NITROSYL COMPLEXES——SYNTHESIS, CHARACTERIZATION AND CRYSTAL STRUCTURES OF (η~5-C_5_H_5)M(CO)_2NO AND (η~5-C_5H_5)M(μ_3-NH)(μ_2-NO)(μ_2-CO)Fe_2(CO)_6(M=Mo OR W)

Treatment of tricarbonyl cyclopentadienyl metal (molybdenum or tungsten) chloride with equal molar amounts of tricarbonyl nitrosyl iron sodium at room temperature yielded the following complexes: orange (η5-C5H5)M(CO)2NO, violet [(η5-C5H5)M(CO)3]2 and black (η5-C5H5)M(μ3-NH)(μ2-NO)(μ2-CO)Fe2(CO)6. These were separated and purified by column chromatography. The last black complexes have a new cluster-type structure with a face-bridging imido(μ3-NH) ligand which was obtained by reducing the nitrosyl (NO) group. In this paper, synthesis and characterization of the title complexes are reported along with their crystal structures which are of two types.     

CRYSTAL STRUCTURE OF (C_5H_9C_5H_4)_3Er_4(μ_2-Cl)_6(μ_3-Cl)(μ_4-O)-(THF)_3

<正> The crystal of (C5H9C5H4)Er4(μ2-Cl)6(μ3-Cl)(μ4-O)(THF)3 belongs to triclinic space group P1, with a = 11. 909(4), b = 15.343(5), c=15. 619 (4)(?), α=104. 18(2), β=98. 81(3), γ= 111. 06(2)°, V = 2486(1)(?)3, Z = 2, Dc = 2. 07g/cm3, λ(MoKα) = 0.71073(?), μ= 73. 4cm-1, F(000) = 1476, Mr=1549. 25. The structure was solved by Patterson and Fourier techniques and refined by block diagonel least-squares method to final R = 0. 078 and Rw = 0. 084 for 5594 reflections with I≥1. 5σ(I). The results revealed that the complex is composed of four Er atoms bridged by seven Cl and one oxygen atoms with three of the Er atoms bonded each to a cyclopentylcyclopentadienyl group, and one Er atom, bonded to three THF. The Er atoms constitute a pseudo-tetrahedron.     

MOLECULAR STRUCTURE OF ORGANOLANTHANIDE COMPLEX [Li(THF)_4]_2[{ (η~5-CH_3C_5H_4)NdCl(μ_2-Cl)NdCl_2(η~5-CH_3C_5H_4)}_2(μ_4-O)]

<正> The crystal structure of the complex [Li(THF)4]2[{(η5-CH3C5H4)Nd-Cl(μ2-Cl)NdCl2(η5-CH3C5H4)}2(μ4-O)] has been determined by X-ray diffraction technique. The crystal is monoclinic of space group C2/c with a = 22. 740(7) ,b= 18. 319 (6),c=18. 330(6) A,β=93. 04(3)°,V = 7624. 93A3,Dc= 1. 55g/cm3,Z = 4,F(000) = 2800,μ=15. 02cm-1. The complex is consisted of two [Li(THF)4]+cations and one [{(η5-CH3C5H4)NdCl(μ2-Cl)NdCl2(η5-CH3C5H4)}2(μ4-O)]2-dianion. The two units [Cη5-CH3C5H4)NdCl(μ2-Cl)NdCl2(η5-CH3C5H4)] in the tetra nnclear-neodymium dianion are connected by a μ4-O bridge with the average Nd -μ4-O 2. 36(1) A ,Nd -C(ring) 2. 76(3)A,Nd-Cl 2. 823(7)A and Nd-μ2-Cl 2. 798(7)A.     

Synthesis and Crystal Structure of an Incomplete Cubane-like Heterobimetallic Cluster [(η~5-C_5Me_5)_2Mo_2(μ_3-S)(μ-S)_3(CuCl)]

The reaction of [(η5-C5Me5)2Mo2(μ3-S)4(CuCl)2] 1 with Na2S in MeCN produced a trinuclear cluster [(η5-C5Me5)2Mo2(μ3-S)(μ-S)3(CuCl)] 2. 2 crystallizes in the monoclinic system, space group P21/c with a = 15.563(3), b = 8.9547(18), c = 17.846(4) , β = 101.29(3)o, V = 2438.9(9) 3, Z = 4, Dc = 1.878 g/cm3, T = 193(2) K, C20H30ClCuMo2S4, Mr = 689.60, F(000) = 1376, μ(MoKα) = 2.335 mm–1, S = 1.050, R = 0.0305 and wR = 0.0688 for 4033 observed reflections with I > 2σ(I). In the structure of 2, one [(η5-C5Me5)2Mo2(μ-S)2S2] moiety and one CuCl unit are assembled into an incomplete cubane-like [Mo2S4Cu] core framework, in which the Cu center adopts a distorted tetrahedral geometry coordinated by one μ3-S atom, two μ-S atoms and one terminal chloride. The two Mo…Cu contacts are 2.7519(7) and 2.7689(8) , respectively.     

Synthesis and Characterization of Tetrametal Cluster [Co2(CO)6][μ-HC2CH2OCH2C2H-μ][Co2(CO)6]

The treatment of dipropargyl ether HC2CH2OCH2C2H with Co2(CO)8 resulted in the formation of a novel cluster [Co2(CO)6][μ-HC2CH2OCH2C2H-μ][Co2(CO)6]. The cluster was characterized by C/H analyses, IR and 1H NMR spectroscopy and X-ray crystal structure determination. It belongs to monoclinic system, space group P21/c with the following crystallographic parameters: a=18.149(2), b=7.0111(7), c=20.897(2)(A), β=115.318(7)°, V=2403.7(5)(A)3, Z=4, Mr=665.97, Dc=1.840 g·cm-3, F(000)=1304.00, μ(Mo-Kα)=27.76 cm-1 and R=0.030 for 2372 observed reflections.     

RESEARCH ON TRINUCLEAR MOLYBDENUM CLUSTER COMPOUNDS——CRYSTAL AND MOLECULAR STRUCTURE OF (C_5H_7S_2)_3[Mo_3(μ_3-S)(μ_2-S_2)_3Cl_7]

The space group of the title crystal is Pnam. The unit cell parameters are: a=17.226(4), b=14.306(7), c=15.074(4); z=4, Dobs=2.03g·cm~(-3), D_(calc)=2.063g·cm~(-3). The crystal structure was solved by the heavy atom method and refined by the full-matrix least-squares until the final conventional discrepancy factor was reduced to 0.068. The results of the investigation show that the crystal consists of trinuclear Mo cluster anions [Mo_3(μ_3-S)(μ_2-S_2)_3Cl_7]~(3-) together with planar (C_5H_7S_2)~ monocations, surrounding the cluster anions. In the cluster anion three Mo atoms, forming a nearly equilateral triangle (2.755(1), 2.755(1), 2.743(1)), are bound by a μ_3-S bridging atom, while each pair of Mo atoms is bridged further by a μ_2-S_2 ligand. Besides, each Mo atom is coordinated to two terminal Cl atoms, so that the coordination sphere of each Mo atom approaches closely a pentagonal bipyramid. In addition, a single Gl atom, not at all involved in the coordination spheres of the three Mo atom     

STUDY ON STRUCTURE OF SULFUR-BRIDGED TRANSITION METAL CLUSTERS——CRYSTAL AND MOLECULAR STRUCTURE OF (C_5H_7S_2)_3 [Mo_3(μ_3-S)_2 (μ_2-Cl)_3Cl_6]

The compound (C_5H_7S_2)_3[Mo_3 (μ_3--S)_2 (μ_2--Cl)_3 Cl_6] forms dark black octahedralcrystals with the following crystallographic parameters: a=22.885(10)A, c=25.407(7)A. Thespace group is I4_1/α, Z=16, ρ_(obs.)=2.073g/cm~3, ρ_(calc.)=2.126g/cm~3. The 1621 independentreflections with I≥3σ were collected on a CAD-4 four-circle diffractometer by means of MoK_αradiation. The structure was solved by three-dimensional Patterson synthesis and refined byleast squares technique to a final discrepancy factor of R=0.056. The results of our investiga-tion reveal that the molecule consists of a trinuclear Mo cluster anion and three planar mono-cations.surrounding the cluster anion. As the core of the anion the triad of Mo atoms, possessing anequilateral triangular configuration, is linked by two μ3--S bridging atoms and three μ2--Clatoms. Besides, two terminal Cl atoms are attached to each Mo atom to form an octahedralconfiguration. The diamagnetism and the short Mo-Mo distances (2.556 A, 2.641 A, 2.653 A)clearly indicate the existence of Mo-Mo bonds. The anion as a whole has an idealized D_(3h) localsymmetry.     

THE SYNTHESIS AND STRUCTURE OF Mo_3(μ_3-O)(μ-S)_3(μ-O_2CC_6H_5)_2(dtp)_2(Py)

<正> The title compound was prepared by reaction of Mo3(μ3-O) (μ-S)3-(dtp)4 (H2O) with C6H5COONa and pyridine. C27 H35 Mo3NO9P2S7, Mr = 1091.76, monoclinic, space group P21/c, a = 15. 057(5), b = 13. 052(5), c = 21. 662(6)(?), β=105.97(2)°, V = 4093(2) (?)3, Z = 4, Dc= 1. 77gcm-3, F(000) = 2176, μ(MoKα)=13.50cm-1, final R =0. 068 for 2835 observations. The molecular configuration is new and of approximate C, symmetry, and there are two C6H5COO bridging groups in the structure. The three Mo atoms form an isosceles triangle with three sides of 2. 567(3), 2. 576(3), and 2. 649(3)(?), where the two Mo-Mo bonds bridged by C6H5COO group are much shorter than that without. Something about the reactivity of Mo clusters is discussed.     

CRYSTAL STRUCTURE OF TRINUCLEAR MOLYBDENUM CLUS-TER Mo_3(μ_3-S)(μ_2-S_2)_3(Et_2dtc)_3I SYNTHESIZED BY SOLID STATE REACTION

<正> The title cluster compound was synthesized by solid state reaction at low heating temperature. C15H30IMo3N3S13, Mr=1083. 93, monoclinic, P21/n, a = 14.396(3), b = 13. 116(3), c=18. 142(8)A;β= 91. 36(3)°; V =3425(2)A3, Z = 4. Dc = 2. 10g/cm3, F(000) = 2112,μ(MoKa) = 27. 29cm-1, final R = 0. 034, Rw = 0. 051 for 1845 unique observed reflections (I≥3σ(I)). The three Mo atoms form an equilateral triangle with an average side length of 2. 714 A. Each Mo atom is coordinated with seven S atoms forming a distorted pentagonal bipyramid. The iodine atom connects with three S atoms (out of the CMo3] plane) with an average distance of 3. 205 A.The title compound has similar cubane-like cluster core [Mo3S7I] as the compound Mo3(μ3-S)(μ2-S2)3(dtp)3I(dtp= (EtO)2PS2).     

Synthesis, Characterization and Crystal Structure of [Co2Mo2(μ4-C2HPh)(μ-CO)4(CO)4(η5-C5H4C(O)Me)2]

The reaction of μ-alkyne-bridged dimolybdenum compound [Mo2(μ-C2HPh)(CO)4(η5-C5H4C(O)Me)2] 1 with Co2(CO)8 in refluxing toluene gave a new butterfly compound [Co2Mo2(μ4-C2HPh)(μ-CO)4(CO)4(η5-C5H4C(O)Me)2] 2 which was fully characterized by elemental analysis, IR, 1H NMR and X-ray single crystal diffraction techniques. 2 crystallized in monoclinic system, C30H20Co2Mo2O10, Mr=850.23, space group P21/a(#14), a=14.165(5), b=12.498(2), c=16.204(2)(A), β = 96.50(2)°, V = 2850(1)(A)3, Z = 4, Dc = 1.981 g cm-3, F(000)=1672, μ(MoKα)=20.41 cm-1, final R=0.030, Rw=0.039 for 4831 observable reflections with I>2σ(I). The structure contains a Co2Mo2 butterfly core, and each Mo-Co bond is spanned by an asymmetric semi-bridging carbonyl ligand.     

The synthesis and crystal structure of a trinuclear molybdenum cluster compound [Mo3(μ-O)(μ-S)3(μ(μ-OAc)2(dtp)2·(Py)]·0.5H2O

[Mo₃,OS₃(dtp)₄(H₂O)] reacts with NaOAc·3H₂O in Py to give the title compound. The crystal data are as follows: [Mo₂OS₃)(OAc)₂(dtp)₂·Py]?0.5H,O(dtp = [S₃P(OC₂H₅)₂]^?, Py = C₅H₅N); M = 976.64; triclinic; space group P1 ; a=11.704(5), b=14.169(7), c= 11.688 (5) Å α=109.94(4) β = 91.53(4), γ = 91.93(4)°; V= 1819(1) Ų; Z=2; D_c = 1.78 g·cm^?3 λ(Mo Kα) = 0.71069 Å μ=15.15 cm^?1; F(000) = 970 T=296 K; final R=0.071 for 1652 reflections with I>3σ(I). In the molecule, the [Mo₃OS₃] core is surrounded by two bridging OAc groups and two terminal chelate dtp groups attached to the {Mo₃} triangle in a symmetric style, and the Py ligand is coordinated to the Mo atom at the apex of {Mo₃} triangle with the nitrogen. This novel configuration is obtained for the first time with Mo—N bond length being 2.27 (2) Å and three Mo—Mo bond lengths 2.584 (4), 2.587 (4) and 2.657(4) Å, respectively. As a whole, the molecule has a virtual C₂ symmetry.     

Synthesis and structure of the cluster μ3-sulfido-μ-–(diethylphosphorodithioato-S,S'-)-allylthioureocyclo-tris-[ (μ-sulfido)-χ-(diethylphosphorodithioato-S,S')-molybdenum(IV)], Mo3S4(C4H10O2S2P)4(C4H8N2S)

Title compound, M_r =1273.16, was synthesized by a substitution reaction and its crystal is triclinic belonging to space group P1 with cell parameters: a =13.944(2), b =14.143(7), c =14.233(3) Å, α =77.35(3)°, β =69.94(2)°, γ =63.50(3)°, V=2351(1) ų, Z=2, D_c =1.799g cm^?2. Room temperature, graphite-filtered Mo K_α radiation (λ =0.71073Å) was used for data collection. μ =14.988 cm^?1, F(000) =1280, R=0.051 for 7025 observed reflections. The crystal consists of decrete cluster molecules containing a cluster core [Mo₂(μ₃-S)]¹⁰⁺ with three μ-S, one μ-dtp(dtp =[S₂P(OC₂H₅)]^2-), three χ-dtp and one allylthioureo to form a local six-coordinated sphere around each Mo atom. The bonds of cluster skeleton [Mo₃(μ₃-S)(μ-S)₃]⁴⁺, Mo? Mo 2.744～2.766, Mo—(μ₂-S) 2.340～2.342 and Mo—(μ-S)2.272～2.296 Å, are comparable with those found in the related analogues.     

Synthesis and structure of an open-type trinuclear molybdenum cluster compound[Mo_3(μ_3-S)(μ-S)_2-(μ-OAc)(S_2CNC_4H_8)_3(O)_2]·0.5CH_2Cl_2·2H_2O

The title compound was prepared by the reaction of Mo_3S_4(dtp)_4(H_2O)[ctp=S_2P(OEt)_2]with NaOAc·3H_2O and C_4H_8NCS_2NH_4.Crystallographic data:[Mo_3(μ_3-S)(μ-S)_2(μ-OAc)-(S_2CNC_4H_8)_3(O)_2]·0.5CH_2CI_2·2H_2O,Mr=980.18,triclinic,space group P,α=12.360(3),b=16.653(6),c=9.206(2)A,α=101.97(2),β=108.32(2),γ=86.14(3)°.V=1759.6(9)A~3,Z=2,Dc=1.85 g/cm~3,F(000)=962,μ(Mo K_α)=16.53 cm~(-1).Final R=0.044 for 4301 reflections with I≥3σ(I).This compoundmay be regarded as a mixed-valent trinuclear molybdenum cluster{Mo_2(V)Mo(Ⅳ)(μ_3-S)(μ-S)_2-(μ-OAc)(S_2CNC_4H_8)_3(O)_2}.The Mo-Mo distances are 2.783(1),2.833(1)and 3.374(2)A in the Mo_3non-equilateral triangle and there exist only two Mo-Mo bonds.The cluster was obtained by oxi-dation and ligand substitution of{Mo_3(μ_3-S)(μ-S)_3[μ-S_2P(OEt_2)][S_2P(OEt)_2]_3(H_2O)}.     

The Thermal Reaction of the Tris(μ-Hydrido)-Dodecacarbonyltrirhenium (o) Cluster with Triphenylphosphine and Triphenylphosphite

The thermal reaction between the tris(μ-hydrido)dodecacarbonyltrirhenium (o) cluster and triphenylphosphine or triphenylphosphite results in a trisubstituted rhenium cluster as the major product. The results of an X-ray structural analysis of H₃Re₂(CO)₉(PPh₃) are: Mr=1597.57, Monoclinic, P₂₁/n, a=15.598(1), b=9.530(2), c=41.685(3) Å, β=80.60(1)°, V=6113.(1) ų, Z=4, D_c=1.736 g/cm³, CuK_e, λ-1.54056 Å, μ-121.23 cm^?1, F(000)=3059.17, room temperature, R=0.049 for 7361 reflections with I≥2.5σ(I). The molecule has a triangular Re₂ core coordinated propellerwise by three PPh₃ ligands in the molecular plane with virtual C_3k symmetry.     

Research on in part “loosely” coordinated trinuclear molybdenum cluster compounds — the synthesis and crystal structure of {Mo3(μ3-S)(μ-S)3[S2P(OEt)2]4 ṁ P(C6H5)3} (0.86CH2Cl2)

The synthesis and crystal structure of novel trinuclear molybdenum cluster compound with somewhat “loose” coordination site {Mo₃(μ₃-S)(μ-S)₃[S₂P(OEt)₂l₄ ? P(C₆H₅)₃} ? (0.86 CH₂C1₂) have been reported. The cluster crystallizes in the space group with two molecules in a unit cell whose parameters are a=10.472(4), b=14.375(2), c=21.695(3)Å; α=74.04(1)°, β=76.50(2)°, γ=72.22 (2)°, V=2950ų and D_o=1.693 g ? cm^?3. On the basis of 4840 independent reflections with I≥2σ(I), the structure was solved by heavy atom method and Fourier method and refined by full-matrix least-squares techniques to a final R=0.058. The distances between Mo atoms in this cluster are 2.731(1), 2.748(1) and 2.753(1)Å respectively. The meat value of Mo—Mo bond lengths is slightly shorter than those in other trinuclear Mo clusters with “loosely coordinated” site. In addition, the PPh₃ ligand is loosely coordinated to one Mo atom in direction opposite to the μ₃-S with Mo—P bond length of 2.647(3)Å. This is different from the other structurally analogous clusters, in which the loosely coordinated ligand is trans to μ₂-S. A summary of Mo—Mo and Mo—L bonding for such clusters is given.     

Synthesis and crystal structure of a cubanelike tetranuclear molybdenum cluster compound {Mo4)(μ3-S)3)(μ3-O)[S2P(OEt)2]6} · 3CH3CN

A tetranuclear molybdenum cluster compound {Mo₄(μ₃-S)₃(μ₃-O)[S₂P(OEt)₂]₅} · 3CH₃CN was obtained by the reaction of MoCl₃ · 3H₂O with P₂S₅ in ethanol and then recrystallization from acetonitrile. The compound crystallizes in the trigonal system belonging to the space group R3 with the following cell dimensions: a = b = c = 12.852 (3) Å, α = β = γ = 108.37 (2)°, V =1697.3ų Z = 1, D_c. = 1.693g.cm^?3. The structure was solved by heavy atom method and refined by full-matrix least-squares to R = 0.072 for 1781 reflections with I≥3σ(I). The results of the structure determination indicate that the cluster skeleton possesses a cubanelike cluster core formed by four Mo atoms located in a distored tetrahedron with three S atoms and one O atom as its triple bridging atoms. There are two sets of bond distances, i.e. 2.700 (1) and 2.831(1) Å in the six Mo—Mo bonds. Taking the Mo cluster core as a whole, it has a formal oxidation state of +14, leaving ten electrons to form the metal-metal bonds. Thus each Mo—Mo bond has an average bond order of 5/6.     

Crystal and molecular structure of (μ-p-CH3C6H4C2S) (μ-n-C3H7S)Fe2(CO)6Co2(CO)6

The crystal and molecular structure of the complex containing cobalt-carbon and iron-sulfur cluster cores, (μ-p-CH₃C₆H₄C₂S) (μ-n-C₃H₇S)Fe₂(CO)₆Co₂(CO)₆, has been determined by X-ray diffraction method. The crystals are triclinic, space group P&1bar;, with a — 9.139(2), b=9.610(1), c-17.183(2) Å, α = 84.36(1), β-89.45(1), γ=88.15(1)°, V-1501.0 ų; Z=2, D_c=1.74 g/cm³. R=0.072, Rw=0.081. The results of the structure determination show a cobalt-carbon cluster core formed through the reaction of (μ-p-CH₃C₆H₄C₂S)(μ-n-C₃H₇S)Fe₂(CO)₆ with Co₂(CO)₈. In the cobalt-carbon cluster core, the bond length of the original C≡C lengthened to 1.324 Å which is close to the typical value of carbon-carbon double bond. The groups connecting the carbons of the cluster core are in cis position and lie on the opposite side of cobalt atoms. In this complex, the conformation of —SC₃H₇ is e-type, while that of —SC₂C₆H₄CH₃ is a-type.     

A cobalt-selenium cluster with triethylphosphine: preparation and structure of Co6(μ3-Se)8(PEt3)6·THF

Co₆(μ₃—Se)₆ (PEt₃)₆·THF, MW = 1766.4, space group R3 , has the trigonal parameters, a= 11.890(2)° Å, α = 92.72(2)°, V = 1670.4 ų, Z = 1. Mo Ka radiation, λ = 0.71073 Å, Dc = 1.749 g/cm³, μ = 59.18 cm^?3, F(000) = 870, R = 0.058 and Rw= 0.067 for 1529 observed unique reflections with I>3α(I). The molecular structure consists of an idealized octahedral Co₄—core. The Co—Co distances fall in the range of 2.909—2.912 Å.     

Preparation and Characterization of a Heteronuclear μ-Alkyne Complex; X-Ray Crystal Structure of MoCo(CO)4(PPh3){μ-HCCSiMe3)(η5-C5H5)

Reaction of MoCo(CO)₅(PPh₃)₂(η⁵-C₅H₅) (1a) with trimethylsilylacetylene in tetrahydrofuran at 58° C yielded two acetylene bridged heterobimetallic compounds, MoCo(CO)₄(PPh₃){μ-HC?CSiMe₃}(η⁵-C₅H₅) (4) and MoCo (CO)₅{μ-HC?CSiMe₃}(η⁵-C₅H₅)(5). (4) was characterized by mass, infrared, ¹H, ¹³C and ³¹P NMR spectra. The X-ray crystal structure of (4) was determined:triclinic, P-1, a=8.821(1) Å, b=11.315(3) Å, c=17.029(2) Å, α=70.73(1)°, β=78 .72(1)°, γ=86.10(2)°,V =1573.4(6) ų, Z=2, R = 3.92%,R_w = 6.06% for 4285 (F > 4σ (F)) observed reflections. The core of this molecule is a quasi-tetrahedron containing Mo, Co and two carbons of acetylene. The triphenylphosphine ligand is attached to cobalt rather than molybdenum center.     

Preparation and Characterization of an Unusual Di-Cobalt Complex; X-Ray Crystal Structure of Co(CO)2(μ-CO)(μ:η2,η4-C4Ph4)Co(CO)2(PPh3)

Reaction of [MoCo(CO)₅(PPh₃)₂(η⁵-C₅H₅)] (1) with diphenylacetylene in tetrahydrofuran at 50 °C yielded two heterobimetallic compounds, [MoCo(CO)₄.(PPh₃){μ-PhC ? CPh}(η⁵-C₅H₅)] (4) and [MoCo(CO)₅{μ-PhC ? CPh} (η⁵-C₅H₅)] (5). However, an unexpected product, Co(CO)₂(μ-CO)(μ:η₂,η⁴-C₄Ph₄)Co(CO)₂(PPh₃) (6), was observed while attempting to grow the crystals for structural determination of 4. The X-ray crystal structure of 6 was determined: triclinic, $ {\rm P}\bar 1 $, a = 11.654(2) Å, b = 12.864(2) Å, c = 13.854(2) Å, α = 89.67(2)°, β = 86.00(2)°, γ= 83.33(2)°, V = 2057.9(6) ų Z=2. In 6, two cobalt fragments are at apical and basal positions of the pseudo-pentagonal pyramidal structure, respectively. The electron count for the apical cobalt fragments is 20, which is rather unusual. It is believed that 6 was formed after the fragmentation and recombination of the fragmented species of 4.