine (bipy) to form 2 equiv of (η⁵-C₅Me₅)(η⁸-C₈H₈)U(Me₂bipy-κ²N,N′) and (η⁵-C₅Me₅)(η⁸-C₈H₈)U(bipy-κ²N,N′), respectively. X-ray crystallography, infrared spectroscopy, and density functional theory calculations indicate that the products are best described as U⁴⁺ complexes of bipyridyl radical anions. Hence, only one of the (C₈H₈)²⁻ ligands in 1 acts as a reductant and delivers 2 electrons per equiv of 1. Since the reduction potentials of uncomplexed (C₈H₈)²⁻, Me₂bipy, and bipy are −1.86, −2.15, and −2.10 V vs SCE, respectively, it is likely that prior coordination of the bipyridine reagents enhances the electron transfer.     

Hydrido-derivatives of [Ir4(CO)10(diphosphine)]: X-ray analyses of [Ir4H(CO)9(μ-Ph2PH(CH3)PPh2)] and [Ir4 H(CO)9(μ-Ph2P(CH2)nPPh2)] (n = 2, 3)

Some novel hydrido-anions of general formula [Ir₄H(CO)₉(μ-L-L)]⁻ (L-L = Ph₂PCH(CH₃)PPh₂, Ph₂P(CH₂)₂PPh₂, Ph₂P(CH₂)₃PPh₂ and Ph₂AsCH₂AsPh₂) have been obtained by the reaction of [Ir₄(CO)₁₀(μ-L-L)] with the base 1,8-diazabicyclo[5.4.0]undec-7-ene in wet dichloromethane. According to IR and ¹H, ³¹P and ¹³C NMR data at low temperature, these anionic derivatives display a single conformation in solution: three edge-bridging COs around the triangular basal face and both the hydride and the bidentate ligands located in axial positions relative to this face. The structures of four compounds were established by X-ray diffraction studies, which confirmed the configuration proposed on the basis of spectroscopic data.     

Oxidation of [Ir(η-C5Me5)(C8H4S8)] and crystal structures of [IrI(η-C5Me5)(C8H4S8)](I3) and [IrI(η-C5Me5)(C8H4S8)](I3)1/2(I7)1/2

[Ir(η⁵-C₅Me₅)(C₈H₄S₈)] (1) [ = 2-{(4,5-ethylenedithio)-1,3-dithiole-2-ylidene}-1,3-dithiole-4,5-dithionate(2−)] was reacted with iodine in dichloromethane to afford one-electron- and two-electron-oxidized species [IrI(η⁵-C₅Me₅)(C₈H₄S₈)] (2), [IrI(η⁵-C₅Me₅)(C₈H₄S₈)](I₃) (3) and [IrI(η⁵-C₅Me₅)(C₈H₄S₈)](I₅) (4). The oxidized species exhibit electrical conductivities of (1.1-5.0) × 10⁻⁶ S cm⁻¹ measured for compacted pellets at room temperature. The X-ray crystal structures of the two-electron-oxidized complexes 3 and 4 revealed the Ir-I bonds for both of them and the presence of for 3 and ions for 4 as the counter anions. They have many S-S and S-I non-bonding contacts to form two-dimensional molecular interaction sheets in the solid state.     

Synthesis and characterisation of [Pt2(μ-S)(μ-I)(PPh3)4] - A cationic iodo analogue of the metalloligand [Pt2(μ-S)2(PPh3)4]

Reaction of [Pt₂(μ-S)₂(PPh₃)₄] with a number of transition metal-iodo complexes leads to the formation of the cationic iodo analogue [Pt₂(μ-S)(μ-I)(PPh₃)₄]⁺, identified using electrospray ionisation mass spectrometry (ESI MS). Synthetic routes to this complex were developed, using the reaction of [Pt₂(μ-S)₂(PPh₃)₄] with either [PtI₂(PPh₃)₂] or elemental iodine. The complex was characterised by NMR spectroscopy, ESI MS and an X-ray structure determination, which reveals the presence of a planar, disordered {Pt₂SI}⁺ core. Monitoring the iodine reaction by ESI MS allows the identification of various iodine species, including the short-lived intermediate [Pt₂(μ-S)₂(PPh₃)₄I]⁺, which allows a mechanism for the reaction to be proposed.     

Potassium(I)thallium(I) heterometallic 3D polymeric mixed-anions compound, succinate-nitrate [K2Tl(μ-C4H4O4)(μ-NO3)]n

A three-dimensional polymeric K^ITl^I heterometallic compound [K₂Tl(μ-C₄H₄O₄)(μ-NO₃)]_n, with mixes succinate and nitrate ligands, has been synthesized and characterized. Its single-crystal X-ray structure shows two types of K⁺-ions with coordination numbers of seven and eight and one Tl⁺-ion with a coordination number of five. However, the arrangement of O-atoms for Tl^I suggests a gap or hole in the coordination geometry around this atom. This ‘hole’ is possibly occupied by a stereochemically ‘active’ electron lone pair of thallium atoms. Two hydrogen atoms of succinate situated 3.26 Å above the proposed site on the lone pair of Tl^I is oriented in such a way that it might be thought to be forming weak Tl-Lp?H-C hydrogen bond or agostic interactions, thus attaining of environment TlO₅H₂.     

Octahedral (cyclopentadienyl)rhodium clusters [Rh6Cp6(μ6-C)] and [Rh6Cp6(μ3-CO)2]: Synthesis, structures and electrochemistry

The 86-electron dicationic octahedral rhodium clusters containing Cp (Cp = C₅H₅) ligands and either an interstitial carbon atom, [Rh₆Cp₆(μ₆-C)]²⁺ ([1]²⁺), or two carbonyl groups, [Rh₆Cp₆(μ₃-CO)₂]²⁺ ([2]²⁺), were synthesized in low yields by reactions of Rh₃Cp₃(μ-CO)₃ with RhCp(C₂H₄)₂ or [RuCp^∗(MeCN)₃]⁺ (Cp^∗ = C₅Me₅), respectively. The structures of [1]²⁺ and [2]²⁺ were determined by X-ray diffraction. Their electrochemical behavior proved that they possess a rather extended electron transfer activity. In accordance with DFT calculations, the nearly octahedral structure of [1]²⁺ and [2]²⁺ is retained both upon oxidation (2+/3+) and the first reduction (2+/+); however, the second reduction (+/0) results in the breaking of one (for [1]⁰) or two (for [2]⁰) Rh-Rh bonds. In the case of the related Dahl’s nickel cluster Ni₆Cp₆ the nearly octahedral structure is retained upon all redox steps (3+/2+/+/0/−/2−).     

Thallium(III) complexes of the metalloligands [Pt2(μ-S)2(PPh3)4] and [Pt2(μ-Se)2(PPh3)4]

Reactions of [Pt₂(μ-S)₂(PPh₃)₄] with the diarylthallium(III) bromides Ar₂TlBr [Ar = Ph and p-ClC₆H₄] in methanol gave good yields of the thallium(III) adducts [Pt₂(μ-S)₂(PPh₃)₄TlAr₂]⁺, isolated as their salts. The corresponding selenide complex [Pt₂(μ-Se)₂(PPh₃)₄TlPh₂]BPh₄ was similarly synthesised from [Pt₂(μ-Se)₂(PPh₃)₄], Ph₂TlBr and NaBPh₄. The reaction of [Pt₂(μ-S)₂(PPh₃)₄] with PhTlBr₂ gave [Pt₂(μ-S)₂(PPh₃)₄TlBrPh]⁺, while reaction with TlBr₃ gave the dibromothallium(III) adduct [Pt₂(μ-S)₂(PPh₃)₄TlBr₂]⁺[TlBr₄]⁻. The latter complex is a rare example of a thallium(III) dihalide complex stabilised solely by sulfur donor ligands. X-ray crystal structure determinations on the complexes [Pt₂(μ-S)₂(PPh₃)₄TlPh₂]BPh₄, [Pt₂(μ-S)₂(PPh₃)₄TlBrPh]BPh₄ and [Pt₂(μ-S)₂(PPh₃)₄TlBr₂][TlBr₄] reveal a greater interaction between the thallium(III) centre and the two sulfide ligands on stepwise replacement of Ph by Br, as indicated by shorter Tl-S and Pt?Tl distances, and an increasing S-Tl-S bond angle. Investigations of the ESI MS fragmentation behaviour of the thallium(III) complexes are reported.     

Influence of CH3 group of μ-N-C-S ligand on the properties of [Fe2(C4H5N2S)2(NO)4] complex

Bi-nuclear neutral sulfur-nitrosyl iron complex [Fe₂(SR)₂(NO)₄] (I) has been obtained by replacement of thiosulfate ligands in dianion [Fe₂(S₂O₃)₂(NO)₄]²⁻ by 1-methyl-imidazole-2-yl. From X-ray analysis data, the complex has centrosymmetrical dimeric structure, with the iron atoms being linked via μ-N-C-S bridge. From Mossbauer spectroscopy, isomeric shift δ_Fe is 0.180(1) mm/s and quadrupole splitting ΔE_Q is 0.928(2) mm/s at T = 290 K. By comparative studying the mass-spectra in the gaseous phase of solid samples decomposition and kinetics of NO release in 1% aqueous solutions of dimethylsulfoxide, using of the ligand with CH₃ substituent in position 1 of imidazole-2-thiol was shown to yield a more stable donor of nitrogen monoxide than earlier obtained analog with imidazole-2-thiol, [Fe₂(C₃H₃N₂S)₂(NO)₄].     

New palladacyclopentadiene complexes containing an N,P-donor setting. Crystal structure of [Pd{C4(COOMe)4}(o-Ph2PC6H4CHNPr)]

The synthesis of palladacyclopentadiene derivatives with the mixed-donor bidentate ligands o-Ph₂PC₆H₄CHNR (N^∧P) has been achieved. The new complexes of general formula [Pd{C₄(COOMe)₄}(o-Ph₂PC₆H₄CHNR)] [R=Me (1), Et (2), ⁱPr (3), ^tBu (4), NHMe (5)] have been prepared by reaction between the precursor [Pd{C₄(COOMe)₄}]_n and the corresponding iminophosphine. The polymer complex [Pd{C₄(COOMe)₄}]_n also reacts with pyridazine (C₄H₄N₂) to give the insoluble dinuclear complex [Pd{C₄(COOMe)₄}(μ-C₄H₄N₂)]₂ (6), which has been successfully employed as precursor in the synthesis of pyridazine-based palladacyclopentadiene complexes. The reaction of 6 with tertiary phosphines yielded complexes containing an N,P-donor setting of formula [Pd{C₄(COOMe)₄}(C₄H₄N₂)(L)] (L=PPh₃ (7), PPh₂Me (8), P(p-MeOC₆H₄)₃ (9), P(p-FC₆H₄)₃ (10)). The new complexes were characterized by partial elemental analyses and spectroscopic methods (IR, ¹H, ¹⁹F and ³¹P NMR). The molecular structure of complex 3 has been determined by a single-crystal diffraction study, showing that the iminophosphine acts as chelating ligand with coordination around the palladium atom slightly distorted from the square-planar geometry.     

Hyponitrite complexes: Crystal and molecular structure of [Ru2(CO)4(μ-PBu2)(μ-Ph2PCH2PPh2)(μ-η-ONNOMe)]

The new trans-hyponitrite derivative complex [Ru₂(CO)₄(μ-P^tBu₂)(μ-dppm)(μ-η²-ONNOMe)] (2, dppm = Ph₂PCH₂PPh₂) was prepared by deprotonation of [Ru₂(CO)₄(μ-H)(μ-P^tBu₂)(μ-dppm)(μ-η²-ONNOMe)][BF₄] (1) with the base DBU (1.8-diazabicyclo[5.4.0]undec-7-ene). The latter complex salt has been obtained in an improved synthesis starting from the trans-hyponitrite complex [Ru₂(CO)₄(μ-H)(μ-P^tBu₂)(μ-dppm)(μ-η²-ONNO)]. Compound 2 has been characterized by spectroscopic methods as well as by X-ray diffraction and represents the first neutral complex bearing a deprotonated monoester of the hyponitrous acid as the bridging ligand.