Zweikernkomplexe des Typs Mn2(CO)8E(CF3)2E′R (E = P,As; E′ = S,Se, Te): Darstellung und Struktur

Perfluoromethyl Element Ligands. XLII Binuclear Complexes of the Type Mn₂(CO)₈E(CF₃)₂E′R (E = P, As; E′ = S, Se, Te): Synthesis and Structure Complexes of the type Mn₂(CO)₈E(CF₃)₂E′R, in which the groups E(CF₃)₂ and E′R act as bridging ligands, are prepared either by direct reactions of Mn₂(CO)₁₀ with (F₃C)₂EE′R (E = P, As; E′ = S, Se, Te) or by substitution of the iodine bridge in the representatives Mn₂(CO)₈ E(CF₃)₂I (E = P, As) with mercury compounds Hg(E′R)₂. As a rule the binuclear systems contain four‐membered heterocycles (Mn₂EE′). However, the reactions of Mn₂(CO)₁₀ with (F₃C)₂PE′P(CF₃)₂ (E′ = S, Se) yield five‐membered rings [Mn₂P(E′P)]. The compounds have been characterized by spectroscopic (NMR, IR, MS), analytic (C, H) and X‐ray diffraction investigations. The pyramidal Mn₂E′R fragment shows dynamic behaviour in solution via inversion between two identical structures.     

Molekulare Verbindungen mit SiAl4-, SiAl3- und GeAl4-Einheiten: Synthese und Struktur von Si(AlCl2 · OEt2)4, Ge(AlCl2 · OEt2)4 und HSi(Cp*AlBr)3

Molecular Compounds containing SiAl₄, SiAl₃, and GeAl₄ Units: Sythesis and Structure of Si(AlCl₂ · OEt₂)₄, Ge(AlCl₂ · OEt₂)₄, and HSi(Cp*AlBr)₃ In the scope of our investigations of the reactivity and the potential for synthesis of solutions of Al^I halides we performed reactions between these solutions and SiCp or GeCp, respectively. From these reactions we could isolate an unusual cluster with a central Al₁₄Si unit, described elsewhere, and the compounds Si(AlCl₂ · OEt₂)₄, Ge(AlCl₂ · OEt₂)₄, and HSi(Cp*AlBr)₃, which will be presented and discussed here. In these species the Si respectively the Ge atoms are connected to 4 respectively 3 Al atoms. This bonding results in strong negative polarized Si/Ge centres. The change of the polarization with respect to “normal” Si–R or Ge–R linking leads to a drastic weakening of the Si–R respectively the Ge–R bonds.     

Verbindungen des Typs (F3C)2EE′R mit Pseudohalogen-Charakter (E = P,As; E′ = S,Se, Te)

Perfluoromethyl Element Ligands. XLI. [1] Compounds of the Type (F₃C)₂EE′R with Pseudohalide Character (E = P, As; E′ = S, Se, Te) Perfluoromethyl phosphorus and -arsenic compounds of the type (F₃C)₂EE′R (E = P, As; E′ = S, Se, Te; R = organic group) are prepared either by dismutation (metathesis) of E₂(CF₃)₄ with (RE′)₂ or by reaction of the iodine compounds (F₃C)₂EI with mercury(II) organosulfanides Hg(SR)₂ and characterized by spectroscopic (¹H, ¹⁹F, ³¹P-NMR; IR; MS) as well as analytical investigations (C, H).     

Synthesis and Structures of Homoleptic Methyl‐ and Phenylthiomethylaluminium Complexes [Al(CH2SR)3](R = Me,Ph)

Sulfur‐substituted methylmercury compounds [Hg(CH₂SR)₂]( 1a, R = Me; 1b, R = Ph ) react with aluminium amalgam in refluxing toluene with transmetallation to give homoleptic tris(thiomethyl)aluminium complexes [Al(CH₂SR)₃]( 2a, R = Me; 2b, R = Ph ) (degree of conversion: >80%, isolated yields: 2a 63%, 2b 41%). Their identities were confirmed by NMR spectros‐copy (¹H, ¹³C) and X‐ray crystal structure analyses. In crystals of compound 2a the aluminium atoms possess a trigonal‐bipyramidal arrangement with the coordination polyhedron defined by three carbon and two sulfur atoms. Two of the three CH₂SMe ligands are bridging ligands (μ‐η²; 1kC:2kS), the third one is terminal bound (η¹; kC). The structure is polymeric. Crystals are threaded by helical chains built up of six‐membered Al₂C₂S₂ rings. Crystals of 2b are built up of centrosymmetrical dimers with six‐membered Al₂C₂S₂ rings having bridging CH₂SPh ligands (μ‐η²; 1kC:2kS). On each Al atom two terminal (η¹; kC)CH₂SPh ligands are bound. They exhibit quite different Al‐C‐S angles (116.7(4) and 106.5(3)?). Similar values (114.32115.7? and 109.52109.9?) were found in ab initio calculations of model compounds [{Al(CH₂SR)₃}₂]( 3a, R=H; 3b, R=Me; 3c, R=CH=CH₂ ). A conformational energy diagram for rotation of one of the terminal CH₂SH ligand in the parent compound 3a around the Al‐C bond is discussed in terms of repulsive interactions of lone electron pairs of sulfur atoms.     

Die Tris(triisopropylsilyl)pnikogene: Synthese und Charakterisierung von [E(SiiPr3)3] (E = P,As, Sb)

The Tris(triisopropylsilyl)pnikogenes: Synthesis and Characterisation of [E(Si ⁱ Pr₃)₃] (E = P, As, Sb) The compounds [E(SiⁱPr₃)₃] (E = P, As, Sb) ( 1 – 3 ) were prepared in high yields by the reaction of (Na/K)₃E with ⁱPr₃SiCl in DME. They were characterised by ¹H‐, ¹³C‐, ²⁹Si‐ and ³¹P‐NMR spectroscopy, mass spectrometry and single crystal X‐ray diffraction. Compound 1 , recently obtained in a different way, shows an unusual trigonal planar coordination of the central phosphorus atom. However, 2 and 3 , featuring increasing covalence radii of the central atoms, show an increasingly pyramidal structure. 1 – 3 crystallise isotyp in the cubic spacegroup Pa 3, the lattice constants are: 1 : a = 1860.1(2) pm, 2 : a = 1873.6(2) pm, 3 : a = 1897.1(2) pm.     

Fluoroborates by Cleavage of Trimethylamine‐bis(trifluoromethyl)boranes with NEt3 × 3 HF. Structures of C6F5(CF3)2B · NMe3, K[C6H5CH2(CF3)2BF], and K[C6H5(CF3)2BF]

Trimethylamine‐bis(trifluoromethyl)boranes R(CF₃)₂B · NMe₃ (R = cis/trans‐CF₃CF=CF ( 1/2 ), HC≡C ( 3 ), H₂C=CH ( 4 ), C₂H₅ ( 5 ), C₆H₅CH₂ ( 6 ), C₆F₅ ( 7 ), C₆H₅ ( 8 )) react with NEt₃ × 3 HF depending on the nature of R at 155–200 °C under replacement of the trimethylamine ligand to form the corresponding fluoro‐bis(trifluoromethyl)borates [R(CF₃)₂BF]^– ( 1 a/2 a – 8 a ). The structures of 7 , K[C₆H₅CH₂(CF₃)₂BF] ( K‐6 a ), and K[C₆H₅(CF₃)₂BF] ( K‐8 a ) have been investigated by single‐crystal X‐ray diffraction. In 7 the CF₃ groups make short repulsive contacts with NMe₃ and C₆F₅ entities – the B–CF₃ bonds being unusually long. The B–F bond lengths of K‐6 a and K‐8 a (1.446(3) and 1.452(2) Å, respectively) are long for a fluoroborate.     

Einfluß der Ringatome auf die Struktur von Triel‐Pentel‐Heterozyklen – Synthese und Molekülstrukturen von [Me2InAs(SiMe3)2]2 und [Me2InSb(SiMe3)2]3

Influence of the Ring Atoms on the Structure of Triel‐Pentel Heterocycles – Synthesis and X‐Ray Crystal Structures of [Me₂InAs(SiMe₃)₂]₂ and [Me₂InSb(SiMe₃)₂]₃ Triel‐pentel heterocycles [Me₂InE(SiMe₃)₂]_x have been prepared by dehalosilylation reactions from Me₂InCl and E(SiMe₃)₃ (E = As, x = 2; E = Sb, x = 3) and characterised by NMR spectroscopy and by X‐ray crystal structure analyses. In addition the X‐ray crystal structures of [Me₂GaAs(SiMe₃)₂]₂ and [Me₂InP(SiMe₃)₂]₂ are reported. The compounds complete a family of 13 identically substituted heterocycles [Me₂ME(SiMe₃)₂]_x (M = Al, Ga, In; E = N, P, As, Sb, Bi; x = 2, 3), whose structures were investigated depending on the ring atoms M and E. The tendencies that have been observed concerning the ring sizes can be explained by the interplay of the atomic radii of the central atoms and the sterical demand of the ligands. After a formal separation of the M–E bonds in σ bonds and dative bonds the characteristic differences and trends in the endocyclic and exocyclic bond angles of both centres M and E can be interpreted on the basis of a simple Lewis acid/base adduct model.     

Darstellung und Strukturen von Mg‐Komplexen mit α, ω‐Dicarboxylato‐Liganden (Dicarboxylat = Succinat,Glutarat und Suberat)

Syntheses and Structures of Magnesium Complexes with α, ω‐Dicarboxylato Ligands; Dicarboxylate = Succinate, Glutarate, and Suberate Crystals of (Tetraaqua)(succinato)magnesium ( 1 ), (Tetraaqua)(glutarato)magnesium ( 2 ) und (Triaqua)(suberato)magnesium ( 3 ) were obtained by layering an aqueous solution of the respective sodium salt with a solution of MgCl₂ in isopropanol. In 1 a chain structure is realized. Mg(H₂O)₄ units are bridged in trans orientation by α, ω‐bonded succinate groups. 2 contains also chains. Glutarato groups are bonded in a cis fashion to Mg(H₂O)₄ units. They form bridges by using their two α O atoms. 3 represents a layer structure. The basic structural motives are α, α, ω‐bonded suberate, and fac‐Mg(H₂O)₃ units. All three structures contain efficient H bridging systems. The connection between the symmetry of the polymeric groups (chains or layer) and the symmetry of the underlying space groups is discussed. 1 : Space group P2₁/c, Z = 4, lattice constants at 20 °C: a = 7.441(2), b = 14.827(2), c = 7.771(2) Å; β = 99.77(3)°, R1 = 0.052. 2 : Space group C2/c, Z = 8, lattice constants at 20 °C: a = 12.867(2), b = 7.109(1), c = 21.683(3) Å; β = 107.33(2)°; R₁ = 0.032. 3 : Space group P2₁/a, Z = 4, lattice constants at 20 °C: a = 9.174(2), b = 8.071(2), c = 15.960(3) Å; β = 104.29(2)°; R₁ = 0.052.     

Neue viergliedrige GaE‐ und InE‐Ringverbindungen: Synthese und Kristallstruktur von [Et2InE(SiMe3)2]2 und [GaCl(PtBu2Me)E(SiMe3)]2 (E = P,As)

New GaE and InE Four Membered Ring Compounds: Syntheses and Crystal Structures of [Et₂InE(SiMe₃)₂]₂ and [GaCl(P t Bu₂Me)E(SiMe₃)]₂ (E = P, As) Et₃In · PR₃ (R = Et, iPr) reacts with H₂ESiMe₃ under liberation of C₂H₆ and EH₃ to form the cyclic compounds [Et₂InE(SiMe₃)₂]₂ ( 1 a : E = P, 1 b : E = As). 1 consists of a planar four membered In₂E₂ ring in which the indium and phosphorus or arsenic atoms are four coordinated. In contrast, the phosphorus/arsenic atoms in [GaCl(PtBu₂Me)E(SiMe₃)]₂ ( 2 a : E = P, 2 b : E = As) only have the coordination number three. 2 results from the reaction of GaCl₃ · PtBu₂Me with As(SiMe₃)₃ or Li₂PSiMe₃ respectively, and displays a folded four membered Ga₂E₂ ring as central structural motif. 1 and 2 have been characterised by single crystal X‐ray diffraction analysis as well as ¹H and ³¹P{¹H} NMR spectroscopy.     

Neue Synthesen von Me2SbX (X = Cl,I) und Kristallstrukturen von Me2SbI und [(Me3Si)2CH]2SbCl

Novel Syntheses of Me₂SbX (X = Cl, I) and Crystal Structures of Me₂SbI and [(Me₃Si)₂CH]₂SbCl The crystal structures of Me₂SbI (Me = CH₃) and [(Me₃Si)₂CH]₂SbCl have been determined by X‐ray methods. Both molecules are pyramidal. The Me₂SbI molecules are associated to chains through short intermolecular Sb…I distances (366,7(1) pm) with linear I–Sb…I units (171,87(4)°) and bent Sb–I…Sb bridges (116,83(3)°).     

Darstellung,Struktur und magnetische Eigenschaften von Erdalkali-Mangan-Verbindungen AMnX mit A = Mg,Ca, Sr,Ba und X = Si,Ge, Sn

Preparation, Structure, and Magnetic Properties of the Alkaline Earth Manganese Compounds AMnX with A = Mg, Ca, Sr, Ba and X = Si, Ge, Sn The new compounds MgMnGe, MgMnSn, CaMnSi, CaMnSn, and SrMnSn were prepared by reaction of the elements. They crystallize tetragonally with the anti-PbFCl type structure (space group P4/nmm). The lattice constants see ”︁Inhaltsübersicht”︁”︁. Using a Faraday balance, magnetic measurements in the range 4.2 to 800 K were performed with the new substances and with the already known compounds CaMnGe, SrMnGe, and BaMnGe. They indicate metamagnetic behaviour at low temperatures. At high temperatures twodimensional magnetic interactions between the manganese atoms seem to persist. The construction of an unexpensive heating device for the Faraday balance is described.     

Syntheses,Structures, and Properties of the Complexes [Ag(N∧S)n](X), N∧S = 1‐Methyl‐2‐(alkylthiomethyl)‐1H‐benzimidazoles,X = PF6 (n = 2) or PO2F2 (n = 1)

The complexes [Ag(η²‐N∧S)₂](PF₆), N∧S = 1‐methyl‐2‐(methylthiomethyl)‐1H‐benzimidazole, mmb (complex 1 ) or 1‐methyl‐2‐(tert‐butylthiomethyl)‐1H‐benzimidazole, mtb (complex 2 ), and [Ag(μ,η²‐mmb)(μ,η²‐O₂PF₂)] (complex 3 ) were synthesized and characterized by X‐ray crystallography. Long Ag–S (ca. 2.70 Å) and shorter Ag–N bonds (ca. 2.23 Å) are part of characteristically distorted tetrahedral coordination arrangements at the silver(I) ions in 1 and 2 . Unexpectedly, the comparison with the copper analogue [Cu(η²‐mmb)₂](PF₆) reveals a more tetrahedral and less linear coordination arrangement for the corresponding silver species. Compound 3 as obtained by hydrolysis of the PF₆ ion or by the use of AgPO₂F₂ exhibits bridging mmb and η²‐difluorophosphate ligands in a chain‐type structure.     

Ab‐initio‐Berechnung des Tetracarbonatoscandat‐Ions in Na5[Sc(CO3)4] · 2 H2O. Einkristallstrukturbestimmung,Schwingungsspektren und thermischer Abbau

Ab Initio Calculation of the Tetracarbonatoscandate‐Ion in Na₅[Sc(CO₃)₄] · 2 H₂O. Single Crystal Structure Determination, Vibrational Spectra, and Thermal Decomposition Normal modes of the tetracarbonatoscandate‐ion, [Sc(CO₃)₄]^5–, were determined by ab initio calculations and were compared with experimental data of Infrared‐ and Raman‐spectra of the compound Na₅[Sc(CO₃)₄] · 2 H₂O. A necessary redetermination of the structure with single crystal x‐ray diffraction data (tetragonal, P42₁c (Nr. 114), Z = 2, a = 746,37(4) pm, c = 1157,0(2) pm, V_EZ = 644,5(1) 10⁶ pm³) allows the discussion of existing hydrogen bonds. Determination of the thermal behaviour indicates a two‐stage decomposition reaction, but no corresponding intermediate could be isolated.     

Reaction of the Pincer‐type Ligand {2, 6‐[P(O)(OEt)2]2‐4‐tert‐Bu‐C6H2}— with [Ph3C]+[PF6]—. Unprecedented Rearrangement and Carbon‐Carbon Bond Formation

The reaction of the organolithium derivative {2, 6‐[P(O)(OEt)₂]₂‐4‐tert‐Bu‐C₆H₂}Li ( 1 ‐Li) with [Ph₃C]⁺[PF₆]^— gave the substituted biphenyl derivative 4‐[(C₆H₅)₂CH]‐4′‐[tert‐Bu]‐2′, 6′‐[P(O)(OEt)₂]₂‐1, 1′‐biphenyl ( 5 ) which was characterized by ¹H, ¹³C and ³¹P NMR spectroscopy and single crystal X‐ray analysis. Ab initio MO‐calculations reveal the intramolecular O···C distances in 5 of 2.952(4) and 2.988(5)Å being shorter than the sum of the van der Waals radii of oxygen and carbon to be the result of crystal packing effects. Also reported are the synthesis and structure of the bromine‐substituted derivative {2, 6‐[P(O)(OEt)₂]₂‐4‐tert‐Bu]C₆H₂}Br ( 9 ) and the structure of the protonated ligand 5‐tert‐Bu‐1, 3‐[P(O)(OEt)₂]₂C₆H₃ ( 1 ‐H). The structures of 1 ‐H, 5 , and 9 are compared with those of related metal‐substituted derivatives.     

Sterically Tuned P‐Phosphanylamino Phosphaalkenes (Me3Si)2C=PN(R)PPh2 and (iPrMe2Si)2C=PN(R)PPh2

Deprotonation of the aminophosphanes Ph₂PN(H)R 1a – 1h [R = tBu ( 1a ), 1‐adamantyl ( 1b ), iPr ( 1c ), CPh₃ ( 1d ), Ph ( 1e ), 2,4,6‐Me₃C₆H₂ (Mes) ( 1f ), 2,4,6‐tBu₃C₆H₂ (Mes*) ( 1g ), 2,6‐iPr₂C₆H₃ (DIPP) ( 1h )], followed by reactions of the phosphanylamide salts Li[Ph₂PNR] 2a , 2b , 2g , and 2h with the P‐chlorophosphaalkene (Me₃Si)₂C=PCl, and of 2a – 2g with (iPrMe₂Si)₂C=PCl, gave the isolable P‐phosphanylamino phosphaalkenes (Me₃Si)₂C=PN(R)PPh₂ 3a , 3b , 3g , and (iPrMe₂Si)₂C=PN(R)PPh₂ 4a – 4g . ³¹P NMR spectra, supported by X‐ray structure determinations, reveal that in compounds 2a , 2b , 3a , and 3b , with bulky N‐alkyl groups the Si₂C=P–N–P skeleton is non‐planar (orthogonal conformation), whereas 3g , 3h , and 4g with bulky N‐aryl groups exhibit planar conformations of the Si₂C=P–N–P skeleton. Solid 3g and 4g exhibit cisoid orientation of the planar C=P–N–C units (planar I) but in solid 3h the transoid rotamer is present (planar II). From 3g , 4d , and 4g mixtures of rotamers were detected in solution by pairs of ³¹P NMR patterns ( 3h : line broadening).     

Alternativ-Liganden. XXV. Neue Chelatliganden des Typs Me2ESiMe2(CH2)2E′Me2 (E=P,As; E′=N,P, As)

Alternative Ligands. XXV. New Chelating Ligands of the Type Me₂ESiMe₂(CH₂)₂E′Me₂ (E=P, As; E′=N, P, As) Chelating ligands of the type Me₂EsiMe₂(CH₂)₂E′ Me₂, have been prepared by the following routes: Starting from Me₂Si(Vi)Cl, the compounds with E=N and E′ =N ( 1 ), P ( 2 ), As ( 3 ) are obtained in yields of 65 to 78% by aminolysis to yield Me₂NSiMe₂Vi, followed by the LiE′ Me₂ catalyzed addition of He′Me₂ to the vinyl group. The intermediates ClSiMe₂(CH₂)E′Me₂ [E′=N ( 4 ), P ( 5 ), As ( 6 )] are produced by the reactions of 1 to 3 with PhPCl₂. 5 and 6 can be prepared in a purer form by the photochemical addition of HPMe₂ and HAsMe₂, respectively, to the vinyl group of Me₂Si(Vo)Cl. 4 to 6 react with LiEMe₂, in situ prepared from n-BuLi and HEMe₂, to yield the ligands Me₂ESiMe₂(CH₂)₂E′Me₂ ( 7–12 ) (E=P, As; E′=N, P, As). The new compounds have been characterized by analytical and spectroscopic investigations (NMR, MS).     

Bildung und Struktur des iso-Tetraphosphans P(PtBu2)3: ein Molekül mit einem planaren,dreibindigen P-Atom

Formation and Structure of the iso -Tetraphosphane P(P^tBu₂)₃: a Molecule with a Planar Three-coordinated P Atom The iso-tetraphosphane P(P^tBu₂)₃ ( 1 ) was obtained by irradiating ^tBu₂P–P=P(Me)^tBu₂ ( 3 ). 1 forms hexagonal crystals (space group P6₃/m) with a = 1005,63(8), c = 1621,4(2) pm, Z = 2. The P(P^tBu₂)₃ molecules are arranged in a hexagonally close packed lattice. The four P atoms in each molecule are coplanar with P–P bond distances 219.08(4) pm and P–P–P angles 120°. The observed planar geometry is in accordance with ab initio calculations.     

Phosphaniminato‐Komplexe von Rhenium(VII). Synthesen und Kristallstrukturen von [ReO3(NPR3)] (R = Ph,Et) sowie von [ReO(OSiMe3)3(Me3SiNPEt3)]

Phosphoraneiminato Complexes of Rhenium(VII). Syntheses and Crystal Structures of [ReO₃(NPR₃)] (R = Ph, Et) and of [ReO(OSiMe₃)₃(Me₃SiNPEt₃)] The phosphoraneiminato complexes [ReO₃(NPR₃)] with R = Ph ( 1 ) and R = Et ( 2 ) are made from dirhenium heptaoxide and the silylated phosphoraneimines Me₃SiNPR₃. The complexes 1 and 2 as well as the red silanolate [ReO(OSiMe₃)₃(Me₃SiNPEt₃)] ( 3 ), which is formed as a by‐product in the synthesis of 2 , are characterized crystallographically. 1 and 2 are monomeric molecules, in which the phosphoraneiminato ligands NPR₃^– realize short ReN bonds of 179.3 pm ( 1 ) and 178.6 pm ( 2 ), respectively, with large ReNP bond angles of 162.0° ( 1 ) and 160.6° ( 2 ), respectively. In the rhenium(V) complex 3 the oxoligand occupies the apical position of the tetragonal pyramidal coordination of the rhenium atom, while the oxygen atoms of the Me₃SiO^– groups take the basic positions along with the nitrogen atom of the phosphaneimine molecule.     

Darstellung und Aufbau der Lanthanoidfluorid‐catena‐Trisilicate M3F[Si3O10] (M = Dy,Ho, Er) im Fluorthalenit‐Typ (Y3F[Si3O10])

Synthesis and Constitution of Fluorothalenite‐Type (Y₃F[Si₃O₁₀]) Fluoride catena‐ Trisilicates M₃F[Si₃O₁₀] with the Lanthanides (M = Dy, Ho, Er) By the reaction of the sesquioxides M₂O₃ with the corresponding trifluorides MF₃ (M = Dy, Ho, Er), SiO₂ and CsCl as flux (molar ratio: 1 : 1 : 3 : 6; 700 °C, 7 d) in evacuated silica tubes and gastight sealed metal capsules made of platinum, niobium or tantalum, respectively, single crystals of the fluoride silicates M₃F[Si₃O₁₀] (monoclinic, P2₁/n; Z = 4; M = Dy: a = 734.06(6), b = 1116.55(9), c = 1040.62(8) pm, β = 97.281(7)°; M = Ho: a = 730.91(6), b = 1111.68(9), c = 1037.83(8) pm, β = 97.238(7)°; M = Er: a = 727.89(6), b = 1107.02(9), c = 1035.21(8) pm, β = 97.209(7)°) were obtained. The most important building groups in the crystal structures of the thalenite type are “isolated” [FM₃]⁸⁺ triangles and catena‐trisilicate anions [Si₃O₁₀]^8–, which contain three [SiO₄] tetrahedra linked to a chain fragment via common corners. This has the shape of a horseshoe where both the terminal tetrahedra show different conformations (eclipsed and staggered) relative to the central unit. Therefore a chelatizing coordination on the same M³⁺ cation via oxygen atoms of both terminal [SiO₄] groups is possible. The narrow area of existence of these fluoride silicates within the lanthanide series will be discussed and structural comparisons with other catena‐trisilicates are presented.