Pyrazole Ligands Research Articles

Substituents' Effect on the Photophysics of Trinuclear Copper(I) and Silver(I) Pyrazolate-Phosphine Cages.

A series of structurally similar trinuclear macrocyclic copper(I) and silver(I) pyrazolate complexes bearing various short-bite diphosphine R2PCH(R')PR2 ligands are reported. Upon diphosphine coordination, the planar geometry of the initial complexes undergoes bending along the line between two metal atoms coordinated to the phosphorus moieties. The complexes based on dcpm ligands (R = cyclohexyl, R' = H, Ph) do not exhibit dynamic behavior in solution at room temperature on the 31P NMR time scale as it was previously observed for similar trinuclear copper complexes bearing the dppm (R = Ph, R' = H) scaffold. All copper(I) complexes exhibit thermally activated delayed fluorescence (TADF) behavior in the solid state. Importantly, the use of aliphatic substituents on the phosphorus atoms instead of aromatic ones leads to an almost double increase in the quantum efficiency (ΦPL) of photoluminescence by eliminating nonradiative decay from the 3LCPh states of the dppm aromatic rings. The higher donating ability of the substituents in the pyrazolate ligand (CF3 vs CH3) lowers the energy of the metal-centered excited state, allowing for a significant metal impact on the T1 state. Finally, the Ag(I) complex displays an emission efficiency of approximately 14%, being the highest among known trinuclear silver(I) pyrazolate homometallic derivatives.

Regioselective ligand substitution in square-planar Pt(II) complexes bearing N^C and C^C chelating ligands with pyrazole derivatives

The regioselectivity of the ligand substitution reaction of square-planar Pt(II) complexes with chelating ligands pyridyl-NHC (Py-NHC), phenyl-NHC (Ph-NHC), and phenyl-pyridine (ppy, Ph-Py) was studied. The reaction of the dichlorido Pt(II) complex bearing these chelating ligands in the presence of pyrazole (pz) ligands afforded mono-pyrazole adducts [PtCl(ChL)(pz)]n+ (n = 0 and 1; ChL=chelating ligand (Py-NHC, Ph-NHC, Ph-py)). The structures of the PtCl(pz) complexes with the Py-NHC, Ph-NHC, and Ph-Py chelating ligands were confirmed by X-ray crystallographic analysis. The regioselectivity of the ligand substitution reaction between the pyrazole moieties and chloride ions can be controlled by the strength of the trans effect of the chelating ligands. Based on the structures of the products, a clear trend of Py < NHC < Ph was observed in the trans effect of the chelating ligands. This finding provides a rational synthetic procedure for asymmetric Pt(II) complexes with chelating ligands in a stepwise manner.

Nitrosyl and Thionitrosyl Complexes of Technetium and Rhenium and Their Reactions with Hydrotris(pyrazolyl)borates.

The very limited number of structurally known thionitrosyl complexes of technetium was increased by the synthesis of [TcII(NS)Cl3(PPh3)2] (3) and [TcII(NS)Cl3(PPh3)(OPPh3)] (4) and their reaction products with hydrotris(pyrazolyl)borates, {HB(pzR)3}-. Similar reactions were conducted with [TcI(NO)Cl2(PPh3)2(CH3CN)] and related rhenium thionitrosyls. Remarkably, most such reactions result in a rapid cleavage of the boron-nitrogen bonds of the ligands and the formation of pyrazole complexes of the two group 7 metals. Only one compound with an intact {HB(pzR)3}- ligand could be isolated: the technetium(I) complex [TcI(NO)Cl(PPh3){HB(pz)3}] (2). Other products show the coordination of one or four neutral pyrazole ligand(s) in the coordination spheres of technetium generated by thermal decomposition of the pyrazolylborates [TcI(NO)Cl2(PPh3)2(pzH)] (1) and [TcI(NS)Cl(pzHMe2)4]+ (5). Reactions with the corresponding thionitrosylrhenium complex [ReII(NS)Cl3(PPh3)2] require higher temperatures and only compounds with one pyrazole ligand, [ReI(NS)Cl2(PPh3)(pzHR)] (6a-6c), were isolated. The products were studied spectroscopically and by X-ray diffraction.

Novel binuclear copper(II) complexes with sulfanylpyrazole ligands: synthesis, crystal structure, fungicidal, cytostatic, and cytotoxic activity.

New binuclear copper(II) [Cu(II)] tetraligand complexes (six examples) with sulfanylpyrazole ligands were synthesized. Electron spin resonance (ESR) studies have shown that in solution the complexes are transformed to the mononuclear one. Fungicidal properties against Candida albicans were found for the Cu complexes with benzyl and phenyl substituents. An in vitro evaluation of the cytotoxic properties of Cu chelates against HEK293, Jurkat, MCF-7, and THP-1 cells identified the Cu complex with the cyclohexylsulfanyl substituent in the pyrazole core as the lead compound, whereas the Cu complex without a sulfur atom in the pyrazole ligand had virtually no cytotoxic or fungicidal activity. The lead Cu(II) complex was more active than cisplatin. Effect of the S-containing Cu complex on apoptosis and cell cycle distribution has been investigated as well.

Trans-Di-bromido-tetra-kis-(5-methyl-1H-pyrazole-κN2)manganese(II).

The title compound, trans-di-bromido-tetra-kis-(5-methyl-1H-pyrazole-κN 2)manganese(II), [MnBr2(C4H6N2)4] or [Mn(3-MePzH)4Br2] (1) crystallizes in the triclinic P space group with the cell parameters a = 7.6288 (3), b = 8.7530 (4), c = 9.3794 (4) Å and α = 90.707 (4), β = 106.138 (4), γ = 114.285 (5)°, V = 542.62 (5) Å3, T = 120 K. The asymmetric unit contains only half the mol-ecule with the manganese atom is situated on a crystallographic inversion center. The 3-MePzH ligands are present in an AABB type manner with two methyl groups pointing up and the other two down. The supra-molecular architecture is characterized by several inter-molecular C-H⋯N, N-H⋯Br, and C-H⋯π inter-actions. Earlier, a polymorphic structure of [Mn(3-MePzH)4Br2] (2) with a similar geometry and also an AABB arrangement for the pyrazole ligands was described [Reedijk et al. (1971 ▸). Inorg. Chem. 10, 2594-2599; a = 8.802 (6), b = 9.695 (5), c = 7.613 (8) Å and α = 105.12 (4), β = 114.98 (4), γ = 92.90 (3)°, V = 558.826 (5) Å3, T = 295 K]. A varying supra-molecular pattern was reported, with the structure of 1 featuring a herringbone type pattern while that of structure 2 shows a pillared network type of arrangement along the a axis. A nickel complex [Ni(3-MePzH)4Br2] isomorphic to 1 and the analogous chloro derivatives of FeII, CoII and CuII are also known.

Palladium‐Catalyzed Chemoselective Amination of Chloro(hetero)aryl Triflates Enabled by Alkyl‐Pyrazole‐Based Phosphine Ligands

AbstractThis study describes the palladium‐catalyzed chemoselective amination of chloro(hetero)aryl triflates in the C−Cl bond for the first time. A newly designed and synthesized alkyl‐pyrazole‐based phosphine ligand, L26 (BirdPhos), which is featured with the pyrazole ligand core and the cyclohexyl group at the C3 and C5 positions, was key to the success of this reaction. A variety of chloro(hetero)aryl triflates coupled with aromatic, aliphatic, and heterocyclic amines smoothly afforded the corresponding products up to 97% yields with general chemoselectivity (&gt;99%) in the C−Cl bond. Drug analogs were also synthesized via the chemoselective intermolecular amination. This study attempts to investigate the structure‐activity relationship of this new class of ligand through experimental and density functional theory (DFT) calculations.

Ruthenium Pyrazole Complexes: A Family of Highly Active Metallodrugs for Photoactivated Chemotherapy.

Ruthenium complexes bearing bis pyrazole (pzH) ligands, cis-[Ru(bpy)2(R-pzH)2]2+ (bpy = 2,2'-bipyridine, R = -H, -Cl), were examined as photoactivated anticancer prodrugs. A dicationic pyrazole complex deprotonated to give monocationic pyrazole-pyrazolate complexes, cis-[Ru(bpy)2(R-pz-)(R-pzH)]+, in an aqueous solution with pKa values of 9.5 and 7.2 for R = H and R = Cl, respectively. Upon deprotonation, relative quantum yields of photosubstitution decreased while lipophilicity of the complexes increased according to the measurements of water-octanol coefficients. The ruthenium complex with 4-chloropyrazole ligands displayed high cytotoxicity upon light irradiation (IC50 = 0.060 ± 0.016 μM) toward lung cancer cells, which was 7 times higher than that in the dark (IC50 = 0.44 ± 0.07 μM). Additional experiments for the ruthenium R-pyrazole complexes indicated that (1) selective photodissociation of the 4-chloropyrazole ligand occurs from cis-[Ru(bpy)2(4-Clpz-)(4-ClpzH)]+, (2) photoinduced ligand dissociation is dominant rather than photoinduced generation of singlet oxygen (1O2), and (3) induction of cell death occurs via the intrinsic pathway of apoptosis.

Phenylbenzothiazole-Based Platinum(II) and Diplatinum(II) and (III) Complexes with Pyrazolate Groups: Optical Properties and Photocatalysis.

Based on 2-phenylbenzothiazole (pbt) and 2-(4-dimethylaminophenyl)benzothiazole (Me2N-pbt), mononuclear [Pt(pbt)(R'2-pzH)2]PF6 (R'2-pzH = pzH 1a, 3,5-Me2pzH 1b, 3,5-iPr2pzH 1c) and diplatinum (PtII-PtII) [Pt(pbt)(μ-R'2pz)]2 (R'2-pz = pz 2a, 3,5-Me2pz 2b, 3,5-iPr2pz 2c) and [Pt(Me2N-pbt)(μ-pz)]2 (3a) complexes have been prepared. In the presence of sunlight, 2a and 3a evolve, in CHCl3 solution, to form the PtIII-PtIII complexes [Pt(R-pbt)(μ-pz)Cl]2 (R = H 4a, NMe2 5a). Experimental and computational studies reveal the negligible influence of the pyrazole or pyrazolate ligands on the optical properties of 1a-c and 2a,b, which exhibit a typical 3IL/3MLCT emission, whereas in 2c the emission has some 3MMLCT contribution. 3a displays unusual dual, fluorescence (1ILCT or 1MLCT/1LC), and phosphorescence (3ILCT) emissions depending on the excitation wavelength. The phosphorescence is lost in aerated solutions due to sensitization of 3O2 and formation of 1O2, whose determined quantum yield is also wavelength dependent. The phosphorescence can be reversibly photoinduced (365 nm, ∼ 15 min) in oxygenated THF and DMSO solutions. In 4a and 5a, the lowest electronic transitions (S1-S3) have mixed characters (LMMCT/LXCT/L'XCT 4a and LMMCT/LXCT/ILCT 5a) and they are weakly emissive in rigid media. The 1O2 generation property of complex 3a is successfully used for the photooxidation of p-bromothioanisol showing its potential application toward photocatalysis.

Octahedral molybdenum iodide clusters with pyrazole or pyrazolate ligands.

Due to the combination of useful physicochemical properties (luminescence, X-ray contrast, etc.), octahedral molybdenum halide cluster complexes [Mo6X8L6]n have been the subject of active investigation during the last decades. The most common methods for synthesizing new compounds with organic ligands involve the use of silver salts of organic acids or the substitution of terminal methylate ligands. However, these methods often necessitate the use of special conditions, such as an inert atmosphere, absolute solvents, and silver salts, which can be costly. In contrast, aqua-hydroxo complexes formed by hydrolysis of many complexes are considered final unreactive products, despite the tendency for them to form. This work proposes a simple and affordable method for the preparation of hexaaqua and hexahydroxo iodide clusters of molybdenum from [Mo6I14]2- in a single step. Furthermore, the possibility of using such compounds as starting complexes for the synthesis of clusters with organic ligands such as pyrazole is discussed. The paper presents synthetic approaches, detailed characterization both in solid and in solution, and a study of the reactivity and luminescence properties of the obtained compounds.

Pyrazolate vs. phenylethynide: direct exchange of the anionic bridging ligand in a cyclic trinuclear silver complex.

Cyclic trinuclear Ag(I) pyrazolate interacts with phenylacetylene forming a mix-ligand complex in which one pyrazolate ligand is changed to phenylethynide. The CC- fragment coordinates only to two silver(I) atoms via one carbon atom demonstrating unique μ2-η1 σ-coordination with close Ag-C bond lengths and Ag-C-C angles. The complex exhibits blue emission under UV irradiation.

57Fe nuclear resonance vibrational spectroscopic studies of tetranuclear iron clusters bearing terminal iron(iii)-oxido/hydroxido moieties.

57Fe nuclear resonance vibrational spectroscopy (NRVS) has been applied to study a series of tetranuclear iron ([Fe4]) clusters based on a multidentate ligand platform (L3-) anchored by a 1,3,5-triarylbenzene linker and pyrazolate or (tertbutylamino)pyrazolate ligand (PzNH t Bu-). These clusters bear a terminal Fe(iii)-O/OH moiety at the apical position and three additional iron centers forming the basal positions. The three basal irons are connected with the apical iron center via a μ4-oxido ligand. Detailed vibrational analysis via density functional theory calculations revealed that strong NRVS spectral features below 400 cm-1 can be used as an oxidation state marker for the overall [Fe4] cluster core. The terminal Fe(iii)-O/OH stretching frequencies, which were observed in the range of 500-700 cm-1, can be strongly modulated (energy shifts of 20-40 cm-1 were observed) upon redox events at the three remote basal iron centers of the [Fe4] cluster without the change of the terminal Fe(iii) oxidation state and its coordination environment. Therefore, the current study provides a quantitative vibrational analysis of how the remote iron centers within the same iron cluster exert exquisite control of the chemical reactivities and thermodynamic properties of the specific iron site that is responsible for small molecule activation.

An octa-nuclear nickel(II) pyrazolate cluster with a cubic Ni8 core and its methyl- and n-octyl-functionalized derivatives.

The mol-ecular and crystal structure of a discrete [Ni8(μ4-OH)6(μ-4-Rpz)12]2- (R = H; pz = pyrazolate anion, C3H3N2 -) cluster with an unprecedented, perfectly cubic arrangement of its eight Ni centers is reported, along with its lower-symmetry alkyl-functionalized (R = methyl and n-oct-yl) derivatives. Crystals of the latter two were obtained with two identical counter-ions (Bu4N+), whereas the crystal of the complex with the parent pyrazole ligand has one Me4N+ and one Bu4N+ counter-ion. The methyl derivative incorporates 1,2-di-chloro-ethane solvent mol-ecules in its crystal structure, whereas the other two are solvent-free. The compounds are tetra-butyl-aza-nium tetra-methyl-aza-nium hexa-μ4-hydroxido-dodeca-μ2-pyrazolato-hexa-hedro-octa-nickel, (C16H36N)(C4H12N)[Ni8(C3H3N2)12(OH)6] or (Bu4N)(Me4N)[Ni8(μ4-OH)6(μ-pz)12] (1), bis-(tetra-butyl-aza-nium) hexa-μ4-hydroxido-dodeca-μ2-(4-methyl-pyrazolato)-hexa-hedro-octa-nickel 1,2-di-chloro-ethane 7.196-solvate, (C16H36N)2[Ni8(C4H5N2)12(OH)6]·7.196C2H4Cl2 or (Bu4N)2[Ni8(μ4-OH)6(μ-4-Mepz)12]·7.196(ClCH2CH2Cl) (2), and bis-(tetra-butyl-aza-nium) hexa-μ4-hydroxido-dodeca-μ2-(4-octylpyrazolato)-hexa-hedro-octa-nickel, (C16H36N)2[Ni8(C11H19N2)12(OH)6] or (Bu4N)2[Ni8(μ4-OH)6(μ-4-nOctpz)12] (3). All counter-ions are disordered (with the exception of one Bu4N+ in 3). Some of the octyl chains of 3 (the crystal is twinned by non-merohedry) are also disordered. Various structural features are discussed and contrasted with those of other known [Ni8(μ4-OH)6(μ-4-Rpz)12]2- complexes, including extended three-dimensional metal-organic frameworks. In all three structures, the Ni8 units are lined up in columns.

Crystal Structure, Anticancer, and Antimicrobial Evaluation of a Novel Substituted Pyrazole Ligand and Its Co(II), Ni(II), and Zn(II) Complexes

Crystal Structure, Anticancer, and Antimicrobial Evaluation of a Novel Substituted Pyrazole Ligand and Its Co(II), Ni(II), and Zn(II) Complexes

A mini review on applications of pyrazole ligands in coordination compounds and metal organic frameworks

The fastest growing class of materials in chemistry is that of the metal organic frameworks (MOF’s), consisting of metal nodes and organic linkers as building units. There exist numerous inorganic ligands to coordinate with metal ions but certainly various organic linkers are also available. Amongst the broadly available heterocyclic compounds, pyrazole functionalities have achieved greater attention owing to their wide-ranging applications. Recently, applications of pyrazole ligands in coordination compounds and structured porous materials such as metal organic frameworks have undoubtedly created considerable interest in developing newer frameworks. Utility of these hybrid compounds in adsorption, fluorescent and colorimetric analysis, drug delivery, catalysis and sensing/detection of analyte are receiving significant interest. Reference to the enormous background on the synthesis and biological resourcefulness of pyrazoles, this review proposes a brief survey on applications of pyrazole ligands in coordination compounds and metal organic frameworks.

Chemical Diversity of Mo5S5 Clusters with Pyrazole: Synthesis, Redox and UV-vis-NIR Absorption Properties.

The chemistry of transition metal clusters has been intensively developed in the last decades, leading to the preparation of a number of compounds with promising and practically useful properties. In this context, the present work demonstrates the preparation and study of the reactivity, i.e., the possibility of varying the ligand environment, of new square pyramidal molybdenum chalcogenide clusters [{Mo5(μ3-S)i4(μ4-S)i(μ-pz)i4}(pzH)t5]1+/2+ (pzH = pyrazole, i = inner, t = terminal). The one-step synthesis starting from the octahedral Mo6Br12 cluster as well as the substitution of the apical pyrazole ligand or the selective bromination of the inner pyrazolate ligands were demonstrated. All the obtained compounds were characterized in detail using a series of physicochemical methods both in solid state (X-ray diffraction analysis, etc.) and in solution (nuclear magnetic resonance spectroscopy, mass spectrometry, etc.). In this work, redox properties and absorption in the ultraviolet-visible and near-infrared region of the obtained compounds were studied.

Halo-bridged Cu(II) complexes with pyridine–pyrazole ligands: Influence of halogen and ligand lipophilicity on antimicrobial activity

A series of five coordinated, doubly halo bridged Cu(II) complexes [Cu2(μ-X)2(Ln)2×2] (X=Cl and Br; n = 1, 2 and 3), CuL1Cl, CuL1Br, CuL2Cl, CuL2Br, CuL3Cl and CuL3Br (where L1 = 2-((1H-pyrazol-1-yl)methyl)pyridine, L2 = 2-((3-methyl-1H-pyrazol-1-yl)methyl)pyridine, L3 = 2-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)pyridine) have been synthesized and structurally characterized using FT-IR and UV–Vis spectroscopy analysis. Moreover, CuL1Cl, CuL1Br and CuL3Cl structures have also been confirmed by single crystal X-ray structural analysis. Copper ion in CuL1Cl, CuL1Br and CuL3Cl displayed distorted squared pyramidal geometry and formed dimers via bridged μ‑chloro/μ‑bromo coordination in the crystal lattice. The intermolecular C–H···Cl hydrogen bonding interaction further connected the dimers of CuL1Cl and CuL3Cl with four neighboring dinuclear complex units. Copper complexes were evaluated for their antimicrobial activity against Gram-negative and Gram-positive bacteria. Interestingly, copper complexes exhibited selective antimicrobial activity against Gram-positive bacteria. Further the increasing ligand lipophilicity showed the increase of antibacterial activity of copper complexes. The lipophilicity of ligands was regulated by number of methyl substituents on the ligands. The chloro bridged copper complexes exhibited relatively higher antibacterial activity compared to the bromo bridged copper complexes. Thus, the present structure-activity of simple copper complexes might provide insight on the structural design of new antimicrobial agents based on coordination complexes.

Rapid microwave synthesis of tetrahedral pyrazole/Co(II) complex: [N[sbnd]H···Cl] synthon, XRD/HSA-interactions, DFT/TD-DFT, physiochemical, antifungal, antibacterial, and POM bio-calculations

A new coordination complex, CoCl2(NNH)2, was synthesized by complexation of the pyrazole (NNH) ligand with CoCl2 in less than 5 min with a high yield (>85%). The synthesis was done using microwave radiation in water solvent where no side products were detected. The complexation was confirmed using UV–visible, CHN-analysis, and FT-IR analysis and compared to theoretical IR, DFT and TD-DFT calculations. CoCl2(NNH)2 crystallizes in a C2/c space group with distorted tetrahedral center as shown by single-crystal X-ray diffraction, two types of heteromeric [NH···Cl] synthons have been recorded, such interactions type and ratios were confirmed also via Hirsfield surface analysis (HSA). The desired complex was evaluated for its antibacterial activity against four classes of gram-positive and gram-negative bacteria, and its antifungal activity against four types of fungi. The complex's physio-chemical properties, toxicity risk, lipophilicity, and pharmacophore sites were studied using the POM (Petra-Osiris-Molinspiration) bio-computational theory.

Enzyme-indole Pyrazole-capped SeNPs based Electrochemical Biosensor for Sensitive Detection of Adenosine Triphosphate

Abstract: In this study, an electrochemical biosensor for the indirect detection of Adenosine triphosphate (ATP) was developed, which was based on the immobilization of the multiwalled carbon nanotubes (MWCNTs) decorated with pyrazole-capped selenium nanoparticles (TRPIDC-CH3 SeNPs) and dual enzyme reaction (hexokinase and glucose oxidase) onto the surface of a bare glassy carbon electrode (GCE) as a working electrode. As confirmed byUltraviolet–visible spectroscopy (UV-Vis), Fourier transform infrared (FTIR) and High-resolution electron microscope (HRTEM), the TRPIDC-CH3 SeNPs successfully green synthesised using Allium sativum cloves and indole pyrazole ligand. The electrochemical study of ATP was performed using cyclic voltammetry (CV) and square wave voltammetry (SWV) techniques on a modified electrode for indirect detection of ATP where the required strong electroactive was [Fe(CN)6]3-/4-. The phosphate buffer solution (PBS; 0.1 M) was used as a supporting electrolyte at pH 7 containing 1 mM K4[Fe(CN)6]/K3[Fe(CN)6] as the redox probe operated at an average potential of 0.23 V. The electrochemical enzymic biosensor showed outstanding sensitivity, good stability, and satisfactory reproducibility with an average RSD of 2.30%. The ATP was quantifiable in spiked tablets with a limit of detection (LOD) of 0.015 mM and a limit of quantification (LOQ) of 0,050 mM.

Recent Developments in Reactions and Catalysis of Protic Pyrazole Complexes.

Protic pyrazoles (N-unsubstituted pyrazoles) have been versatile ligands in various fields, such as materials chemistry and homogeneous catalysis, owing to their proton-responsive nature. This review provides an overview of the reactivities of protic pyrazole complexes. The coordination chemistry of pincer-type 2,6-bis(1H-pyrazol-3-yl)pyridines is first surveyed as a class of compounds for which significant advances have made in the last decade. The stoichiometric reactivities of protic pyrazole complexes with inorganic nitrogenous compounds are then described, which possibly relates to the inorganic nitrogen cycle in nature. The last part of this article is devoted to outlining the catalytic application of protic pyrazole complexes, emphasizing the mechanistic aspect. The role of the NH group in the protic pyrazole ligand and resulting metal-ligand cooperation in these transformations are discussed.

Spectroscopic and Magnetic Studies of Co(II) Scorpionate Complexes: Is There a Halide Effect on Magnetic Anisotropy?

The observation of single-molecule magnetism in transition-metal complexes relies on the phenomenon of zero-field splitting (ZFS), which arises from the interplay of spin-orbit coupling (SOC) with ligand-field-induced symmetry lowering. Previous studies have demonstrated that the magnitude of ZFS in complexes with 3d metal ions is sometimes enhanced through coordination with heavy halide ligands (Br and I) that possess large free-atom SOC constants. In this study, we systematically probe this "heavy-atom effect" in high-spin cobalt(II)-halide complexes supported by substituted hydrotris(pyrazol-1-yl)borate ligands (TptBu,Me and TpPh,Me). Two series of complexes were prepared: [CoIIX(TptBu,Me)] (1-X; X = F, Cl, Br, and I) and [CoIIX(TpPh,Me)(HpzPh,Me)] (2-X; X = Cl, Br, and I), where HpzPh,Me is a monodentate pyrazole ligand. Examination with dc magnetometry, high-frequency and -field electron paramagnetic resonance, and far-infrared magnetic spectroscopy yielded axial (D) and rhombic (E) ZFS parameters for each complex. With the exception of 1-F, complexes in the four-coordinate 1-X series exhibit positive D-values between 10 and 13 cm-1, with no dependence on halide size. The five-coordinate 2-X series exhibit large and negative D-values between -60 and -90 cm-1. Interpretation of the magnetic parameters with the aid of ligand-field theory and ab initio calculations elucidated the roles of molecular geometry, ligand-field effects, and metal-ligand covalency in controlling the magnitude of ZFS in cobalt-halide complexes.