Strong Ligand Field Research Articles

Magneto-structural correlations in self-assembled spin-transition nano-architectures of the [][2×2]-grid-type

Abstract The magnetism of a series of tetranuclear complexes of the [ Fe II 4 L 4 ]( BF 4 ) 8 [2×2] -grid-type was investigated, revealing the occurrence of spin-transition (ST) behavior within this class of compounds. The phenomenon depends directly on the nature of the substituent R1 of the ligand L . All FeII ions in compounds with R1 substituents favoring strong ligand fields ( R 1 = H ; OH ) remain completely in the diamagnetic low-spin state. Only the complex bearing R1=Ph exhibits thermally induced ST behavior.

New Unsymmetrical Oxovanadium(IV) Complexes of Mixed‐Donor Amido and Oxo Ligands

The novel, unsymmetrical, mixed‐donor, tetradentate amido, and oxo complexes of V(IV) of the general formula VOL x were synthesized by a nontemplate method and characterized by physico‐chemical methods. The ligands were prepared by a two‐stage aldol condensation reaction in the molar ratio 1:1 of 2‐hydroxybenzaldehyde, 2′‐hydroxyacetophenone, or 2′,4′‐dihydroxyacetophenone and 1,2‐diaminopropane of the general formula HA x (x = 1–3) as precursors. The tetradentate mixed donor ligands of the general formula H2L x (x = 1–6) were obtained by condensation reaction of precursors with appropriate ketone or aldehyde. The synthesis of the ligand system, described here, is an efficient and easy method with the advantages of high yield, selectivity, and short reaction times. Only a brown polymeric form was obtained for VOL2 and green monomeric forms were obtained for VOL x , x = 1, 3–5, while in the presence of strong field ligands only monomeric forms were obtained. These observations suggest that monomeric forms of oxovanadium(IV) complexes exhibit a coordination number of five with N2O3 donor sets and the polymeric form exhibits a coordination number of six with N2O4 donor sets. Analytical data, 1H NMR, IR, UV–VIS spectra, and elemental analyses confirm the nature of the products.

Growth and properties of strainedVOxthin films with controlled stoichiometry

We have succeeded in growing epitaxial films of rocksalt VOx on MgO(001) substrates. The oxygen content as a function of oxygen flux was determined using 18O2-RBS and the vanadium valence using XAS. The upper and lower stoichiometry limits found are similar to the ones known for bulk material (0.8<x<1.3). From the RHEED oscillation period a large number of vacancies for both vanadium and oxygen were deduced, i.e. ~16% for stoichiometric VO. These numbers are, surprisingly, very similar to those for bulk material and consequently quite strain-insensitive. XAS measurements reveal that the vacancies give rise to strong low symmetry ligand fields to be present. The electrical conductivity of the films is much lower than the conductivity of bulk samples which we attribute to a decrease in the direct overlap between t2g orbitals in the coherently strained layers. The temperature dependence of the conductivity is consistent with a variable range hopping mechanism.

Formation, Absorption and Emission Spectra of Cr4+ Ions in Li2O-SiO2 System Transparent Glass-Ceramics

The formation mechanism, optical absorption and emission spectra of Cr4+ ions-containing Li2O-SiO2 system transparent glass-ceramics were investigated. In this material, the main crystalline phase was Li2O•2SiO2, and the percent crystallinity and crystal size were 67-72 and 20-33 nm, respectively. The remarkable change in color and absorption spectra was observed upon crystallization. The characteristic emission of tetrahedrally coordinated Cr4+ ions was identified in the near infrared region, 1000-1600 nm. It was found that tetrahedrally coordinated Cr4+ ions exist in this transparent glass-ceramics from absorption and emission measurement. The Cr4+ ions exist in residual high SiO2 glassy phase, and their ligand field parameters are estimated to be: 10Dq=10,610 cm-1, B=690 cm-1 and Dq/B=1.54. This Dq/B value is just below the crossing point of 3T2 and 1E levels (Dq/B=1.6). The Cr4+ ions in Li2O-SiO2 system transparent glass-ceramics occupy the little stronger ligand field sites than those in aluminate glass reported previously (Dq/B=1.2). It is considered that the Cr4+ ions can be formed by the reduction of Cr6+ ions (chromate ion [CrO4]2-) associated with decreasing the basicity of residual glassy phase during crystallization. In this process the behavior of Li+ ions plays a significant role. The [CrO4] formed is equivalent to [SiO4] and substitutes [SiO4] sites in the residual high SiO2 glassy phase.

Supramolecular spintronic devices: spin transitions and magnetostructural correlations in [Fe4IIL4]8+ [2x2]-grid-type complexes.

The magnetism of a series of tetranuclear complexes of the [Fe4IIL4]8+ [2x2]-grid-type was investigated, revealing the occurrence of spin transition behavior within this class of compounds. The phenomenon depends directly on the nature of the substituent R(1) in the 2-position on the central pyrimidine group of the ligand L. All Fe(II) ions in compounds with R(1) substituents favoring strong ligand fields (R(1)=H; OH) remain completely in the diamagnetic low-spin state. Only complexes bearing R(1) substituents attenuating the ligand field by steric (and to a lesser extent electronic) effects (R(1)=Me; Ph) exhibit spin transition behavior triggered by temperature. In general, gradual and incomplete transitions without hysteresis were observed for magnetically active complexes. The systems described provide approaches to the development of (supra)molecular spintronics.

Highly phosphorescence iridium complexes and their application in organic light-emitting devices.

A new series of iridium(III) mixed ligand complexes TBA[Ir(ppy)(2)(CN)(2)] (1), TBA[Ir(ppy)(2)(NCS)(2)] (2), TBA[Ir(ppy)(2)(NCO)(2)] (3), and [Ir(ppy)(2)(acac)] (4) (ppy = 2-phenylpyridine; acac = acetoylacetonate, TBA = tetrabutylammonium cation) have been developed and fully characterized by UV-vis, emission, IR, NMR, and cyclic voltammetric studies. The lowest energy MLCT transitions are tuned from 463 to 494 nm by tuning the energy of the HOMO levels. These complexes show emission maxima in the blue, green, and yellow region of the visible spectrum and exhibit unprecedented phosphorescence quantum yields, 97 +/- 3% with an excited-state lifetimes of 1-3 micros in dichloromethane solution at 298 K. The near-unity quantum yields of these complexes are related to an increased energy gap between the triplet emitting state and the deactivating e(g) level that have been achieved by meticulous selection of ligands having strong ligand field strength. Organic light-emitting devices were fabricated using the complex 4 doped into a purified 4,4'-bis(carbazol-9-yl)biphenyl host exhibiting a maximum of the external quantum efficiencies of 13.2% and a power efficiency of 37 lm/W for the 9 mol % doped system.

Spectroscopic comparison of the five-coordinate [Cu(SMeIm)(HB(3,5-iPr2pz)3)] with the four-coordinate [Cu(SCPh3)(HB(3,5-iPr2pz)3)]: effect of coordination number increase on a blue copper type site

A variety of spectroscopic techniques have been applied to characterize and compare two model complexes with relevance to red and blue copper centers in metalloproteins, [Cu(SMeIm)(HB(3,5-iPr2pz)3)] (1) and [Cu(SCPh3)(HB(3,5-iPr2pz)3)] (2), which are five- and four-coordinate, respectively. The key spectral differences in low temperature absorption and magnetic circular dichroism (MCD) include an increase in the Sσ→Cu charge transfer (CT) band intensity at 370 nm and a decrease in the absorption intensity at ∼570 nm for the Sπ→Cu CT. The energies of the d→d transitions in 1 are increased relative to 2 reflecting a more tetragonal geometry and a stronger ligand field. S K-edge X-ray absorption spectroscopy (XAS) measurements demonstrate a less covalent thiolate Cu interaction in the HOMO of 1 (15% Sp) compared to 2 (52% Sp). XAS at the Cu L-edge indicates that the Cu d-character in the HOMO of 1 has increased relative to that of 2. The electronic perturbation resulting from the increased coordination number has been evaluated. The thiolate rotates in the NNS plane resulting in increased σ overlap with the Cu. Additionally, the Cu–S bond length increases. The associated reduced covalency of the thiolate can contribute to the function of perturbed blue copper sites in proteins.

A spectrochemical walk: single-site perturbation within a series of six-coordinate ferrous complexes.

A series of ferrous complexes with the pentadentate ligand 2,6-(bis-(bis-2-pyridyl)methoxymethane)pyridine (PY5) was prepared and examined. PY5 binds ferrous iron in a square-pyramidal geometry, leaving a single coordination site accessible for complexation of a wide range of monodentate exogenous ligands: [Fe(II)(PY5)(X)](n+), X = MeOH, H(2)O, MeCN, pyridine, Cl-, OBz-, N(3)-, MeO-, PhO-, and CN-. The spin-states of these ferrous complexes are extremely sensitive to the nature of the single exogenous ligand; the spectroscopic and structural properties correlate with their high-spin (hs) or low-spin (ls) electronic ground state. Systematic metrical trends within six crystallographic structures clearly indicate a preferred conformational binding mode of the PY5 ligand. The relative binding affinities of the exogenous ligands in MeOH indicate that exogenous ligand charge is the primary determinant of the binding affinity; the [Fe(II)(PY5)](2+) unit preferentially binds anionic ligands over neutral ligands. At parity of charge, strong-field ligands are preferentially bound over weak-field ligands. In MeOH, the pK(a) of the exogenously ligated MeOH in [Fe(PY5)(MeOH)](2+) (9.1) limits the scope of exogenous ligands, as strongly basic ligands preferentially deprotonate [Fe(PY5)(MeOH)](2+) to yield [Fe(PY5)(OMe)](1+) rather than ligate to the ferrous center. Exogenous ligation by a strongly basic ligand, however, can be achieved in polar aprotic solvents.

Stereochemistry of new nitrogen containing heterocyclic aldehyde. VIII. Spectral and coordination modes of mixed-ligand of novel ruthenium(III) complexes

Reaction of 8-hydroxy-7-quinolinecarboxaldehyde (LH) with RuCl 3·XH 2O afforded the compounds [RuL 3] and/or [RuL 2ClOH 2]. Reactions of LH with RuCl 3·XH 2O in the presence of N-heterocyclic bases led to the formation of complexes of type [RuL 2·ClPy] and [RuLCl 2( o-Phen)] (Py=pyridine or o-Phen=1,10-phenanthroline). The amine exchange reactions of coordinated Schiff bases in these complexes have been also carried out which give symmetrical tetradentate Schiff base complexes. These complexes were characterized by a combination of elemental analysis, IR, magnetic susceptibility measurements, 1H, 13C NMR and electronic spectral analysis methods. The spectral data were utilized to compute the important ligand field parameters B, β and Dq. Electronic spectra show all complexes are octahedral. The B-values suggest a strong covalency in the metal–ligand σ-bond and the Dq-values indicate a medium–strong ligand field. The stereochemistry and the nature of the complexes are markedly dependent upon the molar ratios of the reactants, the pH of the system and the nature of the anions involved. The ligands behave as bi-(0,0) and tetradentate (N 2,O 2) donors.

Stereochemistry of new nitrogen containing heterocyclic aldehyde. IX. Spectroscopic studies on novel mixed-ligand complexes of Rh(III)

Mono, bis and tris complexes of rhodium(III) oxine (systematic name 8-hydroxy-7-quinolinecarboxaldehyde) and mixed ligand have been prepared. The amine exchange reaction of coordinated Schiff base in these complexes has also been carried out, which gives symmetrical tetradentate Schiff base complexes. The complexes are characterized by elemental and thermal analysis, IR, magnetic and electronic spectral analysis methods were also employed as well as conductivity measurements. An octahedral structure is proposed for all the new complexes in which chloride is attached to the metal ion in 1:1; 1:2 (metal:ligand) ratio. The spectral data were utilized to compute the important ligand field parameter B, β and Dq. The B-values suggest a strong covalency in the metal-ligand σ-bond and the Dq-values indicate a medium strong ligand field. 1H NMR spectra show that the tris (ligand) complex is cis isomer. IR spectra show that the ligand is mono-basic bidentate.

Transition-metal complexes of a bifunctional tetradentate gallium alkoxide ligand.

The mixed gallium transition-metal complexes [FeCl[Ga(2)((t)Bu)(4)(neol)(2)]] (1) and [M[Ga(2)((t)Bu)(4)(neol)(2)]], M = Co (2), Ni (3), Cu (4), have been prepared by the reaction of [Ga(2)((t)Bu)(4)(neol-H)(2)] (neol-H(2) = 2,2-dimethyl-propane-1,3-diol) with the appropriate metal halide and Proton Sponge. Compounds 1-4 have been characterized by NMR (3), UV/vis, and IR spectroscopy and magnetic susceptibility (solution and solid state), and their molecular structures have been confirmed by X-ray crystallography. The molecular structure of compounds 1-4 consists of a tetracyclic core formed from two four-membered and two six-membered rings. The central metal atom adopts a square pyramidal (1) or square planar (2-4) geometry. The magnetic susceptibilities for 1, 2, and 4 are as expected for strong ligand field environments. On the basis of spectroscopic and structural data, the [Ga(2)((t)Bu)(4)(neol)(2)](2-) ligand appears to be more flexible than other chelating ligands; this is proposed to be due to the flexibility in the O-Ga-O bond angle.

Syntheses, structures, and properties of trinuclear complexes [M(bpca)(2)(M'(hfac)(2))(2)], constructed with the complexed bridging ligand [M(bpca)(2)] [M, M' = Ni(II), Mn(II); Cu(II), Mn(II); Fe(II), Mn(II); Ni(II), Fe(II); and Fe(II), Fe(II); Hbpca = Bis(2-pyridylcarbonyl)amine, Hhfac = Hexafluoroacetylacetone

Five trinuclear complexes [M(bpca)(2)(M'(hfac)(2))(2)] (where MM'(2) = NiMn(2), CuMn(2), FeMn(2), NiFe(2), and FeFe(2); Hbpca = bis(2-pyridylcarbonyl)amine; and Hhfac = hexafluoroacetylacetone) were synthesized almost quantitatively by the reaction of [M(bpca)(2)] and [M'(hfac)(2)] in 1:2 molar ratio, and their structures and magnetic properties were investigated. Three complexes, with M' = Mn, crystallize in the same space group, Pna2(1), whereas two complexes, with M' = Fe, crystallize in P4(1), and complexes within each set are isostructural to one another. In all complexes, [M(bpca)(2)] acts as a bis-bidentate bridging ligand to form a linear trinuclear complex in which three metal ions are arranged in the manner M'-M-M'. The central metal ion is in a strong ligand field created by the N(6) donor set, and hence the Fe(II) in the [Fe(bpca)(2)] moiety is in a low-spin state. The terminal metal ions (M') are surrounded by O(6) donor sets with a moderate ligand field, which leads to the high-spin configuration of Fe(II). Three metal ions in all complexes are almost collinear, and metal-metal distances are ca. 5.5 A. The magnetic behavior of NiMn(2) and NiFe(2) shows a weak ferromagnetic interaction between the central Ni(II) ion and the terminal Mn(II) or Fe(II) ions. In these complexes, sigma-spin orbitals of the central Ni(II) ion and those of terminal metal ions have different symmetry about a 2-fold rotation axis through the Ni-N(amide)-M'(terminal) atoms, and this results in orthogonality between the neighboring sigma-spin orbitals and thus ferromagnetic interactions.

Activation of Small Molecules by Cluster Complexes

The results of theoretical, experimental investigations on activation of small molecules on their coordination to cluster complexes of heavy transition metals with weak- and strong-field ligands are presented. Homogeneous catalytic redox reactions of the CO, N2, H2O molecules and the N3- molecular anion in the presence of cluster complexes of low-valent molybdenum and rhenium are studies. The reaction mechanism is established. Three modifications of the homogeneous cluster catalysis of redox reactions of small molecules are described.

Substituents effect on the spectral studies on ruthenium(III) complexes of 5(-4′-derivatives phenyldiazo)-3-phenyl-2-thioxo-4-thiazolidinone

A novel series of Ru(III) complexes with 5(-4′-derivatives phenyldiazo)-3-phenyl-2-thioxo-4-thiazolidinone (HL n) have been prepared. The ligands and structural composition of complexes were confirmed and characterized by various physico–chemical techniques. The spectral data were utilized to compute the important ligand field parameters B, β and Dq. The effect of Hamett's constant on the ligand field parameters is also discussed. The spectral and magnetic results commensurate an octahedral environment around the Ru(III) ion. The B-values suggest a strong covalency in the metal–ligand σ-bond and the Dq-values indicate a medium–strong ligand-field. IR spectra show that the ligand is neutral bidentate forming thereby a six-membered chelating ring and concomitant formation of an intramolecular hydrogen bond. The ligands are present in associated form in solution via intermolecular hydrogen bond and act as neutral bidentate coordinated to ruthenium(III).

Metal binding studies with macrocyclic hexathioethers: Transition metal complexes of 22S6 and 20S6

AbstractThe two macrocyclic hexathioethers, 1,4,7,12,15,18‐hexathiacyclodocosane (22S6) and 1,4,7,11,14,17‐hexathiacycloeicosane (20S6) have been examined with respect to their complexation behavior towards a variety of transition metals ions. In all of these complexes, the two ligands generate relatively strong ligand field, but their field strengths fall between the smaller ring and stronger field hexathioether ligands (such as 18S6) and larger ring and weaker field ligands, such as 24S6. The stability of the complexes towards hydrolysis parallels their reduced field strengths with first row transition metal complexes of both ligands showing enhanced sensitivity to solvolysis reactions compared to the analogous 18S6 complexes. In general, the electrochemical behavior of the hexathioether complexes shows metal centered oxidations which occur at higher potentials than those complexes involving trithioethers. We suggest that the hexathioether ligand is less able to expand and contract during the reduction and oxidation of the metal center. The complex [Pd(22S6)]2+ does not display the unusual spectroscopic and electrochemical properties observed in the analogous 18S6 complex.

A Very Rigid Bis-bispidine Tetraazamacrocycle and Its Unusual Copper(II) Complex.

The absorption maximum of the orange-colored copper(II) complex of the cyclam derivative L, which has two 3,7-diazabicyclo[3.3.1]nonane units, occurs at 390 nm. This is the lowest value for a copper(II) tetraamine known so far, and indicates an extremely strong ligand field. This maximum is shifted by 110 nm (5740 cm-1 , 68 kJ mol-1 ) to lower wavelengths than that of the parent compound [Cu(cyclam)]2+ ; cyclam=1,4,8,11-tetraazacyclotetradecane.

Complexation and conformational analysis studies of 11-membered ring crown trithioethers

We wish to report the synthesis of a series of transition metal complexes with two isomeric 11-membered ring trithioethers, 1,4,7-trithiacycloundecane (11S3-147) and 1,4,8-trithiacycloundecane (11S3-148). With respect to Ni(II), Fe(II), Co(II), Co(III), and Ru(II), the two function as relatively strong field ligands, but their ligand field strengths fall between the stronger field and smaller ring trithioether ligands such as 1,4,7-trithiacyclononane(9S3) and the larger ring and weaker field ligand, 1,5,9-trithiacyclododecane(12S3). Complexes of both ligands with first row transition metals are much more sensitive to solvolysis reactions than the analogous 9S3 complexes. 13C NMR spectroscopy shows that there is little stereoisomer selectivity between the cis and trans diastereoisomers for the bis complexes of 11S3-147. The complex [Pd(11S3-147) 2] 2+ does not display the unusual spectroscopic and electrochemical properties observed in smaller ring Pd(II) complexes. Its absorption spectrum does not show any d–d transitions and an irreversible ligand-centered oxidation is observed. A conformational analysis study for both macrocycles showed that neither has a lowest energy conformation suitable for tridentate coordination. The effects of the lack of pre-organization dominate the complexation behavior of the two macrocyclic ligands.

Fe3+ Coordination and Redox Properties of a Bacterial Transferrin

The Fe(3+) binding site of recombinant nFbp, a ferric-binding protein found in the periplasmic space of pathogenic Neisseria, has been characterized by physicochemical techniques. An effective Fe(3+) binding constant in the presence of 350 microm phosphate at pH 6.5 and 25 degrees C was determined as 2.4 x 10(18) m(-1). EPR spectra for the recombinant Fe(3+)nFbp gave g' = 4.3 and 9 signals characteristic of high spin Fe(3+) in a strong ligand field of low (orthorhombic) symmetry. (31)P NMR experiments demonstrated the presence of bound phosphate in the holo form of nFbp and showed that phosphate can be dialyzed away in the absence of Fe(3+) in apo-nFbp. Finally, an uncorrected Fe(3+/2+) redox potential for Fe-nFbp was determined to be -290 mV (NHE) at pH 6.5, 20 degrees C. Whereas our findings show that nFbp and mammalian transferrin have similar Fe(3+) binding constants and EPR spectra, they differ greatly in their redox potentials. This has implications for the mechanism of Fe transport across the periplasmic space of Gram-negative bacteria.

The binding ability of iron bonded to porphodimethene: structural, magnetic, and electronic relationship to iron porphyrin complexes.

The availability of the parent compound, meso-hexaethylporphodimetheneiron(II), [(Et6N4)Fe] (2), of this report results from a novel synthetic methodology that makes [Et6N4Li2] (1) easily available. The major focus is on how the axial positions, which are the key reactive sites in metalloporphyrins, and the electronic configuration of the metal can be affected by the breakdown of the aromaticity of the porphyrin skeleton and by the nonplanar conformation of the ligand. DFT calculations indicate a 3B1(dz2)1(dyz)1 ground state for 2 versus the 3A2(dxz)1(dyz)1 ground state in the porphyrin analogue. The intermediate-spin state (S = 1) of 2 changed drastically upon addition of one or two axial ligands, as hexacoordination is preferred by iron(II). The hexacoordinate complexes [(Et6N4)Fe(L)(L')] (L = L' = THF, 3; L = L' = Py, 4; L = PhNO, L' = Py, 14) have been isolated and structurally characterized. Strong-field ligands lead to a low-spin diamagnetic state for iron(II), namely for complexes 4-7, 9, and 14, whereas 3 is a typical d6 high-spin complex, as is the pentacoordinate [(Et6N4)Fe(CN)]Bu4N (8). The structural analysis showed common features for 6, 7, 9, and 14: i) a small displacement of the metal from the N4 plane, and ii) an N4 cavity, larger than that in the corresponding porphyrins, affecting the Fe-N bond lengths. The 1H NMR spectrum is quite diagnostic of the two-fold symmetry in the diamagnetic hexacoordinate complexes, revealing either a D2h or a C2v symmetry. The CO stretching frequency (1951 cm(-1)) in complex 6 probes the good electron density at the metal. The one-electron oxidation of 2 led to pentacoordinate iron(III) derivatives [(Et6N4)Fe(Cl)] (10), [(Et6N4)2Fe2(mu-O)] (11), and [(Et6N4)2Fe2(mu-p-OC6H4-O)] (12). Complex 10 is a typical high-spin iron(III) (5.85muB at 298 K), while 11 and 12 behave as antiferromagnetic coupled iron(III) (J = -9.4cm(-1), 12, and J = -115cm(-1), 11). In complexes 10, 11, and 12 iron is sitting in a quite distorted square pyramidal geometry, in which the ligand displays a very distorted roof conformation with different degrees of ruffling. Distinctive structural and magnetic features have been found for the nitrosyl derivative [(Et6N4)Fe-NO], which has a low-spin state (S = 1/2) and the following structural parameters: Fe-N-O, 147.3(2) degrees; Fe-N, 1.708(2) A; N-O, 1.172(3) A. A comparative structural, magnetic, and theoretical analysis of the compounds listed above has been made with the analogous porphyrin derivatives. The detailed structural investigation has been mapped through the X-ray analysis of 2, 7, 8, 9, 11, 13, and 14.

Spectroscopic and Electronic Structural Studies of Blue Copper Model Complexes. 2. Comparison of Three- and Four-Coordinate Cu(II)−Thiolate Complexes and Fungal Laccase

To evaluate the importance of the axial ligand in blue Cu centers, the electronic structure of a three-coordinate model compound LCuSCPh3 (2, L = β-diketiminate ligand) is defined using low-temperature absorption, magnetic circular dichroism (MCD), X-ray absorption spectroscopy (XAS), and resonance Raman (rR) profiles coupled with density functional calculations. Using these excited-state spectroscopic methods the electronic structure of 2 is compared to that of a four-coordinate blue Cu model compound LCuSCPh3 (1, L = tris(pyrazolyl)hydroborate ligand) and the three-coordinate blue Cu center in fungal laccase. The spectral features of 2 are substantially altered from those of 1 and reflect a trans influence of the β-diketiminate ligand that involves a strong interaction with the Cu dx2-y2 orbital, concomitant with a decreased S pπ interaction with the Cu dx2-y2 orbital. The lack of an axial ligand coupled with the influence of this equatorial donor leads to many of the perturbed spectral features of 2 relative to 1 including: the shift of the S pπ → Cu CT transition to lower energy and its reduced intensity, the stronger ligand field, the larger nitrogen covalency in the HOMO at the expense of thiolate covalency, and the decreased excited-state distortion. From XAS, it also is clear that loss of the axial pyrazole ligand in 2 relative to 1 leads to an increase in the effective nuclear charge of the three-coordinate Cu in 2, which demonstrates that the axial ligand modulates the electronic structure. From a comparison of 2 to fungal laccase it is evident that even in the absence of an axial ligand the electronic structure can be modulated by differential charge donation from the equatorial ligands. These studies show that elimination of the axial ligand of the blue copper site can affect the covalency of the thiolate−Cu bond and potentially modify the electron-transfer properties of the site.