Iron(II)-alkoxide and -aryloxide complexes of a tris(thioether)borate ligand: synthesis, molecular structures, and implications on the origin of instability of their iron(II)-catecholate counterpart.

The phenyltris[(tert-butylthio)methyl]borate ligand, [PhTttBu], has been studied extensively as a platform for coordination, organometallic, and bioinorganic chemistry, especially with 3d metals. While [PhTttBu]Co(3,5-DBCatH) (3,5-DBCatH is 3,5-di-tert-butylcatecholate), a CoII-monoanionic catecholate complex, was successfully isolated to model the active site of cobalt(II)-substituted homoprotocatechuate 2,3-dioxygenase (Co-HPCD) [Wang et al. (2019). Inorg. Chim. Acta, 488, 49-55], its iron(II) counterpart, [PhTttBu]Fe(3,5-DBCatH), was not accessible via similar synthetic routes. Switching the nucleophile from catecholate to alkoxide or aryloxide, however, led to the successful isolation of three highly air-sensitive FeII-alkoxide and -aryloxide complexes, namely, (triphenylmethoxo){tris[(tert-butylsulfanyl)methyl]phenylborato-κ3S,S',S''}iron(II), [Fe(C21H38BS3)(C19H15O)], (2), (2,6-dimethylphenolato){tris[(tert-butylsulfanyl)methyl]phenylborato-κ3S,S',S''}iron(II), [Fe(C21H38BS3)(C8H9O)], (3), and bis{µ-tris[(tert-butylsulfanyl)methyl]phenylborato-κ3S,S':S''}bis[(phenolato-κO)iron(II)] toluene disolvate, [Fe2(C21H38BS3)2(C6H5O)2]·2C7H8, (4). In the solid state, compounds (2) and (3) are monomeric, with [PhTttBu] acting as a tridentate ligand. In contrast, compound (4) crystallizes as a dimeric complex, wherein each [PhTttBu] ligand binds to an iron centre with two thioethers and binds to the other iron centre with the third thioether. The molecular structures of (2)-(4) demonstrate a diversity in the binding modes of [PhTttBu] and highlight its potential use for assembling multinuclear complexes. In addition, the successful isolation of (2)-(4), as well as the structural information of a [PhTttBu] modification product, namely, bis{µ-tris[(tert-butylsulfanyl)methyl](2-oxidophenolato)borato-κO,O',S,S':O'}dicobalt(II), [Co2(C21H37BO2S3)2], (5), obtained from the reaction of [PhTttBu]CoCl with potassium monoanionic catecholate, shed light on the origin of the instability of [PhTttBu]Fe(3,5-DBCatH).

Molecular and Electronic Structures and Single-Molecule Magnet Behavior of Tris(thioether)-Iron Complexes Containing Redox-Active α-Diimine Ligands.

Incorporating radical ligands into metal complexes is one of the emerging trends in the design of single-molecule magnets (SMMs). While significant effort has been expended to generate multinuclear transition metal-based SMMs with bridging radical ligands, less attention has been paid to mononuclear transition metal-radical SMMs. Herein, we describe the first α-diiminato radical-containing mononuclear transition metal SMM, namely, [κ2-PhTttBu]Fe(AdNCHCHNAd) (1), and its analogue [κ2-PhTttBu]Fe(CyNCHCHNCy) (2) (PhTttBu = phenyltris(tert-butylthiomethyl)borate, Ad = adamantyl, and Cy = cyclohexyl). 1 and 2 feature nearly identical geometric and electronic structures, as shown by X-ray crystallography and electronic absorption spectroscopy. A more detailed description of the electronic structure of 1 was obtained through EPR and Mössbauer spectroscopies, SQUID magnetometry, and DFT, TD-DFT, and CAS calculations. 1 and 2 are best described as high-spin iron(II) complexes with antiferromagnetically coupled α-diiminato radical ligands. A strong magnetic exchange coupling between the iron(II) ion and the ligand radical was confirmed in 1, with an estimated coupling constant J < -250 cm-1 (J = -657 cm-1, DFT). Calibrated CAS calculations revealed that the ground-state Fe(II)-α-diiminato radical configuration has significant ionic contributions, which are weighted specifically toward the Fe(I)-neutral α-diimine species. Experimental data and theoretical calculations also suggest that 1 possesses an easy-axis anisotropy, with an axial zero-field splitting parameter D in the range from -4 to-1 cm-1. Finally, dynamic magnetic studies show that 1 exhibits slow magnetic relaxation behavior with an energy barrier close to the theoretical maximum, 2|D|. These results demonstrate that incorporating strongly coupled α-diiminato radicals into mononuclear transition metal complexes can be an effective strategy to prepare SMMs.

Synthesis and Reactivity Studies of a Series of Nickel(II) Arylchalcogenolates.

Two series of high-spin nickel complexes, [TpPh,Me]Ni(EAr) (E = O, Se, Te; Ar = C6H5) and [TpPh,Me]Ni(SeC6H4-4-X) (X = H, Cl, Me, OMe), were prepared by metathetical reaction of the nickel(II) halide precursor with sodium salts of the corresponding chalcogen, NaEAr. X-ray crystallographic characterization and spectroscopic studies have established the geometric and electronic structures of these complexes. The observed spectroscopic and structural characteristics reveal distinct trends in accordance with the variation of the identity of the arylchalcogenolate and para substituent. Reaction of the [TpPh,Me]Ni(EAr) complexes with methyl iodide proceeded readily, producing the corresponding methylarylchalcogen and [TpPh,Me]NiI. A kinetic and computational analysis of the reaction of [TpPh,Me]Ni(SeC6H5) with MeI supports that the electrophilic alkylation reactions occur via an associative mechanism via a classical SN2 transition state.

Electronic, Magnetic, and Redox Properties and O2 Reactivity of Iron(II) and Nickel(II) o-Semiquinonate Complexes of a Tris(thioether) Ligand: Uncovering the Intradiol Cleaving Reactivity of an Iron(II) o-Semiquinonate Complex.

The iron(II) semiquinonate character within the iron(III) catecholate species has been proposed by numerous studies to account for the O2 reactivity of intradiol catechol dioxygenases, but a well-characterized iron(II) semiquinonate species that exhibits intradiol cleaving reactivity has not yet been reported. In this study, a detailed electronic structure description of the first iron(II) o-semiquinonate complex, [PhTttBu]Fe(phenSQ) [PhTttBu = phenyltris(tert-butylthiomethyl)borate; phenSQ = 9,10-phenanthrenesemiquinonate; Wang et al. Chem. Commun. 2014, 50, 5871-5873], was generated through a combination of electronic and Mössbauer spectroscopies, SQUID magnetometry, and density functional theory (DFT) calculations. [PhTttBu]Fe(phenSQ) reacts with O2 to generate an intradiol cleavage product, diphenic anhydride, in 16% yield. To assess the dependence of the intradiol reactivity on the identity of the metal ion, the nickel analogue, [PhTttBu]Ni(phenSQ), and its derivative, [PhTttBu]Ni(3,5-DBSQ) (3,5-DBSQ = 3,5-di-tert-butyl-1,2-semiquinonate), were prepared and characterized by X-ray crystallography, mass spectrometry, 1H NMR and electronic spectroscopies, and SQUID magnetometry. DFT calculations, evaluated on the basis of the experimental data, support the electronic structure descriptions of [PhTttBu]Ni(phenSQ) and [PhTttBu]Ni(3,5-DBSQ) as high-spin nickel(II) complexes with antiferromagnetically coupled semiquinonate ligands. Unlike its iron counterpart, [PhTttBu]Ni(phenSQ) decomposes slowly in an O2 atmosphere to generate 14% phenanthrenequinone with a negligible amount of diphenic anhydride. [PhTttBu]Ni(3,5-DBSQ) does not react with O2. This dramatic effect of the metal-ion identity supports the hypothesis that a metal(III) alkylperoxo species serves as an intermediate in the intradiol cleaving reactions. The redox properties of all three complexes were probed using cyclic voltammetry and differential pulse voltammetry, which indicate an inner-sphere electron-transfer mechanism for the formation of phenanthrenequinone. The lack of O2 reactivity of [PhTttBu]Ni(3,5-DBSQ) can be rationalized by the high redox potential of the metal-ligated 3,5-DBSQ/3,5-DBQ couple.

Steric and electronic factor comparisons in hydrotris(3-phenylpyrazolyl)borate nickel(II) aryloxides.

Hydrotris(pyrazolyl)borate (Tp) ligands, also known as scorpionates, are potent tridentate donors that effectively bind metal ions in a face-capping array. Hydrotris(3-phenylpyrazolyl)borate enforces a tetrahedral environment on NiII to model metalloenzymes. The syntheses and structural characterizations of a number of [hydrotris(3-phenylpyrazolyl)borato]nickel(II) aryloxides were performed to provide insight into the environment of the model active site; these compounds are chlorido[hydrotris(3-phenylpyrazolyl-κN2)borato](3-phenyl-1H-pyrazole-κN2)nickel(II) chloroform monosolvate, [Ni(C27H22BN6)Cl(C9H8N2)]·CHCl3, (2), [hydrotris(3-phenylpyrazolyl-κN2)borato](phenolato-κO)nickel(II), [Ni(C27H22BN6)(C6H5O)], (3), (2,6-dimethylphenolato-κO)[hydrotris(3-phenylpyrazolyl-κN2)borato]nickel(II) [Ni(C27H22BN6)(C8H9O)], (4), (4-tert-butylphenolato-κO)[hydrotris(3-phenylpyrazolyl-κN2)borato]nickel(II), [Ni(C27H22BN6)(C10H13O)], (5), and [hydrotris(3-phenylpyrazolyl-κN2)borato](phenolato-κO)(tetrahydrofuran-κO)nickel(II) tetrahydrofuran monosolvate, [Ni(C27H22BN6)(C6H5O)(C4H8O)]·C4H8O, (6). Alkyl groups, e.g. tert-butyl in (5) and methyl in (4), electronically activate the aryloxide group to intramolecular π-π stacking but can be frustrated by steric encumbrance at the interacting ring faces. The flexibility at the nickel coordination site, afforded by the uncrowded B atom of the TpPh ligand, allows tetrahydrofuran coordination in the phenolate compound.

Synchrotron-based Nickel Mössbauer Spectroscopy.

We used a novel experimental setup to conduct the first synchrotron-based (61)Ni Mössbauer spectroscopy measurements in the energy domain on Ni coordination complexes and metalloproteins. A representative set of samples was chosen to demonstrate the potential of this approach. (61)NiCr2O4 was examined as a case with strong Zeeman splittings. Simulations of the spectra yielded an internal magnetic field of 44.6 T, consistent with previous work by the traditional (61)Ni Mössbauer approach with a radioactive source. A linear Ni amido complex, (61)Ni{N(SiMe3)Dipp}2, where Dipp = C6H3-2,6-(i)Pr2, was chosen as a sample with an "extreme" geometry and large quadrupole splitting. Finally, to demonstrate the feasibility of metalloprotein studies using synchrotron-based (61)Ni Mössbauer spectroscopy, we examined the spectra of (61)Ni-substituted rubredoxin in reduced and oxidized forms, along with [Et4N]2[(61)Ni(SPh)4] as a model compound. For each of the above samples, a reasonable spectrum could be obtained in ∼1 d. Given that there is still room for considerable improvement in experimental sensitivity, synchrotron-based (61)Ni Mössbauer spectroscopy appears to be a promising alternative to measurements with radioactive sources.

Acetate and acetamide complexes of [Ni(Me4[12]aneN4)]PF6: a tale of two ligands.

Although there are many examples of acetate complexes, acetamide complexes are virtually unknown. A side-by-side comparison in (acetato-κ(2)O,O')(1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane-κ(4)N)nickel(II) hexafluoridophosphate, [Ni(C2H3O2)(C12H28N4)]PF6, (1), and (acetamidato-κ(2)O,O')(1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane-κ(4)N)nickel(II) hexafluoridophosphate, [Ni(C2H4NO)(C12H28N4)]PF6, (2), shows the steric equivalence between these two ligands, suggesting that acetamide could be considered as a viable acetate replacement for electronic tuning.

Five-coordinate M(II)-semiquinonate (M = Fe, Mn, Co) complexes: reactivity models of the catechol dioxygenases.

A series of five-coordinate M(II)-semiquinonate (M = Fe, Mn, Co) complexes were synthesized and characterized, including the first example of a mononuclear Fe(II)-semiquinonate. Intermediates were observed in the reactions of M(II)-phenSQ (M = Fe, Co) with O2. Evidence for the relevance of these intermediates to the intradiol catechol dioxygenases was obtained by characterization of the oxidized semiquinone-derived product, muconic anhydride, resulting from the reaction of [PhTt(tBu)]Co(II)(3,5-DBSQ) with O2.

C-H activation by a diselenido dinickel(II) complex.

Addition of selenium to the nickel(I) complex, [Ni(Me4[12]aneN4)(CO)]PF6, effects a redox reaction leading to the diselenido dinickel(II) complex, {[(Ni(Me4[12]aneN4)]2(Se2)}(PF6)2, in 70% crystalline yield. The product's structure features a µ-η(2):η(2)-Se2 ligand with Se-Se bond length of 2.379(13) Å. Upon mild heating, {[(Ni(Me4[12]aneN4)]2(µ-η(2):η(2)-Se2)}(PF6)2 oxidizes 9,10-dihydroanthracene or 1,4-cyclohexadiene forming the terminal hydroselenide, [Ni(Me4[12]aneN4)(SeH)]PF6, and anthracene or benzene, respectively. [Ni(Me4[12]aneN4)(SeH)]PF6 cleanly converts back to the diselenido dinickel(II) adduct upon addition of a phenoxy radical.

Coordination chemistry of poly(thioether)borate ligands.

This review traces the development and application of the tris(thioether)borate ligands, tripodal ligands with highly polarizable thioether donors. Areas of emphasis include the basic coordination chemistry of the mid-to-late first row transition metals (Fe, Ni, Co, Cu), and the role of the thioether substituent in directing complex formation, the modeling of zinc thiolate protein active sites, high-spin organo-iron and organo-cobalt chemistry, the preparation of monovalent complexes of Fe, Co and Ni, and dioxygen and sulfur activation by monovalent nickel complexes.

Spectroscopic and computational studies of a series of high-spin Ni(II) thiolate complexes.

The electronic structures of a series of high-spin Ni(II)-thiolate complexes of the form [PhTt(tBu)]Ni(SR) (R = CPh(3), 2; C(6)F(5), 3; C(6)H(5), 4; PhTt(tBu) = phenyltris((tert-butylthio)methyl)borate) have been characterized using a combined spectroscopic and computational approach. Resonance Raman (rR) spectroscopic data reveal that the nu(Ni-SR) vibrational feature occurs between 404 and 436 cm(-1) in these species. The corresponding rR excitation profiles display a striking de-enhancement behavior because of interference effects involving energetically proximate electronic excited states. These data were analyzed in the framework of time-dependent Heller theory to obtain quantitative insight into excited state nuclear distortions. The electronic absorption and magnetic circular dichroism spectra of 2-4 are characterized by numerous charge transfer (CT) transitions. The dominant absorption feature, which occurs at approximately 18,000 cm(-1) in all three complexes, is assigned as a thiolate-to-Ni CT transition involving molecular orbitals that are of pi-symmetry with respect to the Ni-S bond, reminiscent of the characteristic absorption feature of blue copper proteins. Density functional theory computational data provide molecular orbital descriptions for 2-4 and allow for detailed assignments of the key spectral features. A comparison of the results obtained in this study to those reported for similar Ni-thiolate species reveals that the supporting ligand plays a secondary role in determining the spectroscopic properties, as the electronic structure is primarily determined by the metal-thiolate bonding interaction.

Spectroscopic and computational studies of a trans-mu-1,2-disulfido-bridged dinickel species, [{(tmc)Ni}(2)(S(2))](OTf)(2): comparison of end-on disulfido and peroxo bonding in (Ni(II))(2) and (Cu(II))(2) species.

A powerful means of enhancing our understanding of the structures and functions of enzymes that contain nickel-sulfur bonds, such as Ni superoxide dismutase, acetyl-coenzyme A synthase/carbon monoxide dehydrogenase, [NiFe] hydrogenase, and methyl-CoM reductase, involves the investigation of model compounds with similar structural and/or electronic properties. In this study, we have characterized a trans-mu-1,2-disulfido-bridged dinickel(II) species, [{(tmc)Ni}(2)(S(2))](2+) (1, tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) by using electronic absorption, magnetic circular dichroism (MCD), and resonance Raman (rR) spectroscopic techniques, as well as density functional theory (DFT) and time-dependent DFT computational methods. Our computational results, validated on the basis of the experimental MCD data and previously reported (1)H NMR spectra, reveal that 1 is best described as containing two antiferromagnetically coupled high-spin Ni(II) centers. A normal coordinate analysis of the rR vibrational data was performed to quantify the core bond strengths, yielding force constants of k(Ni-S) = 2.69 mdyn/A and k(S-S) = 2.40 mdyn/A. These values provide a useful basis for a comparison of metal-S/O bonding in 1 and related Ni(2)(O(2)), Cu(2)(O(2)), and Cu(2)(S(2)) dimers. In both the disulfido and the peroxo species, the lower effective nuclear charge of Ni(II) as compared to Cu(II) results in a decreased covalency, and thus relatively weaker metal-S/O bonding interactions in the Ni(2) dimers than in the Cu(2) complexes.

Spectroscopic and computational studies of a mu-eta(2):eta(2)-disulfido-bridged dinickel(II) species, [{(PhTt(tBu))Ni}(2)(mu-eta(2):eta(2)-S(2))]: comparison of side-on disulfido and peroxo bonding in (Ni(II))(2) and (Cu(II))(2) species.

In this study, a combined spectroscopic and computational approach has been employed to generate a detailed description of the electronic structure of a binuclear side-on disulfido (Ni(II))(2) complex, [{(PhTt(tBu))Ni}(2)(mu-eta(2):eta(2)-S(2))] (1, where PhTt(tBu) = phenyltris[(tert-butylthio)methyl]borate). The disulfido-to-Ni(II) charge-transfer transitions that dominate the electronic absorption spectrum have been assigned on the basis of time-dependent density functional theory (DFT) calculations. Resonance Raman spectroscopic studies of 1 have revealed that the S-S stretching mode occurs at 446 cm(-1), indicating that the S-S bond is weaker in 1 than in the analogous mu-eta(2):eta(2)-S(2) dicopper species. DFT computational data indicate that the steric bulk of PhTt(tBu) stabilize the side-on core enough to prevent its conversion to the electronically preferred bis(mu-sulfido) (Ni(III))(2) structure. Hence, 1 provides an interesting contrast to its O(2)-derived analogue, [{(PhTt(tBu))Ni}(2)(mu-O)(2)], which was shown previously to assume a bis(mu-oxo) (Ni(III))(2) "diamond core". By a comparison of 1 to analogous disulfidodicopper and peroxodinickel species, new insight has been obtained into the roles that the metal centers, bridging ligands, and supporting ligands play in determining the core structures and electronic properties of these dimers.

A high-spin organometallic Fe-S compound: structural and Mössbauer spectroscopic studies of [phenyltris((tert-butylthio)methyl)borate]Fe(Me).

The synthesis and structure of the pseudotetrahedral, sulfur-rich, high-spin organoiron(II) [phenyltris((tert-butylthio)methyl)borate]Fe(Me), [PhTt(tBu)]Fe(Me), 1, are reported. Low-temperature Mössbauer spectroscopic studies reveal an isomer shift of delta = 0.60(3) mm/s and DeltaE(Q) = 0.00(1) mm/s and an S = 2 ground multiplet with a negative zero-field splitting, D = -33(3) cm(-1), E/D approximately = 0.01. The small separation of the ground doublet, Delta approximately = 0.01 cm(-1), allows for observation of X-band EPR signals at g(eff) approximately = 10 (g(z) = 2.6, g(x,y) = 2.00). The relatively large negative zero-field splitting and a highly anisotropic magnetic hyperfine tensor, containing a large orbital z component, {-10(4), -10(4), +33.8(2) MHz}, are concordant with the presence of unquenched orbital angular momentum. Density functional theory (DFT) calculations predict that the lowest-lying orbitals have predominantly d(xy)- and d(x(2)-y(2))-like character, separated by an energy gap small enough to allow mixing through spin-orbit coupling, to generate a negative zero-field splitting, consistent with the experimental observations. The experimental and DFT-calculated isomer shifts are in good agreement (delta(calcd) = 0.5 mm/s). The unusual (for a high-spin ferrous site) null electric field gradients can be qualitatively explained in the frame of the spin-orbit coupling mixing. The very small Fermi contact component of the magnetic hyperfine tensor (A(FC)(exp) = -9 MHz) is not well described by the DFT approach (A(FC)(calcd) = +2 MHz). To our knowledge, this is the first study of a sulfur-coordinated high-spin organoiron(II) complex.

Synthetic analogs for evaluating the influence of N-H...S hydrogen bonds on the formation of thioester in acetyl coenzyme A synthase.

A series of square planar methylnickel(II) complexes, (dppe)Ni(Me)(SAr) (dppe = 1,2-bis(diphenylphosphino)ethane); 2. Ar = phenyl; 3. Ar = pentafluorophenyl; 4. Ar = o-pivaloylaminophenyl; 5. Ar = p-pivaloylaminophenyl; (depe)Ni(Me)(SAr), (depe = 1,2-bis(diethylphosphino)ethane); 7. Ar = phenyl; 8. Ar = pentafluorophenyl; 9. Ar = o-pivaloylaminophenyl; 10. Ar = p-pivaloylaminophenyl), were synthesized via the reaction of (dppe)NiMe(2) (1) and (depe)NiMe(2) (6) with either the corresponding thiol or disulfide. These complexes were characterized by various spectroscopic methods including (31)P NMR, (1)H NMR, (13)C NMR and infrared spectroscopies and in most cases by X-ray diffraction analyses. Solid state and solution measurements establish that 4 and 9 contain intramolecular N-H...S bonds. Carbonylation of the complexes 2-4, 7-10 leads to (dRpe)Ni(CO)(2) and MeC(O)SAr via the intermediacy of the acylnickel adducts, (dRpe)Ni(C(O)Me)(SAr), detected at low temperature by (31)P NMR spectroscopy. Consistent with experimental observations, density functional theory results reveal that the intramolecular hydrogen bond in 9 stabilizes the acylnickel adduct compared with its non-hydrogen-bonded adduct, 10. Oxidative addition of MeC(O)SC(6)F(5) to (dRpe)Ni(COD) followed by spontaneous decarbonylation proceeds in variable yields generating 3 and 8.

Synthesis and spectroscopic identification of a mu-1,2-disulfidodinickel complex.

Reduction of elemental sulfur by a monovalent nickel precursor leads to a trans-1,2-mu-disulfidodinickel(II) complex assigned based on a combination of advanced spectroscopic methods in conjunction with density functional theory calculations. The disulfido linkage is characterized by an intense optical S --> Ni charge transfer transition at 650 nm, which causes a significant distortion of the Ni(2)S(2) core along an isotope-sensitive v(S-S) mode at 474 cm(-1), as demonstrated by the resonance Raman excitation profile of this vibrational feature.

A Series of Cyanide-Bridged Binuclear Complexes.

A series of cyanide-bridged binuclear complexes, ('S(3)')Ni-CN-M[Tp(tBu)] ('S(3)' = bis(2-mercaptophenyl)sulfide, Tp(tBu) = hydrotris(3-tert-butylpyrazolyl)borate, M = Fe (2-Fe), Co (2-Co), Ni (2-Ni), Zn (2-Zn)) was prepared by the coupling of K[('S(3)')Ni(CN)] with [Tp(tBu)]MX. The isostructural series of complexes was structurally and spectroscopically characterized. A similar coupling strategy was used to synthesize the anionic copper(I) analogue, Et4N{('S3')Ni-CN-Cu[Tp(tBu)]}, 2-Cu.An alternative synthesis was devised for the preparation of the linkages isomers of 2-Zn, i.e. of cyanide-bridged linkage isomers. X-ray diffraction, (13)C NMR and IR spectral studies established that isomerization to the more stable Ni-CN-Zn isomer occurs. DFT computational results buttressed the experimental observations indicating that the cyanide-bridged isomer is ca. 5 kcal/mol more stable than its linkage isomer.

Influence of ligand electronic effects on the structure of monovalent cobalt complexes.

The syntheses, spectroscopic properties, and structures of the monovalent cobalt complexes, [PhTt (tBu)]Co(L), 1-L {PhTt (tBu) = phenyltris[( tert-butylthio)methyl]borate; L = PPh 3, PMe 3, PEt 3, PMe 2Ph, PMePh 2, P(OPh) 3, CNBu (t)}, are described. Complexes 1-L are prepared via the sodium amalgam reduction of [PhTt (tBu)]CoCl in the presence of L. The complexes display magnetic moments and paramagnetically shifted (1)H NMR spectra consistent with triplet, S = 1, ground states. The molecular geometries, determined by X-ray diffraction methods, reveal that some of the complexes display structures in which the L donor is moved off of the inherent 3-fold axis. In the most extreme cases (e.g., 1-P(OPh) 3 or 1-CNBu ( t )), the geometries can be described as cis-divacant octahedra. The origin of the geometric distortions is a consequence of the electronic characteristics of L as first deduced by Detrich et al. for [Tp (Np)]Co(L) ( J. Am. Chem. Soc. 1996, 118, 1703). The results establish a linear correlation between the magnitude of the structural distortion and the electronic parameter of the phosphine donor.