Homoleptic Fe(III) and Fe(IV) Complexes of a Dianionic C3-Symmetric Scorpionate.

High-valent iron species have been implicated as key intermediates in catalytic oxidation reactions, both in biological and synthetic systems. Many heteroleptic Fe(IV) complexes have now been prepared and characterized, especially using strongly π-donating oxo, imido, or nitrido ligands. On the other hand, homoleptic examples are scarce. Herein, we investigate the redox chemistry of iron complexes of the dianonic tris-skatylmethylphosphonium (TSMP2-) scorpionate ligand. One-electron oxidation of the tetrahedral, bis-ligated [(TSMP)2FeII]2- leads to the octahedral [(TSMP)2FeIII]-. The latter undergoes thermal spin-cross-over both in the solid state and solution, which we characterize using superconducting quantum inference device (SQUID), Evans method, and paramagnetic nuclear magnetic resonance spectroscopy. Furthermore, [(TSMP)2FeIII]- can be reversibly oxidized to the stable high-valent [(TSMP)2FeIV]0 complex. We use a variety of electrochemical, spectroscopic, and computational techniques as well as SQUID magnetometry to establish a triplet (S = 1) ground state with a metal-centered oxidation and little spin delocalization on the ligand. The complex also has a fairly isotropic g-tensor (giso = 1.97) combined with a positive zero-field splitting (ZFS) parameter D (+19.1 cm-1) and very low rhombicity, in agreement with quantum chemical calculations. This thorough spectroscopic characterization contributes to a general understanding of octahedral Fe(IV) complexes.

Analysis of vibronic coupling in a 4f molecular magnet with FIRMS.

Vibronic coupling, the interaction between molecular vibrations and electronic states, is a fundamental effect that profoundly affects chemical processes. In the case of molecular magnetic materials, vibronic, or spin-phonon, coupling leads to magnetic relaxation, which equates to loss of magnetic memory and loss of phase coherence in molecular magnets and qubits, respectively. The study of vibronic coupling is challenging, and most experimental evidence is indirect. Here we employ far-infrared magnetospectroscopy to directly probe vibronic transitions in [Yb(trensal)] (where H3trensal = 2,2,2-tris(salicylideneimino)trimethylamine). We find intense signals near electronic states, which we show arise due to an "envelope effect" in the vibronic coupling Hamiltonian, which we calculate fully ab initio to simulate the spectra. We subsequently show that vibronic coupling is strongest for vibrational modes that simultaneously distort the first coordination sphere and break the C3 symmetry of the molecule. With this knowledge, vibrational modes could be identified and engineered to shift their energy towards or away from particular electronic states to alter their impact. Hence, these findings provide new insights towards developing general guidelines for the control of vibronic coupling in molecules.

Observability of Paramagnetic NMR Signals at over 10 000 ppm Chemical Shifts.

We report an experimental observation of 31 P NMR resonances shifted by over 10 000 ppm (meaning percent range, and a new record for solutions), and similar 1 H chemical shifts, in an intermediate-spin square planar ferrous complex [tBu (PNP)Fe-H], where PNP is a carbazole-based pincer ligand. Using a combination of electronic structure theory, nuclear magnetic resonance, magnetometry, and terahertz electron paramagnetic resonance, the influence of magnetic anisotropy and zero-field splitting on the paramagnetic shift and relaxation enhancement is investigated. Detailed spin dynamics simulations indicate that, even with relatively slow electron spin relaxation (T1 ≈10-11  s), it remains possible to observe NMR signals of directly metal-bonded atoms because pronounced rhombicity in the electron zero-field splitting reduces nuclear paramagnetic relaxation enhancement.

Spectroscopic Investigation of a Metal-Metal-Bonded Fe6 Single-Molecule Magnet with an Isolated S = 19/2 Giant-Spin Ground State.

The metal-metal-bonded molecule [Bu4N][(HL)2Fe6(dmf)2] (Fe6) was previously shown to possess a thermally isolated spin S = 19/2 ground state and found to exhibit slow magnetization relaxation below a blocking temperature of ∼5 K [J. Am. Chem. Soc. 2015, 137, 13949-13956]. Here, we present a comprehensive spectroscopic investigation of this unique single-molecule magnet (SMM), combining ultrawideband field-swept high-field electron paramagnetic resonance (EPR) with frequency-domain Fourier-transform terahertz EPR to accurately quantify the spin Hamiltonian parameters of Fe6. Of particular importance is the near absence of a 4th-order axial zero-field splitting term, which is known to arise because of quantum mechanical mixing of spin states on account of the relatively weak spin-spin (superexchange) interactions in traditional polynuclear SMMs such as the celebrated Mn12-acetate. The combined high-resolution measurements on both powder samples and an oriented single crystal provide a quantitative measure of the isolated nature of the spin ground state in the Fe6 molecule, as well as additional microscopic insights into factors that govern the quantum tunneling of its magnetization. This work suggests strategies for improving the performance of polynuclear SMMs featuring direct metal-metal bonds and strong ferromagnetic spin-spin (exchange) interactions.

Experimental and Theoretical Evidence for an Unusual Almost Triply Degenerate Electronic Ground State of Ferrous Tetraphenylporphyrin.

Iron porphyrins exhibit unrivalled catalytic activity for electrochemical CO2-to-CO conversion. Despite intensive experimental and computational studies in the last 4 decades, the exact nature of the prototypical square-planar [FeII(TPP)] complex (1; TPP2- = tetraphenylporphyrinate dianion) remained highly debated. Specifically, its intermediate-spin (S = 1) ground state was contradictorily assigned to either a nondegenerate 3A2g state with a (dxy)2(dz2)2(dxz,yz)2 configuration or a degenerate 3Egθ state with a (dxy)2(dxz,yz)3(dz2)1/(dz2)2(dxy)1(dxz,yz)3 configuration. To address this question, we present herein a comprehensive, spectroscopy-based theoretical and experimental electronic-structure investigation on complex 1. Highly correlated wave-function-based computations predicted that 3A2g and 3Egθ are well-isolated from other triplet states by ca. 4000 cm-1, whereas their splitting ΔA-E is on par with the effective spin-orbit coupling (SOC) constant of iron(II) (≈400 cm-1). Therfore, we invoked an effective Hamiltonian (EH) operating on the nine magnetic sublevels arising from SOC between the 3A2g and 3Egθ states. This approach enabled us to successfully simulate all spectroscopic data of 1 obtained by variable-temperature and variable-field magnetization, applied-field 57Fe Mössbauer, and terahertz electron paramagnetic resonance measurements. Remarkably, the EH contains only three adjustable parameters, namely, the energy gap without SOC, ΔA-E, an angle θ that describes the mixing of (dxy)2(dxz,yz)3(dz2)1 and (dz2)2(dxy)1(dxz,yz)3 configurations, and the ⟨rd-3⟩ expectation value of the iron d orbitals that is necessary to estimate the 57Fe magnetic hyperfine coupling tensor. The EH simulations revealed that the triplet ground state of 1 is genuinely multiconfigurational with substantial parentages of both 3A2g (<88%) and 3Eg (>12%), owing to their accidental near-triple degeneracy with ΔA-E = +950 cm-1. As a consequence of this peculiar electronic structure, 1 exhibits a huge effective magnetic moment (4.2 µB at 300 K), large temperature-independent paramagnetism, a large and positive axial zero-field splitting, strong easy-plane magnetization (g⊥ ≈ 3 and g∥ ≈ 1.7) and a large and positive internal field at the 57Fe nucleus aligned in the xy plane. Further in-depth analyses suggested that g⊥ ≫ g∥ is a general spectroscopic signature of near-triple orbital degeneracy with more than half-filled pseudodegenerate orbital sets. Implications of the unusual electronic structure of 1 for CO2 reduction are discussed.

A Synergy and Struggle of EPR, Magnetometry and NMR: A Case Study of Magnetic Interaction Parameters in a Six-Coordinate Cobalt(II) Complex.

Herein, we combine for the first time SQUID magnetometry, cw-EPR, THz-EPR, and paramagnetic NMR spectroscopies to study the magnetic properties of a high-spin cobalt(II) heteroscorpionate complex. Complementary information provided by these methods allowed precise determination of the magnetic interaction parameters, thereby removing the ambiguity inherit to single-method studies. We systematically investigate the extent to which information about the magnetic interaction parameters can be deduced from reduced data sets. The detailed study revealed significant different magnetic properties in solid state and solution. To further exploit the information content of the solution NMR experimental results, we introduce the new concept of reduced paramagnetic shift. It allows for the determination of the magnetic axes and, subsequently, full NMR signal assignment. It is shown that even in complicated cases, in which common NMR analytics (integral intensities, relaxation factors, etc.) fail, it yields robust results.

Purely Spectroscopic Determination of the Spin Hamiltonian Parameters in High-Spin Six-Coordinated Cobalt(II) Complexes with Large Zero-Field Splitting.

Accurate determination of the spin Hamiltonian parameters in transition-metal complexes with large zero-field splitting (ZFS) is an actual challenge in studying magnetic and spectroscopic properties of high-spin transition metal complexes. Recent critical papers have convincingly shown that previous determinations of these parameters, based only on the magnetic data, have low accuracy and reliability. A combination of X-band electron paramagnetic resonance (EPR) spectroscopy and SQUID magnetometry seems to be a more convincing and accurate approach. However, even in this case, the accuracy of the determination of the spin Hamiltonian parameters is strongly limited. In this work, we propose a purely spectroscopic approach, in which three complementary EPR spectroscopic techniques are used to unambiguously with high accuracy determine the spin Hamiltonian parameters for transition-metal complexes with S = 3/2. The applicability of this approach is demonstrated by analyzing the new quasi-octahedral high-spin Co(II) complex [Co(hfac)2(bpy)] (I). Along with the conventional X-band EPR spectroscopy, we also use such advanced techniques as multi-high-frequency EPR spectroscopy (MHF-EPR) and frequency-domain Fourier-transform THz-EPR (FD-FT THz-EPR). We demonstrate that the experimental data derived from the X-band and MHF-EPR EPR spectra allow determination of the g tensor (gx = 2.388, gy = 2.417, gz = 2.221) and the ZFS rhombicity parameter E/D = 0.158. The axial ZFS parameter D = 37.1 cm-1 is measured for I with the aid of FD-FT THZ-EPR spectroscopy, which is able to detect the high-energy EPR transition between the two Kramers doublets. CASSCF/NEVPT2 quantum-chemical calculations of magnetic parameters and magnetic direct current (dc) measurements are performed as well as testing options, and the results obtained in these ways are in good agreement with those derived using the proposed spectroscopic approach.

Examination of the Magneto-Structural Effects of Hangman Groups on Ferric Porphyrins by EPR.

Ferric hangman porphyrins are bioinspired models for haem hydroperoxidase enzymes featuring an acid/base group in close vicinity to the metal center, which results in improved catalytic activity for reactions requiring O-O bond activation. These functional biomimics are examined herein with a combination of EPR techniques to determine the effects of the hanging group on the electronics of the ferric center. These results are compared to those for ferric octaethylporphyrin chloride [Fe(OEP)Cl], tetramesitylporphyrin chloride [Fe(TMP)Cl], and the pentafluorophenyl derivative [Fe(TPFPP)Cl], which were also examined herein to study the electronic effects of various substituents. Frequency-domain Fourier-transform THz-EPR combined with field domain EPR in a broad frequency range from 9.5 to 629 GHz allowed the determination of zero-field splitting parameters, revealing minor rhombicity E/D and D values in a narrow range of 6.24(8) to 6.85(5) cm-1. Thus, the hangman porphyrins display D values in the expected range for ferric porphyrin chlorides, though D appears to be correlated with the Fe-Cl bond length. Extrapolating this trend to the ferric hangman porphyrin chlorides, for which no crystal structure has been reported, indicates a slightly elongated Fe-Cl bond length compared to the non-hangman equivalent.

Multimodeling Approach to Ferromagnetic Spin-Wave Excitations in the High-Spin Cluster Mn18Sr Observed by Inelastic Neutron Scattering.

The magnetism of the mixed-valence high-spin cluster [Mn18SrO8(N3)7Cl(MedhmpH)12(MeCN)6]Cl2 (1) exhibiting intramolecular ferromagnetic interactions was studied using inelastic neutron scattering (INS), and reliable values for the exchange coupling constants were determined based on the quality of simultaneous fits to the INS and magnetic data. The challenge of the huge size of the Hilbert space (3 375 000) and many exchange coupling constants (7 assuming a C3 symmetry) generally encountered in large spin clusters was resolved as follows: (a) The results of the restricted Hilbert space ferromagnetic cluster spin wave theory were compared to the experimental spectroscopic data. The observed INS transitions were thus assigned to spin wave excitations in a bounded ferromagnetic spin cluster and moreover could be visualized in a straightforward way based on this theory. (b) Simultaneously, Quantum Monte Carlo (QMC) calculations of the temperature-dependent magnetic susceptibility with the same parameter set were compared to the experimental data. Application of state-of-the-art QMC algorithms, as available in the open source ALPS package, in ferromagnetic clusters avoids the full Hamiltonian diagonalization without sacrificing calculation accuracy of the magnetic susceptibility down to the lowest temperatures, which was crucial for the successful analysis. The combined fits revealed two exchange-coupling models with equally good overall agreement to the data. Our preferred model was inspired by magnetostructural correlations and is consistent with them. The model involves three different exchange interactions, one describing the interaction between the core MnIII spins Ja = 14.3(1.0) K and two interactions linking the core and the peripheral MnII spins: Jb = 8.3(4) K and J6 = 3.6(4) K. The use of open-source QMC software and our systematic approach to fitting multiple sets of data obtained by different experimental techniques are described in detail and are generally applicable for understanding large ferromagnetically coupled clusters.

Coordination [CoII2] and [CoIIZnII] Helicates Showing Slow Magnetic Relaxation.

The slow magnetic relaxation of CoII ions in the elusive intermediate geometry between the trigonal prism and antiprism has been studied on the new [Co2L3]4+ and [CoZnL3]4+ coordination helicates [L is a bis(pyrazolylpyridine) ligand]. Solution paramagnetic 1H NMR and solid-state magnetization measurements unveil single-molecule-magnet behavior with small axial anisotropy, as predicted previously.

In-depth investigation of large axial magnetic anisotropy in monometallic 3d complexes using frequency domain magnetic resonance and ab initio methods: a study of trigonal bipyramidal Co(ii).

The magnetic properties of 3d monometallic complexes can be tuned through geometric control, owing to their synthetic accessibility and relative structural simplicity. Monodentate ligands offer great potential for fine-tuning the coordination environment to engineer both the axial and rhombic zero-field splitting (ZFS) parameters. In [CoCl3(DABCO)(HDABCO)] (1), the trigonal bipyramidal Co(ii) centre has two bulky axial ligands and three equatorial chloride ligands. An in-depth experimental and theoretical study of 1 reveals a large easy-plane magnetic anisotropy (+ve D) with a negligible rhombic zero-field splitting (E) due to the strict axial symmetry imposed by the C 3 symmetric ligand and trigonal space group. The large easy-plane magnetic anisotropy (D = +44.5 cm-1) is directly deduced using high-field EPR and frequency-domain magnetic resonance (FDMR) studies. Ab initio calculations reveal a large positive contribution to the D term arising from ground state/excited state mixing of the 4E'' states at ∼4085 cm-1 and a minor contribution from the spin-flip transition as well. The nature of the slow relaxation in 1 is elucidated through analysis of the rates of relaxation of magnetisation, taking into account Raman and direct spin-lattice relaxation processes and Quantum Tunnelling of the Magnetisation (QTM). The terms relating to the direct process and QTM were found based on the fit of the field-dependence of τ at 2 K. Subsequently, these were used as fixed parameters in the fit of the temperature-dependence of τ to obtain the Raman terms. This experimental-theoretical investigation provides further insight into the power of FDMR and ab initio methods for the thorough investigation of magnetic anisotropy. Thus, these results contribute to design criteria for high magnetic anisotropy systems.

Detailed electronic structure of a high-spin cobalt(ii) complex determined from NMR and THz-EPR spectroscopy.

Here we report a combined use of THz-EPR and NMR spectroscopy for obtaining a detailed electronic structure of a long-known high-spin complex, cobalt(ii) bis[tris(pyrazolyl)borate]. The lowest inter-Kramers transition was directly measured by THz-EPR spectroscopy, while the energies of higher Kramers doublets were estimated by a recently proposed NMR-based approach. Together, they produced magnetic parameters for a full model that explicitly includes spin-orbit coupling. This approach is applicable to all transition metal ions for which the spin-orbit coupling cannot be treated perturbatively.

A Trigonal Prismatic Cobalt(II) Complex as a Single Molecule Magnet with a Reduced Contribution from Quantum Tunneling.

Herein, we report a new trigonal prismatic cobalt(II) complex that behaves as a single molecule magnet. The obtained zero-field splitting, which is also directly accessed by THz-EPR spectroscopy (-102.5 cm-1 ), results in a large magnetization reversal barrier U of 205 cm-1 . Its effective value, however, is much lower (101 cm-1 ), even though there is practically no contribution from quantum tunneling to magnetization relaxation.

Determination of Large Zero-Field Splitting in High-Spin Co(I) Clathrochelates.

Zero-field splitting (ZFS) of three high-spin Co(I) ( S = 1) clathrochelate complexes was determined by frequency-domain Fourier-transform THz-EPR (FD-FT THz-EPR). The following axial and rhombic ZFS values ( D and E, respectively) were determined: [N( n-Bu)4]CoI(GmCl2)3(BPh)2 (1, D/ hc = +16.43(1) cm-1, E/ hc = 0.0(1) cm-1), [P(Me2N)4]CoI(GmCl2)3(BPh)2 (2, D/ hc = +16.67(4) cm-1, E/ hc = 0.0(1) cm-1), and [P(C6H5)4]CoI(GmCl2)3(BPh)2 (3, D/ hc = +16.72(2) cm-1, E/ hc = 0.24(3) cm-1). Complementary susceptibility χ T measurements and quantum chemistry calculations on 1 revealed hard-axis-type magnetic anisotropy and allowed for a correlation of ZFS and the electronic structure. Increased rhombicity of 3 as compared to 1 and 2 was assigned to symmetry changes of the ligand structure induced by the change of the counterion. 1 and 3 exhibited temperature-dependent ZFS values. Possible reasons for this phenomenon, such as structural changes and weak chain-like intermolecular antiferromagnetic interactions, are discussed.

Crystallographic Evidence for a Sterically Induced Ferryl Tilt in a Non-Heme Oxoiron(IV) Complex that Makes it a Better Oxidant.

Oxoiron(IV) units are often implicated as intermediates in the catalytic cycles of non-heme iron oxygenases and oxidases. The most reactive synthetic analogues of these intermediates are supported by tetradentate tripodal ligands with N-methylbenzimidazole or quinoline donors, but their instability precludes structural characterization. Herein we report crystal structures of two [FeIV (O)(L)]2+ complexes supported by pentadentate ligands incorporating these heterocycles, which show longer average Fe-N distances than the complex with only pyridine donors. These longer distances correlate linearly with log k2 ' values for O- and H-atom transfer rates, suggesting that weakening the ligand field increases the electrophilicity of the Fe=O center. The sterically bulkier quinoline donors are also found to tilt the Fe=O unit away from a linear N-Fe=O arrangement by 10°.

Recent progress in synchrotron-based frequency-domain Fourier-transform THz-EPR.

We describe frequency-domain Fourier-transform THz-EPR as a method to assign spin-coupling parameters of high-spin (S>1/2) systems with very large zero-field splittings. The instrumental foundations of synchrotron-based FD-FT THz-EPR are presented, alongside with a discussion of frequency-domain EPR simulation routines. The capabilities of this approach is demonstrated for selected mono- and multinuclear HS systems. Finally, we discuss remaining challenges and give an outlook on the future prospects of the technique.

Analysis of Magnetic Anisotropy and the Role of Magnetic Dilution in Triggering Single-Molecule Magnet (SMM) Behavior in a Family of CoII YIII Dinuclear Complexes with Easy-Plane Anisotropy.

Three new closely related CoII YIII complexes of general formula [Co(µ-L)(µ-X)Y(NO3 )2 ] (X- =NO3- 1, benzoate 2, or 9-anthracenecarboxylato 3) have been prepared with the compartmental ligand N,N',N''-trimethyl-N,N''-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine (H2 L). In these complexes, CoII and YIII are triply bridged by two phenoxide groups belonging to the di-deprotonated ligand (L2- ) and one ancillary anion X- . The change of the ancillary bridging group connecting CoII and YIII ions induces small differences in the trigonally distorted CoN3 O3 coordination sphere with a concomitant tuning of the magnetic anisotropy and intermolecular interactions. Direct current magnetic, high-frequency and -field EPR (HFEPR), frequency domain Fourier transform THz electron paramagnetic resonance (FD-FT THz-EPR) measurements, and ab initio theoretical calculations demonstrate that CoII ions in compounds 1-3 have large and positive D values (≈50 cm-1 ), which decrease with increasing the distortion of the pseudo-octahedral CoII coordination sphere. Dynamic ac magnetic susceptibility measurements indicate that compound 1 exhibits field-induced single-molecule magnet (SMM) behavior, whereas compounds 2 and 3 only display this behavior when they are magnetically diluted with diamagnetic ZnII (Zn/Co=10:1). In view of this, it is always advisable to use magnetically diluted complexes, in which intermolecular interactions and quantum tunneling of magnetism (QTM) would be at least partly suppressed, so that "hidden single-ion magnet (SIM)" behavior could emerge. Field- and temperature-dependence of the relaxation times indicate the prevalence of the Raman process in all these complexes above approximately 3 K.

Spectroscopic and Computational Studies of Spin States of Iron(IV) Nitrido and Imido Complexes.

High-oxidation-state metal complexes with multiply bonded ligands are of great interest for both their reactivity as well as their fundamental bonding properties. This paper reports a combined spectroscopic and theoretical investigation into the effect of the apical multiply bonded ligand on the spin-state preferences of threefold symmetric iron(IV) complexes with tris(carbene) donor ligands. Specifically, singlet (S = 0) nitrido [{PhB(ImR)3}FeN], R = tBu (1), Mes (mesityl, 2) and the related triplet (S = 1) imido complexes, [{PhB(ImR)3}Fe(NR')]+, R = Mes, R' = 1-adamantyl (3), tBu (4), were investigated by electronic absorption and Mössbauer effect spectroscopies. For comparison, two other Fe(IV) nitrido complexes, [(TIMENAr)FeN]+ (TIMENAr = tris[2-(3-aryl-imidazol-2-ylidene)ethyl]amine; Ar = Xyl (xylyl), Mes), were investigated by 57Fe Mössbauer spectroscopy, including applied-field measurements. The paramagnetic imido complexes 3 and 4 were also studied by magnetic susceptibility measurements (for 3) and paramagnetic resonance spectroscopy: high-frequency and -field electron paramagnetic resonance (for 3 and 4) and frequency-domain Fourier-transform (FD-FT) terahertz electron paramagnetic resonance (for 3), which reveal their zero-field splitting parameters. Experimentally correlated theoretical studies comprising ligand-field theory and quantum chemical theory, the latter including both density functional theory and ab initio methods, reveal the key role played by the Fe 3dz2 (a1) orbital in these systems: the nature of its interaction with the nitrido or imido ligand dictates the spin-state preference of the complex. The ability to tune the spin state through the energy and nature of a single orbital has general relevance to the factors controlling spin states in complexes with applicability as single molecule devices.

Magneto-Structural Correlations in Pseudotetrahedral Forms of the [Co(SPh)4]2- Complex Probed by Magnetometry, MCD Spectroscopy, Advanced EPR Techniques, and ab Initio Electronic Structure Calculations.

The magnetic properties of pseudotetrahedral Co(II) complexes spawned intense interest after (PPh4)2[Co(SPh)4] was shown to be the first mononuclear transition-metal complex displaying slow relaxation of the magnetization in the absence of a direct current magnetic field. However, there are differing reports on its fundamental magnetic spin Hamiltonian (SH) parameters, which arise from inherent experimental challenges in detecting large zero-field splittings. There are also remarkable changes in the SH parameters of [Co(SPh)4]2- upon structural variations, depending on the counterion and crystallization conditions. In this work, four complementary experimental techniques are utilized to unambiguously determine the SH parameters for two different salts of [Co(SPh)4]2-: (PPh4)2[Co(SPh)4] (1) and (NEt4)2[Co(SPh)4] (2). The characterization methods employed include multifield SQUID magnetometry, high-field/high-frequency electron paramagnetic resonance (HF-EPR), variable-field variable-temperature magnetic circular dichroism (VTVH-MCD), and frequency domain Fourier transform THz-EPR (FD-FT THz-EPR). Notably, the paramagnetic Co(II) complex [Co(SPh)4]2- shows strong axial magnetic anisotropy in 1, with D = -55(1) cm-1 and E/D = 0.00(3), but rhombic anisotropy is seen for 2, with D = +11(1) cm-1 and E/D = 0.18(3). Multireference ab initio CASSCF/NEVPT2 calculations enable interpretation of the remarkable variation of D and its dependence on the electronic structure and geometry.