Coordination‐Induced Spin‐State Switching of an Aminyl‐Radical‐Bridged Nickel(II) Porphyrin Dimer between Doublet and Sextet States

A bis(Ni^II‐porphyrinyl)aminyl radical with meso‐C₆F₅ groups was prepared as a spin‐delocalized stable aminyl radical with a doublet spin state. Upon addition of pyridine, both Ni^II centers became hexacoordinated by accepting two axial pyridines, which triggered a spin‐state change of the Ni^II centers from diamagnetic (S=0) to paramagnetic (S=1). The resulting high‐spin Ni^II centers interact with the aminyl radical ferromagnetically to give rise to an overall sextet state (S=5/2). Importantly, this coordination‐induced spin‐state switching can be conducted in a reversible manner, in that washing of the high‐spin radical with aqueous hydrochloric acid regenerates the original doublet radical in good yield.     

Styrene Aziridination by Iron(IV) Nitrides

Thermolysis of the iron(IV) nitride complex [PhB(tBuIm)₃Fe?N] with styrene leads to formation of the high‐spin iron(II) aziridino complex [PhB(tBuIm)₃Fe‐N(CH₂CHPh)]. Similar aziridination occurs with both electron‐rich and electron‐poor styrenes, while bulky styrenes hinder the reaction. The aziridino complex [PhB(tBuIm)₃Fe‐N(CH₂CHPh)] acts as a nitride synthon, reacting with electron‐poor styrenes to generate their corresponding aziridino complexes, that is, aziridine cross‐metathesis. Reaction of [PhB(tBuIm)₃Fe‐N(CH₂CHPh)] with Me₃SiCl releases the N‐functionalized aziridine Me₃SiN(CH₂CHPh) while simultaneously generating [PhB(tBuIm)₃FeCl]. This closes a synthetic cycle for styrene azirdination by a nitride complex. While the less hindered iron(IV) nitride complex [PhB(MesIm)₃Fe?N] reacts with styrenes below room temperature, only bulky styrenes lead to tractable aziridino products.     

Unprecedented Five‐Coordinate Iron(IV) Imides Generate Divergent Spin States Based on the Imide R‐Groups

Three five‐coordinate iron(IV) imide complexes have been synthesized and characterized. These novel structures have disparate spin states on the iron as a function of the R‐group attached to the imide, with alkyl groups leading to low‐spin diamagnetic (S=0) complexes and an aryl group leading to an intermediate‐spin (S=1) complex. The different spin states lead to significant differences in the bonding about the iron center as well as the spectroscopic properties of these complexes. Mössbauer spectroscopy confirmed that all three imide complexes are in the iron(IV) oxidation state. The combination of diamagnetism and ¹⁵N labeling allowed for the first ¹⁵N NMR resonance recorded on an iron imide. Multi‐reference calculations corroborate the experimental structural findings and suggest how the bonding is distinctly different on the imide ligand between the two spin states.     

High‐Spin Iron(I) and Iron(0) Dinitrogen Complexes Supported by N‐Heterocyclic Carbene Ligands

The use of 1,3‐dicyclohexylimidazol‐2‐ylidene (ICy) as ligand has enabled the preparation of the high‐spin tetrahedral iron(I)‐ and iron(0)–N₂ complexes, namely [(ICy)₃Fe(N₂)][BPh₄] ( 1 ) and [(ICy)₃Fe(N₂)] ( 2 ), the electronic structures of which have been established by various spectroscopic characterization and DFT calculations. The frequency of the N–N stretching resonance of the iron(0)–N₂ complex is the lowest among the reported terminal N₂ complexes of iron, signifying the beneficial roles of strongly σ‐donating ligands in combination with the high‐spin low‐valent iron center in promoting N₂‐activation. The iron(0)–N₂ complex can convert reversibly to the low‐spin iron(II)‐N₂ hydride complex [(ICy)₂(ICy′)Fe(N₂)(H)] ( 4 ).     

Effect of Inter‐Porphyrin Distance on Spin‐State in Diiron(III) μ‐Hydroxo Bisporphyrins

The synthesis, structure, and properties of bischloro, μ‐oxo, and a family of μ‐hydroxo complexes (with BF₄^?, SbF₆^?, and PF₆^? counteranions) of diethylpyrrole‐bridged diiron(III) bisporphyrins are reported. Spectroscopic characterization has revealed that the iron centers of the bischloro and μ‐oxo complexes are in the high‐spin state (S=⁵/₂). However, the two iron centers in the diiron(III) μ‐hydroxo complexes are equivalent with high spin (S=⁵/₂) in the solid state and an intermediate‐spin state (S=³/₂) in solution. The molecules have been compared with previously known diiron(III) μ‐hydroxo complexes of ethane‐bridged bisporphyrin, in which two different spin states of iron were stabilized under the influence of counteranions. The dimanganese(III) analogues were also synthesized and spectroscopically characterized. A comparison of the X‐ray structural parameters between diethylpyrrole and ethane‐bridged μ‐hydroxo bisporphyrins suggest an increased separation, and hence, less interactions between the two heme units of the former. As a result, unlike the ethane‐bridged μ‐hydroxo complex, both iron centers become equivalent in the diethylpyrrole‐bridged complex and their spin state remains unresponsive to the change in counteranion. The iron(III) centers of the diethylpyrrole‐bridged diiron(III) μ‐oxo bisporphyrin undergo very strong antiferromagnetic interactions (J=?137.7 cm^?1), although the coupling constant is reduced to only a weak value in the μ‐hydroxo complexes (J=?42.2, ?44.1, and ?42.4 cm^?1 for the BF₄, SbF₆, and PF₆ complexes, respectively).     

Electronic Structure Modulation in an Exceptionally Stable Non‐Heme Nitrosyl Iron(II) Spin‐Crossover Complex

The highly stable nitrosyl iron(II) mononuclear complex [Fe(bztpen)(NO)](PF₆)₂ (bztpen=N‐benzyl‐N,N′,N′‐tris(2‐pyridylmethyl)ethylenediamine) displays an S=1/2?S=3/2 spin crossover (SCO) behavior (T_1/2=370 K, ΔH=12.48 kJ mol^?1, ΔS=33 J K^?1 mol^?1) stemming from strong magnetic coupling between the NO radical (S=1/2) and thermally interconverted (S=0?S=2) ferrous spin states. The crystal structure of this robust complex has been investigated in the temperature range 120–420 K affording a detailed picture of how the electronic distribution of the t_2g–e_g orbitals modulates the structure of the {FeNO}⁷ bond, providing valuable magneto–structural and spectroscopic correlations and DFT analysis.     

Well‐Defined Iron Complexes as Efficient Catalysts for “Green” Atom‐Transfer Radical Polymerization of Styrene,Methyl Methacrylate,and Butyl Acrylate with Low Catalyst Loadings and Catalyst Recycling

Environmentally friendly iron(II) catalysts for atom‐transfer radical polymerization (ATRP) were synthesized by careful selection of the nitrogen substituents of N,N,N‐trialkylated‐1,4,9‐triazacyclononane (R₃TACN) ligands. Two types of structures were confirmed by crystallography: “[(R₃TACN)FeX₂]” complexes with relatively small R groups have ionic and dinuclear structures including a [(R₃TACN)Fe(μ‐X)₃Fe(R₃TACN)]⁺ moiety, whereas those with more bulky R groups are neutral and mononuclear. The twelve [(R₃TACN)FeX₂]_n complexes that were synthesized were subjected to bulk ATRP of styrene, methyl methacrylate (MMA), and butyl acrylate (BA). Among the iron complexes examined, [{(cyclopentyl)₃TACN}FeBr₂] ( 4 b ) was the best catalyst for the well‐controlled ATRP of all three monomers. This species allowed easy catalyst separation and recycling, a lowering of the catalyst concentration needed for the reaction, and the absence of additional reducing reagents. The lowest catalyst loading was accomplished in the ATRP of MMA with 4 b (59 ppm of Fe based on the charged monomer). Catalyst recycling in ATRP with low catalyst loadings was also successful. The ATRP of styrene with 4 b (117 ppm Fe atom) was followed by precipitation from methanol to give polystyrene that contained residual iron below the calculated detection limit (0.28 ppm). Mechanisms that involve equilibria between the multinuclear and mononuclear species were also examined.     

Two square‐pyramidal chromium(V)–nitride complexes: bis­(2‐methyl­quinolin‐8‐olato)nitridochromium(V) and nitridobis(2‐sulfidopyridine N‐oxide)chromium(V)

Two new chromium(V)–nitride complexes with a coordination sphere completed by bidentate ligands have been synthesized and structurally characterized. Bis(2‐methyl­quinolin‐8‐olato)nitridochromium(V), [Cr(C₁₀H₈NO)₂(N)], has the coordination sphere completed by an equatorial N₂O₂ set of ligators. The compound crystallizes with the five‐coordinate complexes at sites with twofold rotational symmetry and all Cr—N bond directions aligned with the crystallographic b axis. Nitridobis(2‐sulfidopyridine N‐oxide)chromium(V), [Cr(C₅H₄NOS)₂(N)], crystallizes with the mol­ecules on general positions and has an equatorial S₂O₂ coordination environment, which is unprecedented among nitride complexes of the first‐row transition metals. In both systems, Cr[triple‐bond]N bonds are short at ca 1.56 Å.     

A Brønsted‐Ligand‐Based Iron Complex as a Molecular Switch with Five Accessible States

A mononuclear Fe^II complex, prepared with a Brønsted diacid ligand, H₂L (H₂L=2‐[5‐phenyl‐1H‐pyrazole‐3‐yl] 6‐benzimidazole pyridine), shows switchable physical properties and was isolated in five different electronic states. The spin crossover (SCO) complex, [Fe^II(H₂L)₂](BF₄)₂ ( 1_A ), exhibits abrupt spin transition at T_1/2=258 K, and treatment with base yields a deprotonated analogue [Fe^II(HL)₂] ( 1_B ), which shows gradual SCO above 350 K. A range of Fe^III analogues were also characterized. [Fe^III(HL)(H₂L)](BF₄)Cl ( 1_C ) has an S=5/2 spin state, while the deprotonated complexes [Fe^III(L)(HL)], ( 1_D ), and (TEA)[Fe^III(L)₂], ( 1_E ) exist in the low‐spin S=1/2 state. The electronic properties of the five complexes were fully characterized and we demonstrate in situ switching between multiple states in both solution and the solid‐state. The versatility of this simple mononuclear system illustrates how proton donor/acceptor ligands can vastly increase the range of accessible states in switchable molecular devices.     

Photomagnetism of a sym‐cis‐Dithiocyanato Iron(II) Complex with a Tetradentate N,N′‐Bis(2‐pyridylmethyl)1,2‐ethanediamine Ligand

A comprehensive study of the magnetic and photomagnetic behaviors of cis‐[Fe(picen)(NCS)₂] (picen=N,N′‐bis(2‐pyridylmethyl)1,2‐ethanediamine) was carried out. The spin‐equilibration was extremely slow in the vicinity of the thermal spin‐transition. When the cooling speed was slower than 0.1 K min^?1, this complex was characterized by an abrupt thermal spin‐transition at about 70 K. Measurement of the kinetics in the range 60–70 K was performed to approach the quasi‐static hysteresis loop. At low temperatures, the metastable HS state was quenched by a rapid freezing process and the critical T(TIESST) temperature, which was associated with the thermally induced excited spin‐state‐trapping (TIESST) effect, was measured. At 10 K, this complex also exhibited the well‐known light‐induced excited spin‐state‐trapping (LIESST) effect and the T(LIESST) temperature was determined. The kinetics of the metastable HS states, which were generated from the freezing effect and from the light‐induced excitation, was studied. Single‐crystal X‐ray diffraction as a function of speed‐cooling and light conditions at 30 K revealed the mechanism of the spin‐crossover in this complex as well as some direct relationships between its structural properties and its spin state. This spin‐crossover (SCO) material represents a fascinating example in which the metastability of the HS state is in close vicinity to the thermal spin‐transition region. Moreover, it is a beautiful example of a complex in which the metastable HS states can be generated, and then compared, either by the freezing effect or by the LIESST effect.     

Hydrogen‐Bonding Interactions Trigger a Spin‐Flip in Iron(III) Porphyrin Complexes

A key step in cytochrome P450 catalysis includes the spin‐state crossing from low spin to high spin upon substrate binding and subsequent reduction of the heme. Clearly, a weak perturbation in P450 enzymes triggers a spin‐state crossing. However, the origin of the process whereby enzymes reorganize their active site through external perturbations, such as hydrogen bonding, is still poorly understood. We have thus studied the impact of hydrogen‐bonding interactions on the electronic structure of a five‐coordinate iron(III) octaethyltetraarylporphyrin chloride. The spin state of the metal was found to switch reversibly between high (S=⁵/₂) and intermediate spin (S=³/₂) with hydrogen bonding. Our study highlights the possible effects and importance of hydrogen‐bonding interactions in heme proteins. This is the first example of a synthetic iron(III) complex that can reversibly change its spin state between a high and an intermediate state through weak external perturbations.     

Synthesis,Magnetic Properties and X‐ray Structure Analysis of a 1‐D Chain Iron(II) Spin Crossover Complex with wide Hysteresis

The reaction of [FeL(MeOH)₂] (L being a tetradentate [N₂O₂]^2? coordinating Schiff base like ligand [([3,3′]‐[1,2‐phenylenebis(iminomethylidyne)]bis(2,4‐pentane‐dionato)(2‐)N,N′,O²,O²′], MeOH = methanol) with 4,4′‐bipyridine (bipy) results in the formation of a new iron(II ) spin crossover coordination polymer of the formula [FeL(bipy)] ( 1 ). T‐dependent susceptibility measurements revealed an abrupt HS ? LS spin transition with an approximately 18 K‐wide thermal hysteresis loop (T_1/2↑ = 237 K and T_1/2↓ = 219 K). The isolation of crystals suitable for X‐ray structure analysis allowed the determination of the motive of the molecule structure of the first 1‐D chain compound with hysteresis in the HS form at 250 K. Despite the low qualtity of the data, we were able to obtain some insight into the interplay of covalent and elastic interactions that are both responsible for the high cooperative interactions during the spin transition in this compound.     

Sulfur‐linked Phenolates as Ligands for the Syntheses of Low‐Nuclearity Iron(III) Complexes

Exploiting thiacalix 4 arene and sulfur‐bridged bisphenolates as ligands for bioinorganic studies involving iron(III) requires the prior development of synthetic routes (varying substituents and reaction conditions) to construct complexes with low nuclearities and accessible coordination sites, which was in the focus of this investigation. Treating p‐tert‐butylthiacalix 4 arene (H₄TC) and 1, 4‐dimethyl‐p‐tert‐butylthiacalix 4 arene (Me₂H₂TC) with Fe[N(SiMe₃)₂]₃ yielded in the formation of the iron(III) complexes [(Me₃SiTC)₂Fe₂] ( 1 ) and [(Me₂TC)₃Fe₂] ( 3 ), respectively. While 1 is a sandwich compound, in 3 one [Me₂TC]^2– unit is bridging two [Me₂TCFe]⁺ moieties. Employing thiobisphenolates as ligands it turned out, that in dependence on the residues R and the preparation method it is possible to selectively access sandwich, anionic or neutral complexes, which were shown to contain central high‐spin iron(III) atoms. The syntheses, structures, and electronic properties of three iron(III) bisphenolate complexes, [^ClL₂Fe]NEt₃H ( 4 ), [^MeLFeCl₂]NEt₃H ( 5 ), and [^tBuLFeCl(thf)] ( 7 ) are discussed.     

2,6‐Bis(pyrazol‐3‐yl)pyridine as Meridional Capping Ligand: Synthesis,Characterization, and Crystal Structure of the First Corresponding Hexanuclear Iron(III) Complex

Based on the 2,6‐bis(pyrazol‐3‐yl)pyridine ligand (H₂bpp) the hexanuclear iron(III) complex [Fe₆(bpp)₄(μ₃‐O)₂(μ‐OMe)₃(μ‐OH)Cl₂] ( 1 ) was synthesized. The reaction with iron(II) chloride and additional pyridine leads to the exclusive formation of the complex through self‐assembly process. Six octahedrally coordinated iron atoms are linked through the pyrazolido groups of four H₂bpp ligands. These are further linked through bridging hydroxido, methoxido, and oxido groups. The complex has been characterized by IR spectroscopy, ESI mass spectrometry, elemental analysis and X‐ray crystallography. Temperature‐dependent magnetic measurements indicate strong antiferromagnetic exchange interaction between the high‐spin iron(III) ions within the complex, which leads to an S = 0 spin ground state. As a result of the two Fe3(μ₃‐O) fragments two frustrated exchange pathways are present. In addition the properties of H₂bpp as a potential capping ligand for the synthesis of heteroleptic trinuclear complexes based on the triaminoguanidine core is investigated.     

C−H Activation from Iron(II)‐Nitroxido Complexes

The reaction of nitroxyl radicals TEMPO (2,2′,6,6′‐tetramethylpiperidinyloxyl) and AZADO (2‐azaadamantane‐N‐oxyl) with an iron(I) synthon affords iron(II)‐nitroxido complexes (^ArL)Fe(κ¹‐TEMPO) and (^ArL)Fe(κ²‐N,O‐AZADO) (^ArL=1,9‐(2,4,6‐Ph₃C₆H₂)₂‐5‐mesityldipyrromethene). Both high‐spin iron(II)‐nitroxido species are stable in the absence of weak C−H bonds, but decay via N−O bond homolysis to ferrous or ferric iron hydroxides in the presence of 1,4‐cyclohexadiene. Whereas (^ArL)Fe(κ¹‐TEMPO) reacts to give a diferrous hydroxide [(^ArL)Fe]₂(μ‐OH)₂, the reaction of four‐coordinate (^ArL)Fe(κ²‐N,O‐AZADO) with hydrogen atom donors yields ferric hydroxide (^ArL)Fe(OH)(AZAD). Mechanistic experiments reveal saturation behavior in C−H substrate and are consistent with rate‐determining hydrogen atom transfer.     

(Aminocarbene)(Divinyltetramethyldisiloxane)Iron(0) Compounds: A Class of Low‐Coordinate Iron(0) Reagents

The combined use of aminocarbene and divinyltetramethyldisiloxane (dvtms) as supporting ligands enables the access of unprecedented low‐coordinate iron(0) alkene compounds [L_nFe(η²:η²‐dvtms)] (L=N‐heterocyclic carbene (NHC) or cyclic (alkyl)(amino)carbene (CAAC), n=1 or 2) from the reactions of FeCl₂ with alkali‐metal reducing agents, free aminocarbene ligands, and dvtms. The iron(0) species deliver their {L_nFe⁰} fragments to perform redox reactions with Ph₂SiH₂, S₈, Se, and DippN₃, furnishing novel aminocarbene‐supported iron(IV) silylene, all‐ferrous iron–sulfur/selenium cubanes, and bis(imido)iron(IV) compounds. These conversions demonstrate the potential synthetic utility of the carbene‐supported iron(0) complexes as a valuable class of low‐coordinate iron(0) reagents.     

Heteroleptic FeII Complexes of 2,2′‐Biimidazole and Its Alkylated Derivatives: Spin‐Crossover and Photomagnetic Behavior

Three iron(II) complexes, [Fe(TPMA)(BIM)](ClO₄)₂?0.5H₂O ( 1 ), [Fe(TPMA)(XBIM)](ClO₄)₂ ( 2 ), and [Fe(TPMA)(XBBIM)](ClO₄)₂ ?0.75CH₃OH ( 3 ), were prepared by reactions of Fe^II perchlorate and the corresponding ligands (TPMA=tris(2‐pyridylmethyl)amine, BIM=2,2′‐biimidazole, XBIM=1,1′‐(α,α′‐o‐xylyl)‐2,2′‐biimidazole, XBBIM=1,1′‐(α,α′‐o‐xylyl)‐2,2′‐bibenzimidazole). The compounds were investigated by a combination of X‐ray crystallography, magnetic and photomagnetic measurements, and Mössbauer and optical absorption spectroscopy. Complex 1 exhibits a gradual spin crossover (SCO) with T_1/2=190 K, whereas 2 exhibits an abrupt SCO with approximately 7 K thermal hysteresis (T_1/2=196 K on cooling and 203 K on heating). Complex 3 is in the high‐spin state in the 2–300 K range. The difference in the magnetic behavior was traced to differences between the inter‐ and intramolecular interactions in 1 and 2 . The crystal packing of 2 features a hierarchy of intermolecular interactions that result in increased cooperativity and abruptness of the spin transition. In 3 , steric repulsion between H atoms of one of the pyridyl substituents of TPMA and one of the benzene rings of XBBIM results in a strong distortion of the Fe^II coordination environment, which stabilizes the high‐spin state of the complex. Both 1 and 2 exhibit a photoinduced low‐spin to high‐spin transition (LIESST effect) at 5 K. The difference in the character of intermolecular interactions of 1 and 2 also manifests in the kinetics of the decay of the photoinduced high‐spin state. For 1 , the decay rate constant follows the single‐exponential law, whereas for 2 it is a stretched exponential, reflecting the hierarchical nature of intermolecular contacts. The structural parameters of the photoinduced high‐spin state at 50 K are similar to those determined for the high‐spin state at 295 K. This study shows that N‐alkylation of BIM has a negligible effect on the ligand field strength. Therefore, the combination of TPMA and BIM offers a promising ligand platform for the design of functionalized SCO complexes.     

Low‐Spin Pseudotetrahedral Iron(I) Sites in Fe2(μ‐S) Complexes

Fe^I centers in iron–sulfide complexes have little precedent in synthetic chemistry despite a growing interest in the possible role of unusually low valent iron in metalloenzymes that feature iron–sulfur clusters. A series of three diiron [(L₃Fe)₂(μ‐S)] complexes that were isolated and characterized in the low‐valent oxidation states Fe^II? S? Fe^II, Fe^II? S? Fe^I, and Fe^I? S? Fe^I is described. This family of iron sulfides constitutes a unique redox series comprising three nearly isostructural but electronically distinct Fe₂(μ‐S) species. Combined structural, magnetic, and spectroscopic studies provided strong evidence that the pseudotetrahedral iron centers undergo a transition to low‐spin S=1/2 states upon reduction from Fe^II to Fe^I. The possibility of accessing low‐spin, pseudotetrahedral Fe^I sites compatible with S^2? as a ligand was previously unknown.     

Magnetic Relaxation in Single‐Electron Single‐Ion Cerium(III) Magnets: Insights from Ab Initio Calculations

Detailed ab initio calculations were performed on two structurally different cerium(III) single‐molecule magnets (SMMs) to probe the origin of magnetic anisotropy and to understand the mechanism of magnetic relaxations. The complexes [Ce^III{Zn^II(L)}₂(MeOH)]BPh₄ ( 1 ) and [Li(dme)₃][Ce^III(cot′′)₂] ( 1 ; L=N,N,O,O‐tetradentate Schiff base ligand; 2 ; DME=dimethoxyethane, COT′′=1,4‐bis(trimethylsilyl)cyclooctatetraenyldianion), which are reported to be zero‐field and field‐induced SMMs with effective barrier heights of 21.2 and 30 K respectively, were chosen as examples. CASSCF+RASSI/SINGLE_ANISO calculations unequivocally suggest that m_J|±5/2〉 and |±1/2〉 are the ground states for complexes 1 and 2 , respectively. The origin of these differences is rooted back to the nature of the ligand field and the symmetry around the cerium(III) ions. Ab initio magnetisation blockade barriers constructed for complexes 1 and 2 expose a contrasting energy‐level pattern with significant quantum tunnelling of magnetisation between the ground state Kramers doublet in complex 2 . Calculations performed on several model complexes stress the need for a suitable ligand environment and high symmetry around the cerium(III) ions to obtain a large effective barrier.     

Stabilization of the Low‐Spin State in a Mononuclear Iron(II) Complex and High‐Temperature Cooperative Spin Crossover Mediated by Hydrogen Bonding

The tetrapyridyl ligand bbpya (bbpya=N,N‐bis(2,2′‐bipyrid‐6‐yl)amine) and its mononuclear coordination compound [Fe(bbpya)(NCS)₂] ( 1 ) were prepared. According to magnetic susceptibility, differential scanning calorimetry fitted to Sorai’s domain model, and powder X‐ray diffraction measurements, 1 is low‐spin at room temperature, and it exhibits spin crossover (SCO) at an exceptionally high transition temperature of T_1/2=418 K. Although the SCO of compound 1 spans a temperature range of more than 150 K, it is characterized by a wide (21 K) and dissymmetric hysteresis cycle, which suggests cooperativity. The crystal structure of the LS phase of compound 1 shows strong N?H???S intermolecular H‐bonding interactions that explain, at least in part, the cooperative SCO behavior observed for complex 1 . DFT and CASPT2 calculations under vacuum demonstrate that the bbpya ligand generates a stronger ligand field around the iron(II) core than its analogue bapbpy (N,N′‐di(pyrid‐2‐yl)‐2,2′‐bipyridine‐6,6′‐diamine); this stabilizes the LS state and destabilizes the HS state in 1 compared with [Fe(bapbpy)(NCS)₂] ( 2 ). Periodic DFT calculations suggest that crystal‐packing effects are significant for compound 2 , in which they destabilize the HS state by about 1500 cm^?1. The much lower transition temperature found for the SCO of 2 compared to 1 appears to be due to the combined effects of the different ligand field strengths and crystal packing.