Molybdenum(0) and Tungsten(0) Complexes with P,S and P,Se Monooxidized Bis(phosphanyl)amine Ligands

Reactions of monooxidized thioyl and selenoyl bis(phosphanyl)amine ligands C₁₀H₇‐1‐N(P(E)Ph₂)(PPh₂) [E = S ( 1 ), Se ( 2 )] with Mo(CO)₄(pip)₂ and W(CO)₄(cod) afforded the complexes [M(CO)₄{ 1 ‐κ²P,S}] [M = Mo ( 3 ), W ( 4 )] and [M(CO)₄{ 2 ‐κ²P,Se}] [M = Mo ( 5 ), W ( 6 )]. Complexes 3 – 6 were characterized by multinuclear NMR (¹H, ¹³C, ³¹P, and ⁷⁷Se NMR) and IR spectroscopy. Crystal‐structure determinations were carried out on 3 , 5 , and 6 , which represent the first examples of structurally characterized complexes of such ligands with group‐6 metal carbonyls.     

Tetranuclear Complexes with {M4O4} (M = CoII,NiII) Cubane‐Like Core: Synthesis,Crystal Structure,and Magnetic Properties

Two tetranuclear clusters of formula [M₄L₄(HOMe)₄] {H₂L = (E)‐1‐[(2‐(hydroxymethyl)phenylimino)methyl]naphthalen‐2‐ol} [M = Co ( 1 ), Ni ( 2 )] were hydrothermally synthesized by reaction of M(OAc)₂ · 4H₂O with H₂L and NaOH in MeOH. X‐ray crystal structure analysis revealed that complexes 1 and 2 are isostructural. In the core of the structures, four M^II ions and four oxygen atoms occupied alternate vertices of [M₄O₄] cubane. The magnetic property measurements of 1 and 2 revealed that overall ferromagnetic M^II ··· M^II exchange interactions exist in both complexes.     

Synthesis and Crystal Structures of Manganese(II) and ­Cobalt(II) Complexes with 2, 6‐Dimethoxybenzoic Acid and Additional N‐Donor Ligands

Three coordination compounds [Mn₃(dmb)₆(H₂O)₄(4, 4′‐bpy)₃(EtOH)]_n ( 1 ) and [M(dmb)₂(pyz)₂ (H₂O)₂] [M^II = Co ( 2 ), Mn ( 3 )] (Hdmb = 2, 6‐dimethoxybenzoic acid, 4, 4′‐bpy = 4, 4′‐bipyridine, pyz = pyrazine) were synthesized and characterized by single‐crystal X‐ray diffraction analysis. Compound 1 consists of infinite 1D polymeric chains, in which the metal entities are bridged by 4, 4′‐bpy ligands. There are four crystallographically independent Mn^II atoms in the linear chain with different coordination modes, which is only scarcely reported for linear polymers. The isostructural crystals of 2 and 3 are composed of neutral mononuclear complexes. In crystal the complexes are combined into chains by intermolecular O–H ··· N hydrogen bonds and π–π interactions between antiparallel pyrazine molecules.     

Expansion of Germanato‐Polyoxovanadate Cluster Cores: Solvothermal Syntheses and Selected Properties of (trenH2)2[{M(tren)}4V15Ge6O48(H2O)] (M = Co and Zn)

Two germanato‐polyoxovanadates with the {V₁₅Ge₆O₄₈} cluster core are extended by covalent bonds to four transition metal amine complexes [M(tren)]²⁺ (M = Co and Zn, tren = tris(2‐aminoethyl)amine). The complexes have bonds to terminal atoms of the Ge₂O₇ units and such expansion of a germanato‐polyxovanadate was never observed before. The characterization of these compounds revealed the presence of two protonated tren molecules charge balancing the negative charges of the [{M(tren)}₄V₁₅Ge₆O₄₈(H₂O)]^4– anion.     

Synthesis,Structure, and Spectroscopic and Electrochemical Properties of Heteroleptic Bis(phthalocyaninato) Rare Earth Complexes with a C4 Symmetry

A series of eleven heteroleptic bis(phthalocyaninato) rare earth double‐deckers [M^III(pc){pc(α‐OC₅H₁₁)₄}] 1 – 11 (M=Y, Sm? Lu; pc=phthalocyaninato; pc(α‐OC₅H₁₁)₄=1,8,15,22‐tetrakis(1‐ethylpropoxy)phthalocyaninato) were prepared as racemic mixtures by [M^III(pc)(acac)]‐induced (acac=acetylacetonato) cyclic tetramerization of 3‐(1‐ethylpropoxy)phthalonitrile in the presence of 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) in refluxing pentanol. These compounds could also be prepared by treating [M^III(pc)(acac)] with the metal‐free phthalocyanine H₂{pc(α‐OC₅H₁₁)₄} in refluxing octanol. The whole series of double‐decker complexes 1 – 11 were characterized by elemental analysis and various spectroscopic methods. The molecular structures of the Sm, Eu, and Er complexes 1, 2 , and 8 , respectively, were also determined by single‐crystal X‐ray diffraction analysis. The effects of the rare earth ion size on the reaction yield, molecular structure, and spectroscopic and electrochemical properties of these complexes were systematically examined.     

Syntheses,Crystal Structures,and Properties of Lead(II), ­Zinc(II), and Manganese(II) Complexes Constructed from 2, 3, 5, 6‐Tetraiodo‐1, 4‐benzenedicarboxylic Acid

Three new coordination compounds, [Pb(HBDC‐I₄)₂(DMF)₄]( 1 ) and [M(BDC‐I₄)(MeOH)₂(DMF)₂]_n (M = Zn^II for 2 and Mn^II for ( 3 ) (H₂BDC‐I₄ = 2, 3, 5, 6‐tetraiodo‐1, 4‐benzenedicarboxylic acid), were synthesized and characterized by elemental analysis, IR spectroscopy, thermogravimetric (TG) analysis, and X‐ray single crystal structure analysis. Single‐crystal X‐ray diffraction reveals that 1 crystallizes in the monoclinic space group C2/c and has a discrete mononuclear structure, which is further assembled to form a two‐dimensional (2D) layer through intermolecular O–H ··· O and C–H ··· O hydrogen bonding interactions. The isostructural compounds 2 and 3 crystallize in the space group P2₁/c and have similar one‐dimensional (1D) chain structures that are extended into three‐dimensional (3D) supramolecular networks by interchain C–H ··· π interactions. The Pb^II and Zn^II complexes 1 and 2 display similar emissions at 472 nm in the solid state, which essentially are intraligand transitions.     

Syntheses,Crystal Structures,and Luminescence Properties of Four Coordination Polymers Based on 1,3,5‐Tris (imidazol‐1‐yl)benzene

Based on the tripodal 1,3,5‐tris(imidazol‐1‐yl)benzene (tib) ligand, four transition metal coordination polymers, namely, {[Ni₃(tib)₂(H₂O)₁₂] · (SO₄)₃}_n ( 1 ), {[Co_1/6(tib)_1/3] · (O)_1/3}_n ( 2 ), and [M(tib)(hip)]_n (M = Mn for 3 , and M = Co for 4 ) (hip = 5‐hydroxyisophthalic acid), were synthesized through solvothermal method. Their structures were defined by single‐crystal X‐ray diffraction analyses and further characterized by elemental analyses (EA), IR spectra, powder X‐ray diffraction (PXRD), and thermogravimetric analyses (TGA). Complex 1 displays a 2D 3‐connected (6³) hcb net. Complex 2 is a 2D (3,6)‐connected (4³)₂(4⁶.6⁶.8³) kgm net. Complex 3 and 4 present similar 2D 4‐connected (4⁴.6²) sql net. Moreover, the solid state luminescence properties of complexes 1 and 3 were investigated.     

Modulation of the Lifetime of Water Bound to Lanthanide Metal Ions in Complexes with Ligands Derived from 1,4,7,10‐Tetraazacyclododecane Tetraacetate (DOTA)

A series of di‐ and tetraamide derivatives of DOTA were synthesized, and their lanthanide(III) complexes were examined by multinuclear ¹H‐, ¹³C‐, and ¹⁷O‐NMR spectroscopy, and compared with literature data of similar, known complexes (Table). All ligands formed structures similar to the parent [Ln^III(DOTA)]^? complexes, with four N‐atoms and four O‐atoms from DOTA and one O‐atom from the inner‐sphere water molecules. Interestingly, the lifetimes τ_M of the inner‐sphere, metal‐bound water molecules vary widely, ranging from nano‐ to milliseconds, depending on the identity of the pendent amide side chains. In general, positively charged [Ln^III(DOTA‐tetraamide)]³⁺ complexes display the longest residence times (high τ_M values), while complexes with additional charged functional groups on the extended amides display much smaller τ_M values, even when the side groups are not directly coordinated to the central Ln³⁺ ions. The design of novel [Ln^III(DOTA‐tetraamide)]³⁺ complexes with a wide, tunable range of τ_M values is of prime importance for the application of fast‐responding, paramagnetic chemical‐exchange‐saturation‐transfer (PARACEST) imaging agents used for the study of physiological and metabolic processes.     

Homo‐ and Heteronuclear Compounds with a Symmetrical Bis‐hydrazone Ligand: Synthesis,Structural Studies,and Luminescent Properties

Nine new coordination compounds have been synthesized by the reaction of salts of bivalent metal ions (a=Zn^II, b=Cu^II, c=Ni^II, d=Co^II) with the bis(benzoylhydrazone) derivative of 4,6‐diacetylresorcinol (H₄L). Three kinds of complexes have been obtained: homodinuclear compounds [M₂(H₂L)₂]?nH₂O ( 1 a , 1 b , 1 c , and 1 d ), homotetranuclear compounds [M₄(L)₂]?n(solv) ( 2 a and 2 c ), and heterotetranuclear compounds [Zn₂M₂(L)₂]?n(solv) ( 2 ab , 2 ac , and 2 ad ). The structures of the free ligand H₄L?2 DMSO and its complexes [Zn₂(H₂L)₂(DMSO)₂] ( 1 a* ), [Zn₄(L)₂(DMSO)₆] ( 2 a* ), and [Zn_0.45Cu_3.55(L)₂(DMSO)₆]?2 DMSO ( 2 ab* ) were elucidated by single‐crystal X‐ray diffraction. The ligand shows luminescence properties and its fluorimetric behavior towards M^II metals (M=Zn, Cu, Ni and Co) has been studied. Furthermore, the solid‐state luminescence properties of the ligand and compounds have been determined at room temperature. ¹H NMR spectroscopic monitoring of the reaction of H₄L with Zn^II showed the deprotonation sequence of the OH/NH groups upon metal coordination. Heteronuclear reactions have also been monitored by using ESI‐MS and spectrofluorimetric techniques.     

Functionalization of Pentaphosphorus Cations by Complexation

The chemistry of polyphosphorus cations has rapidly developed in recent years, but their coordination behavior has remained mostly unexplored. Herein, we describe the reactivity of [P₅R₂]⁺ cations with cyclopentadienyl metal complexes. The reaction of [Cp^ArFe(μ‐Br)]₂ (Cp^Ar=C₅(C₆H₄‐4‐Et)₅) with [P₅R₂][GaCl₄] (R=iPr and 2,4,6‐Me₃C₆H₂ (Mes)) afforded bicyclo[1.1.0]pentaphosphanes ( 1‐R , R=iPr and Mes), showing an unsymmetric “butterfly” structure. The same products 1‐R were formed from K[Cp^Ar] and [P₅R₂][GaCl₄]. The cationic complexes [Cp^ArCo(η⁴‐P₅R₂)][GaCl₄] ( 2‐R [GaCl₄], R=iPr and Cy) and [(Cp^ArNi)₂(η^3:3‐P₅R₂)][GaCl₄] ( 3‐R [GaCl₄]) were obtained from [P₅R₂][GaCl₄] and [Cp^ArM(μ‐Br)]₂ (M=Co and Ni) as well as by using low‐valent “Cp^ArM^I” sources. Anion metathesis of 2‐R [GaCl₄] and 3‐R [GaCl₄] was achieved with Na[BArF₂₄]. The P₅ framework of the resulting salts 2‐R [BArF₂₄] can be further functionalized with nucleophiles. Thus reactions with [Et₄N]X (X=CN and Cl) give unprecedented cyano‐ and chloro‐functionalized complexes, while organo‐functionalization was achieved with CyMgCl.     

Formation and Structural Characterization of Metal Complexes derived from Thiosalicylic Acid

The formation and structural aspects of some metal complexes of thiosalicylic acid (TSA) were studied. The μ‐bridging tetra‐coordinated Ru complex, [Ru(C₆H₄(CO₂)(μ‐S)(H₂O)]₂ ( 1 ) was formed by hydrothermal reaction of TSA with RuCl₃. The complexes [M(dtdb)(phen)(H₂O)]_n ( 2 – 4 ) (M = Zn^II, Co^II, Ni^II, dtdb = 2,2′‐dithiodibenzoate anion, phen = 1,10‐phenanthroline) were obtained by the slow diffusion technique and the in situ S–S bond formation was confirmed by elemental, spectral and X‐ray analysis. Reaction of TSA with CuCl₂ and 2,2′‐bipyridine (bipy) under the slow diffusion technique yielded the dimer [Cu(tdb)(bipy)] ( 5 ) (tdb = thiodibenzoic acid), where the in situ generation of 2,2′‐thiodibenzoic acid was observed.     

Coordination behaviour and two‐dimensional‐network formation in poly[[μ‐aqua‐diaqua(μ5‐propane‐1,3‐diyldinitrilotetraacetato)dilithium(I)cobalt(II)] dihydrate]: the first example of an MII–1,3‐pdta complex with a monovalent metal counter‐ion

The title compound, {[CoLi₂(C₁₁H₁₄N₂O₈)(H₂O)₃]·2H₂O}_n, constitutes the first example of a salt of the [M^II(1,3‐pdta)]²⁻ complex (1,3‐pdta is propane‐1,3‐diyldinitrilotetraacetate) with a monopositive cation as counter‐ion. Insertion of the Li⁺ cation could only be achieved through application of the ion‐exchange column technique which, however, appeared unsuccessful with other alkali metals and the ammonium cation. The structure contains two tetrahedrally coordinated Li⁺ cations, an octahedral [Co(1,3‐pdta)]²⁻ anion and five water molecules, two of which are uncoordinated, and is built of two‐dimensional layers extending parallel to the (010) lattice plane, the constituents of which are connected by the coordinate bonds. O—H_water...O hydrogen bonds operate both within and between these layers. The crystal investigated belongs to the enantiomeric space group P2₁ with only one (Λ) of two possible optical isomers of the [Co(1,3‐pdta)]²⁻ complex. A possible cause of enantiomer separation during crystallization might be the rigidification and polarization of the [M(1,3‐pdta)]²⁻ core, resulting from direct coordination of Li⁺ cations to three out of four carboxylate groups constituting the 1,3‐pdta ligand. The structure of (I) differs considerably from those of the other [M^II(1,3‐pdta)]²⁻ complexes, in which the charge compensation is realized by means of divalent hexaaqua complex cations. This finding demonstrates a significant structure‐determining role of the counter‐ions.     

Syntheses,Structures, and Photoluminescent and Magnetic Properties of Four Novel Complexes Constructed from 2,4,5‐Tri(4‐pyridyl)‐imidazole and Benzene‐1,3,5‐tricarboxylic Acid

Four novel 2D complexes M₂(Hptim)₂(HBTC)₂ [M = Co ( 1 ), Cd ( 2 ), Zn ( 3 ), Mn ( 4 ); Hptim = 2,4,5‐tri(4‐pyridyl)‐imidazole; HBTC^2– = Benzene‐1,3,5‐tricarboxylic acid] were synthesized under solvothermal conditions. The four complexes are isomorphous and present a unique structure with a 1D ladder of [Co₂(HBTC)₂]_n. The 2D network structure of 1 is achieved through bridging Hptim groups, which coordinate to the metal atoms of two adjacent 1D [Co₂(HBTC)₂]_n ladders. Magnetic measurements reveal that dominant antiferromagnetic coupling was observed in compounds 1 and 4 . Compounds 2 and 3 both exhibit strong fluorescent emissions in the solid state and may be suitable candidates for fluorescent materials.     

Dihalomethanes as Bent Bifunctional XB/XB‐Donating Building Blocks for Construction of Metal‐involving Halogen Bonded Hexagons

The dihalomethanes CH₂X₂ (X=Cl, Br, I) were co‐crystallized with the isocyanide complexes trans‐[MX^M₂(CNC₆H₄‐4‐X^C)₂] (M=Pd, Pt; X^M=Br, I; X^C=F, Cl, Br) to give an extended series comprising 15 X‐ray structures of isostructural adducts featuring 1D metal‐involving hexagon‐like arrays. In these structures, CH₂X₂ behave as bent bifunctional XB/XB‐donating building blocks, whereas trans‐[MX^M₂(CNC₆H₄‐4‐X^C)₂] act as a linear XB/XB acceptors. Results of DFT calculations indicate that all XCH₂–X???X^M–M contacts are typical noncovalent interactions with estimated strengths in the range of 1.3–3.2 kcal mol^?1. A CCDC search reveals that hexagon‐like arrays are rather common but previously overlooked structural motives for adducts of trans‐bis(halide) complexes and halomethanes.     

Aluminum Complexes Stabilized by Piperazidine‐Bridged Bis(phenolate) Ligands: Syntheses,Structures, and Application in the Ring‐Opening Polymerization of ε‐Caprolactone

The synthesis and characterization of aluminum alkoxide and alkyl complexes stabilized by piperazidine‐bridged bis(phenolate) ligands are described. Treatment of ligand precursors H₂[ONNO]¹ {H₂[ONNO]¹=1,4‐bis(2‐hydroxy‐3‐tert‐butyl‐5‐methylbenzyl)piperazidine} and H₂[ONNO]² {H₂[ONNO]²=1,4‐bis(2‐hydroxy‐3,5‐di‐tert‐butylbenzyl)piperazidine} with AlEt₂(OCH₂Ph) and AlEt₂(OPr‐i), which were generated in situ by the reactions of AlEt₃ with equivalent of the corresponding alcohols, in a 1:1 molar ratio in THF gave the corresponding aluminum alkoxide complexes [ONNO]¹Al(OCH₂Ph) ( 1 ) and [ONNO]²Al(OPr‐i) ( 2 ), respectively. The reaction of H₂[ONNO]¹ with AlEt₂(OCH₂Ph) in a 1:2 molar ratio in THF afforded a mixture of monometallic aluminum ethyl complex [ONNO]¹AlEt ( 3 ) and complex 1 , which can be isolated by stepwise crystallization. Similarly, H₂[ONNO]² reacted with AlEt₂(OPr‐i) in a 1:2 molar ratio in THF to give a mixture of aluminum ethyl complex [ONNO]²AlEt ( 4 ) and complex 2 . Complexes 1 and 2 were also available via treatment of complexes 3 and 4 with 1 equiv. of benzyl alcohol and isopropyl alcohol, respectively. All of these complexes were fully characterized including X‐ray structural determination. It was found that complexes 1 to 4 can initiate the ring‐opening polymerization of ε‐caprolactone, and complexes 1 and 2 showed higher catalytic activity in comparison with complexes 3 and 4 .     

Structure and Polymorphism of M(thd)3 (M = Al,Cr, Mn,Fe, Co,Ga, and In)

Formation, crystal structure, polymorphism, and transition between polymorphs are reported for M(thd)₃, (M = Al, Cr, Mn, Fe, Co, Ga, and In) [(thd)^– = anion of H(thd) = C₁₁H₂₀O₂ = 2, 2, 6, 6‐tetramethylheptane‐3, 5‐dione]. Fresh crystal‐structure data are provided for monoclinic polymorphs of Al(thd)₃, Ga(thd)₃, and In(thd)₃. Apart from adjustment of the M–O_k bond length, the structural characteristics of M(thd)₃ complexes remain essentially unaffected by change of M. Analysis of the M–O_k, O_k–C_k, and C_k–C_k distances support the notion that the M–O_k–C_k–C_k–C_k–O_k– ring forms a heterocyclic unit with σ and π contributions to the bonds. Tentative assessments according to the bond‐valence or bond‐order scheme suggest that the strengths of the σ bonds are approximately equal for the M–O_k, O_k–C_k, and C_k–C_k bonds, whereas the π component of the M–O_k bonds is small compared with those for the O_k–C_k, and C_k–C_k bonds. The contours of a pattern for the occurrence of M(thd)₃ polymorphs suggest that polymorphs with structures of orthorhombic or higher symmetry are favored on crystallization from the vapor phase (viz. sublimation). Monoclinic polymorphs prefer crystallization from solution at temperatures closer to ambient. Each of the M(thd)₃ complexes subject to this study exhibits three or more polymorphs (further variants are likely to emerge consequent on systematic exploration of the crystallization conditions). High‐temperature powder X‐ray diffraction shows that the monoclinic polymorphs convert irreversibly to the corresponding rotational disordered orthorhombic variant above some 100–150 °C (depending on M). The orthorhombic variant is in turn transformed into polymorphs of tetragonal and cubic symmetry before entering the molten state. These findings are discussed in light of the current conceptions of rotational disorder in molecular crystals.     

Phenylimido Complexes of Technetium and Rhenium with Maleonitriledithiolate

[Tc(NPh)Cl₃(PPh₃)₂] or [Re(NPh)Cl₃(PPh₃)₂] react with two equivalents of Na₂mnt (mnt^2– = 1,2‐dicyanoethene‐1,2‐dithiolate) with formation of anionic complexes of the composition [M(NPh)(mnt)₂]^–. The products can be isolated as large red blocks of their AsPh₄⁺ salts. The complex anions contain square‐pyramidal coordinated metal atoms with the phenylimido ligands in apical positions. The M–N–C bonds are almost linear. A similar phenylimido complex with an additional amino group was synthesized from [Re(NC₆H₄‐4‐NH₂)Cl₃(PPh₃)₂]. The presence of such substituents may allow coupling of the metal complexes to biomolecules such as peptides, proteins, or sugars, provided the M=N bonds are sufficiently stable against hydrolysis.     

Fine Control of the Redox Reactivity of a Nonheme Iron(III)–Peroxo Complex by Binding Redox‐Inactive Metal Ions

Redox‐inactive metal ions are one of the most important co‐factors involved in dioxygen activation and formation reactions by metalloenzymes. In this study, we have shown that the logarithm of the rate constants of electron‐transfer and C−H bond activation reactions by nonheme iron(III)–peroxo complexes binding redox‐inactive metal ions, [(TMC)Fe^III(O₂)]⁺‐M^{n +} (M^{n +}=Sc³⁺, Y³⁺, Lu³⁺, and La³⁺), increases linearly with the increase of the Lewis acidity of the redox‐inactive metal ions (ΔE ), which is determined from the g_zz values of EPR spectra of O₂^.−‐M^{n +} complexes. In contrast, the logarithm of the rate constants of the [(TMC)Fe^III(O₂)]⁺‐M^{n +} complexes in nucleophilic reactions with aldehydes decreases linearly as the ΔE value increases. Thus, the Lewis acidity of the redox‐inactive metal ions bound to the mononuclear nonheme iron(III)–peroxo complex modulates the reactivity of the [(TMC)Fe^III(O₂)]⁺‐M^{n +} complexes in electron‐transfer, electrophilic, and nucleophilic reactions.     

Synthesis,Crystal Structure,and Spectral Properties of Two Manganese(II) and Zinc(II) Complexes with 3,4‐Dimethyl‐5‐(2‐pyridyl)‐1,2,4‐triazole

Two complexes, cis‐[MnL₂(NCS)₂] ( 1 ) and cis‐[ZnL₂(NCS)₂] ( 2 ) with asymmetrical substituted triazole ligands [L = 3,4‐dimethyl‐5‐(2‐pyridyl)‐1,2,4‐triazole], were synthesized and characterized by elemental analysis, UV/Vis and FT‐IR spectroscopy as well as thermogravimetric analyses (TGA), powder XRD, and single‐crystal X‐ray diffraction. In the complexes, each L molecule adopts a chelating bidentate mode by the nitrogen atoms of pyridyl and triazole. Both complexes have a similar distorted octahedral [MN₆] core (M = Mn²⁺ and Zn²⁺) with two NCS^– ions in the cis position.     

Access to Heteroleptic Fluorido‐Cyanido Complexes with a Large Magnetic Anisotropy by Fluoride Abstraction

Silicon‐mediated fluoride abstraction is demonstrated as a means of generating the first fluorido‐cyanido transition metal complexes. This new synthetic approach is exemplified by the synthesis and characterization of the heteroleptic complexes, trans‐[M^IVF₄(CN)₂]^2? (M=Re, Os), obtained from their homoleptic [M^IVF₆]^2? parents. As shown by combined high‐field electron paramagnetic resonance spectroscopy and magnetization measurements, the partial substitution of fluoride by cyanide ligands leads to a marked increase in the magnetic anisotropy of trans‐[ReF₄(CN)₂]^2? as compared to [ReF₆]^2?, reflecting the severe departure from an ideal octahedral (O_h point group) ligand field. This methodology paves the way toward the realization of new heteroleptic transition metal complexes that may be used as highly anisotropic building‐blocks for the design of high‐performance molecule‐based magnetic materials.