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PubMed2005

Vibrational and electronic structure of the dinuclear bis(mu-nitrido) vanadium(v) complex [V(N{N"}2)(mu-N)]2: spectroscopic properties of the M2(mu-N)2 diamond core.

The vibrational and electronic structure of the bis(mu-nitrido) bridged complex [V(N{N"}2)(mu-N)]2 (1) (where [N{N"}2](2-)=[(Me3Si)N{CH(2)CH(2)N(SiMe3)}2](2-)) is analyzed. Assignment of the five modes of the V(2)(mu-N)2 core is based on (15)N isotope shifts and a DFT calculation on the calculated structure I which is an exact reproduction of 1. The three Raman active modes of the planar V(2)(mu-N)2 core are found in the Raman spectrum whereas the two IR allowed vibrations are identified in the infrared spectrum. Furthermore, the electronic structure of is described which complements earlier theoretical studies on the reaction pathway leading to 1(V. M. E. Bates, G. K. B. Clentsmith, F. G. N. Cloke, J. C. Green, H. D. L. Jenkin, Chem. Commun., 2000, 927). Based on the MO scheme of I the UV-vis transitions of 1 are assigned.

Electrons↗
PubMed2005

Local atomic structure of partially ordered NiMn in NiMn/NiFe exchange-coupled layers: 2. Electronic structure calculations.

Local electronic and magnetic structure calculations for NiMn exchange bias alloys are reported for clusters containing NiMn in both the chemically disordered face-centered cubic and the chemically and magnetically ordered L1(0) phases. The results of these calculations are consistent with our local structure measurements that point toward the existence of nanometer-scale ordered clusters at the beginning stages of chemical ordering. The spatial dependence of both the local density of states and the magnetization is strongly influenced by the existence of magnetic order on short length scales, giving rise to an inhomogeneous profile for these quantities across the material with the greatest change at the interface that is still small enough within the domain to imply that the magnetization is still highly developed.

Journal Article↗
PubMed2005

Near-edge electronic structure in NbS2.

The near-edge electronic and structural properties of 2H-NbS(2) were investigated using scanning tunneling microscopy (STM) and density-functional calculations. Geometry optimization of the near-edge structure using density-functional calculations was performed on [1010]- and [1010]-terminated layer edges. Ribbon model systems also included variation of the number of bound sulfur atoms at the edges. Atomic resolution STM data exhibit a pronounced electronic density of states at the outermost edge atomic sites but are otherwise bulk-like in the near-edge region. Optimized NbS2 ribbon structures confirm the STM results indicating that minimal reconstruction occurs and that the edge electronic structure exhibits a significant increase in local density of states compared to bulk. Simulated STM images using extended Hückel tight-binding calculations based on optimized ribbon structures successfully modeled the experimental STM results. The results indicate that the [1010] "Nb" edges are preferentially observed compared to the [1010] "S" edge possibly due to differences in stability.

Journal Article↗
PubMed2002

Photoelectron spectra and electronic structure of some diastereomeric quinuclidine derivatives.

The molecular and electronic structure of three pairs of diastereomeric quinuclidine derivatives (1-6) have been investigated by quantum chemical calculations and UV photoelectron (PE) spectroscopy. Compounds 1-6 each possess a hydroxymethyl group at C2 and an ethyl (1, 2), vinyl (3, 4) or ethynyl group (5, 6) at C5. The diastereomeric pairs differ only in the configuration of carbon C2 to which the hydroxymethyl group is attached. The 1-azabicyclo[2.2.2]octane cages of 1-6 are slightly twisted. The torsion angles show opposite signs in the pseudoenantiomeric pairs and vary only little with the degree of unsaturation in the C5 substituent. In all compounds the hydroxymethyl group forms an intramolecular hydrogen bond with the nitrogen atom of the bicycle in the gas phase (in contrast to the crystalline and liquid state). The PE spectra of the diastereomers are nearly identical. Compared to unsubstituted quinuclidine, the orbital of the nitrogen lone pair electrons n(N) is stabilized by 0.3-0.4 eV, which is a result of partially compensating contributions of the OH...N hydrogen bond and substituent effects. Among these a substantial through-bond interaction of the n(N) with a pi(CC) orbital is detected for 3-6. The marked differences of the properties of 1-6 in the gas phase versus solid state and solution are emphasised. In solution the ethynyl azabicyclics 5 and 6 are more polar than the vinyl analogues 3 and 4.

Cinchona Alkaloids↗
PubMed2006

Spectroscopic properties and electronic structure of five- and six-coordinate iron(II) porphyrin NO complexes: Effect of the axial N-donor ligand.

In this paper, the differences in the spectroscopic properties and electronic structures of five- and six-coordinate iron(II) porphyrin NO complexes are explored using [Fe(TPP)(NO)] (1; TPP = tetraphenylporphyrin) and [Fe(TPP)(MI)(NO)] (2; MI = 1-methylimidazole) type systems. Binding of N-donor ligands in axial position trans to NO to five-coordinate complexes of type 1 is investigated using UV-vis absorption and 1H NMR spectroscopies. This way, the corresponding binding constants Keq are determined and the 1H NMR spectra of 1 and 2 are assigned for the first time. In addition, 1H NMR allows for the determination of the degree of denitrosylation in solutions of 1 with excess base. The influence of the axial ligand on the properties of the coordinated NO is then investigated. Vibrational spectra (IR and Raman) of 1 and 2 are presented and assigned using isotope substitution and normal-coordinate analysis. Obtained force constants are 12.53 (N-O) and 2.98 mdyn/A (Fe-NO) for 1 compared to 11.55 (N-O) and 2.55 mdyn/A (Fe-NO) for 2. Together with the NMR results, this provides experimental evidence that binding of the trans ligand weakens the Fe-NO bond. The principal bonding schemes of 1 and 2 are very similar. In both cases, the Fe-N-O subunit is strongly bent. Donation from the singly occupied pi* orbital of NO into d(z2) of iron(II) leads to the formation of an Fe-NO sigma bond. In addition, a medium-strong pi back-bond is present in these complexes. The most important difference in the electronic structures of 1 and 2 occurs for the Fe-NO sigma bond, which is distinctively stronger for 1 in agreement with the experimental force constants. The increased sigma donation from NO in 1 also leads to a significant transfer of spin density from NO to iron, as has been shown by magnetic circular dichroism (MCD) spectroscopy in a preceding Communication (Praneeth, V. K. K.; Neese, F.; Lehnert, N. Inorg. Chem. 2005, 44, 2570-2572). This is confirmed by the 1H NMR results presented here. Hence, further experimental and computational evidence is provided that complex 1 has noticeable Fe(I)NO+ character relative to 2, which is an Fe(II)NO(radical) complex. Finally, using MCD theory and quantum chemical calculations, the absorption and MCD C-term spectra of 1 and 2 are assigned for the first time.

Binding Sites↗
PubMed2004

New probe of the electronic structure of amorphous materials.

Here we show that electrochemical equilibrium voltage curves of amorphous WO3 and TiO2 coatings exhibit fine structure in striking agreement with the density of states in the conduction bands, as obtained by ab initio calculations for the crystalline counterparts. We suggest that localization of the band states is essential for observing the electronic structure. Our highly sensitive electrochemical method opens new vistas for studying the electronic structure of nonmetallic disordered materials that can be intercalated with an ionic species.

Journal Article↗
PubMed2004

Multiscale simulations of carbon nanotube nucleation and growth: electronic structure calculations.

Several first-principles surface and bulk electronic structure calculations relating to the nucleation and growth of single-wall carbon nanotubes are described. Density-functional theory in various forms is used throughout. In the surface-related calculations, a 38-atom Ni cluster and several low-index Ni surfaces are investigated using pseudopotentials and plane-wave expansions. The energetic ordering of the sites for C atom adsorption is found to be the same, with the Ni(100) facet favored. The bulk diffusion coefficient of C in Ni as a function of cluster size and temperature is calculated from various molecular dynamics approaches. In another group of bulk-related calculations, Gaussian orbital basis sets are used to study a cluster or "flake" containing 14 C atoms. The flake is a segment of three hexagons from an "unrolled" carbon nanotube, with an armchair termination. The binding energies of C, Ni, Co, Fe, Cu, and Au atoms to it were calculated in an effort to gain insight into the mechanism for the high catalytic activity of Ni, Co, and Fe and the lack of it in Cu and Au. The binding energies of Cu and Au are about 1 eV less than those of the three catalytic elements. Similar methods are used to study the initial stages of nanotube growth within the context of classical nucleation theory. Finally, issues relating to the establishment of a fundamental catalytic mechanism are addressed.

Adsorption↗
PubMed2002

Delocalized electronic structure of the thiol sulfur substantially prevents nucleic acid damage induced by neocarzinostatin.

Neocarzinostatin is a potent antitumor antibiotic and is a prodrug, which induces genome damage after activation by a thiol. The prodrug is stored as a protein-bound chromophore that contains an enediyne nucleus. A thiolate attack on the chromophore cyclizes the nucleus and produces radicals that abstract hydrogen from DNA. Because thiol is the only cofactor in the vital activation process, the structure of the thiol plays an important role in the activity of the drug. Here we systematically examine the effect of the electronic structure of some thiols on the efficiency of the drug, and compare particularly aromatic with aliphatic thiols. The values of drug-induced base release from DNA are remarkably different between thiophenol (3.6%) and benzyl mercaptan (12.5%), the activity of which is comparable with those of aliphatic thiols. Cleavage results determined by DNA electrophoresis are consistent with the results of base release; they show that the total number of DNA lesions is more than 3-fold lower for thiophenol than for aliphatic thiols or benzyl mercaptan. We conclude that among aromatic thiols, only those that have delocalized thiol sulfur electrons can substantially reduce the DNA cleavage activity. This result suggests that the effect of an aromatic ring arises from an inductive effect imposed on the thiol sulfur electron through pi-resonance rather than through effects such as aromatic stacking, steric hindrance, or hydrophobic interaction. Replacing thiophenol with substituted derivatives with electron-releasing or -withdrawing groups changes the drug activity and supports the important role of the electronic structure of the thiol sulfur in determining the drug activity.

Acetylcysteine↗
PubMed1981

Spectroscopic studies of ascorbate oxidase. Electronic structure of the blue copper sites.

Low-temperature electronic absorption and room temperature circular dichroism (CD) and magnetic circular dichroism (MCD) spectra are reported for ascorbate oxidase. Bands attributable to d-d electronic transitions in the type 1 (blue) coppers (2B2 ground state) have been observed at 5800, 10000, and 12000 cm-1 (1725, 1000, and 835 nm). The three bands are assigned to the transitions 2B2 leads to 2E, 2B2 leads to 2B1, and 2B2 leads to 2A1, respectively, in the slightly flattened tetrahedral blue copper centers. The ligand field theoretical treatment predicts a type 1 copper reduction potential of 417 mV. The MCD spectrum of ascorbate oxidase contains one major band at 14000 cm-1 (710 nm) and a very weak feature at 13 200 cm-1 (550 nm). The intensities of the CD and MCD bands are consistent with two or three type 1 coppers in each enzyme molecule, and the near-infrared spectroscopic results suggest that the electronic structures of the blue coppers are closely similar.

Ascorbate Oxidase↗
PubMed2005

Revisiting the electronic structure of phosphazenes.

Natural bond orbital (NBO) and topological electron density analyses have been used to investigate the electronic structure of phosphazenes [N3P3R6] (R = H, F, Cl, Br, CH3, CF3, N(C2H4); 2R = O2C6H4), [N4P4Cl8], and H[NPCl2]4H. Using the former, the two most likely phosphazene bonding alternatives, negative hyperconjugation and ionic bonding have been critically evaluated. Ionic bonding, as suggested by topological analysis, was found to be the dominant bonding feature, although contributions from negative hyperconjugation are necessary for a more complete bonding description. Substituent effects on the P-N bond have been assessed and cases of bond length alternation have been rationalized using this combined bonding model, which supersedes previous models involving d-orbital participation, leading to an explanation for the observed bond length alternation found in some linear polyphosphazenes. In addition, common aromaticity indicators, nucleus independent chemical shifts (NICS) and para-delocalization indices (PDI), have been determined for the cyclophosphazenes.

Journal Article↗
PubMed2006

Probing the intrinsic electronic structure of the bis(dithiolene) anions [M(mnt)2]2- and [M(mnt)2]1- (M = Ni, Pd, Pt; mnt = 1,2-S2C2(CN)2) in the gas phase by photoelectron spectroscopy.

A detailed understanding of the electronic structure of transition metal bis(dithiolene) complexes is important because of their interesting redox, magnetic, optical, and conducting properties and their relevance to enzymes containing molybdenum and tungsten bis(dithiolene) centers. The electronic structures of the bis(dithiolene) anions [M(mnt)(2)](n-) (M = Ni, Pd, Pt; mnt = 1,2-S(2)C(2)(CN)(2); n = 0-2) were examined by a combination of photodetachment photoelectron spectroscopy (PES) and density functional theory calculations. The combined experimental and theoretical data provide insight into the molecular orbital energy levels of [M(mnt)(2)](2-) and the ground and excited states of [M(mnt)(2)](1-) and [M(mnt)(2)]. Detachment features from ligand-based orbitals of [M(mnt)(2)](2-) occur at similar energies for each species, independent of the metal center, while those arising from metal-based orbitals occur at higher energies for the heavier congeners. Electronic excitation energies inferred for [M(mnt)(2)](1-) from the PES experiments agree well with those obtained in optical absorption experiments in solution, with the PES experiments providing additional insight into the changes in energy of these transitions as a function of metal. The singly charged anions [M(mnt)(2)](1-) were also prepared and studied independently. Electron detachment from the ground states of these doublet anions accessed the lowest singlet and triplet states of neutral [M(mnt)(2)], thereby providing a direct experimental measure of their singlet-triplet splitting.

Journal Article↗
PubMed1993

Microscopic pathway for the medium-chain fatty acyl CoA dehydrogenase catalyzed oxidative half-reaction: changes in the electronic structures of flavin and CoA derivatives during catalysis.

In a previous communication, we demonstrated that the medium-chain fatty acyl CoA dehydrogenase (MCAD) catalyzed conversion of 3-indolepropionyl CoA (IPCoA) to trans-3-indoleacryloyl CoA (IACoA) proceeds via the formation of an intermediary species X that possesses the electronic properties of reduced flavin and highly conjugated CoA product. Since the steady-state turnover of the enzyme-catalyzed dehydrogenation reaction precisely matches with the rate of formation of X [Johnson, J. K., & Srivastava, D. K. (1993) Biochemistry 32, 8004-8013], the latter species appeared to be the likely site for the transfer of electrons to external electron acceptors (e.g., ferricenium hexafluorophosphate, FcPF6). To probe the microscopic pathway for the oxidative half-reaction, we employed a sequential mixing stopped-flow technique utilizing IPCoA as the enzyme substrate and FcPF6 as the electron acceptor. The time-dependent changes in absorption at 450, 415, and 367 nm were measured upon mixing FcPF6 with previously mixed and aged solutions of MCAD-FAD+IPCoA in the stopped-flow syringes. The kinetic traces show an increase (1/tau 1) followed by a decrease (1/tau 2) in absorption at 450 and 415 nm, and a lag (corresponding to the time regime of 1 u 1) followed by an increase in absorption (1/tau 2) at 367 nm. The relaxation rate constants (1/tau's) thus measured remain unaffected, with variations in the aging time; however, the amplitudes of these phases increase up to the aging time of 5 s, after which the amplitudes attain maxima. For an aging time of 5 s, 1/tau 1 and 1/tau 2 show a linear and a hyperbolic dependence on the FcPF6 concentration, respectively. These, coupled with the complementary studies involving butyryl CoA as a nonchromophoric substrate for this enzyme, lead us to propose the following sequence of events during the MCAD-catalyzed oxidative half-reaction: (1) The enzyme-catalyzed oxidative half-reaction proceeds via the formation of a collision complex between X and FcPF6 during the fast (1/tau 1) relaxation phase. (2) The reduced flavin moiety of X is oxidized via (rapid) transfer of electrons to FcPF6 within the collision complex, without formation of a detectable (metastable) flavin semiquinone intermediate. (3) The transfer of electrons is accompanied by changes in the electronic structures of both the flavin and IACoA moieties within the enzyme-IACoA complex. The electronic structure of this newly formed complex is exactly the same as that formed upon isomerization of the MCAD-FAD-IACoA complex [Johnson, J. K., Wang, Z. X., & Srivastava, D. K. (1992) Biochemistry 31, 10564-10575].(ABSTRACT TRUNCATED AT 400 WORDS)

Acyl Coenzyme A↗
PubMed2005

The electronic structure of the isoelectronic, square-planar complexes [FeII(L)2]2- and [CoIII(L Bu)2]- (L2- and (L Bu)2-=benzene-1,2-dithiolates): an experimental and density functional theoretical study.

The electronic structures of two formally isoelectronic transition-metal dithiolato complexes [Fe(L)2]2- (1) and [Co(L Bu)2]1- (2) both possessing a spin triplet ground state (St=1) have been investigated by various spectroscopic and density functional methods; H2L Bu represents the pro-ligand 3,5-di-tert-butylbenzene-1,2-dithiol and H2L is the corresponding unsubstituted benzene-1,2-dithiol. An axial zero-field splitting (D) of +32 cm(-1) for 2 has been measured independently by SQUID magnetometry, far-infrared absorption, and variable-temperature and variable-field (VTVH) magnetic circular dichroism spectroscopies. A similar D value of +28 cm(-1) is obtained for 1 on the basis of VTVH SQUID measurements. The absorption spectra of 1 and 2 are found, however, to be very different. Complex 1 is light yellow in color with no intense transition in the visible region, whereas 2 is deep blue. DFT calculations establish that the electronic structures of the [Fe(L)2](2-) and [Co(L)2]1- anions are very different and explain the observed differences in their absorption spectra. On the basis of these spectroscopic and theoretical analyses, 1 is best described as containing an intermediate spin FeII ion, whereas for the corresponding cobalt complex, oxidation states describing a d6 (CoIII) or d7 (CoII) electron configuration cannot be unambiguously assigned. The physical origin of the large zero-field splitting in both 1 and 2 is found to be due to the presence of low-energy spin-conserved d-d excitations which lead to a large Dzz through efficient spin-orbit coupling. Differential covalency effects appear to be of limited importance for this property.

Journal Article↗
PubMed2005

Synthesis, characterization, in vitro antitumor activity, DNA-binding properties and electronic structure (DFT) of the new complex cis-(Cl,Cl)[RuIICl2(NO+)(terpy)]Cl.

The complex cis-(Cl,Cl)-[RuCl2(terpy)(NO)]Cl (1) has been synthesized by the reaction of [RuCl3(H2O)2(NO)] with terpyridine (terpy) and characterized by various spectroscopic, analytical techniques and using electronic structure calculation (DFT) methods. The cytotoxic activity and the DNA-binding properties of have also been studied using biochemical techniques. The results establish unequivocally that corresponds to a so-called [RuNO]6 species, which readily releases the nitrosyl ligand upon irradiation with a mercury lamp in acetonitrile solution. DFT calculations provided a satisfactory description of structural, bonding, electronic and related properties of the new compound and throw light on the mechanism of the photo-induced NO release. Screening on A2780 (human ovarian carcinoma) cell lines showed significant cytotoxicity with an IC50 value of 0.49 microM. 31P and 23Na NMR spectroscopy along with electrophoretic mobility studies illustrated that complex primarily binds by coordination to DNA without any pi-interaction between the planar terpy ligand and the DNA bases, while weak electrostatic interactions could not be excluded. Studies on the inhibition of the restriction enzymes DraI and SmaI revealed that prefers the guanine and cytosine bases of DNA.

Antineoplastic Agents↗
PubMed1999

Study of the Molecular Geometry, Electronic Structure, and Thermal Stability of Phosphazene and Heterophosphazene Rings with ab Initio Molecular Orbital Calculations.

Ab initio molecular orbital calculations at the MP2/6-31G level of theory have been used to study the molecular geometry, electronic structure, and the thermal stability of six-membered phosphazene and heterophosphazene rings. The studies included the phosphazene ring [NPCl(2)](3), the carbophosphazene ring [(NCCl)(NPCl(2))(2)], and three thionylphosphazene rings [(NSOX)(NPCl(2))(2)] (X = F, Cl) and [(NSOF)(NPF(2))(2)] and their cations [(NPCl)(NPCl(2))(2)](+), [(NC)(NPCl(2))(2)](+), and [(NSO)(NPY(2))(2)](+) (Y = F, Cl). The ring skeleton of the phosphazene ring, the carbophosphazene ring and of all cation rings adopt a planar conformation; the ring skeletons of the thionylphosphazene rings adopt an envelope conformation. The valence electron charge density of the molecules indicates strong charge separations along their skeleton and is in agreement with Dewar's island delocalization model. The electrostatic potential in the vicinity of the neutral heterophosphazene rings which results from their electronic structure, and the position of the HOMO indicate that a heterolytic cleavage of a ligand and the opening of the ring involving a reaction with a electrophilic cation will most likely occur at the nitrogen atoms close to the heteroatom. The thermal stability of the phosphazene ring with respect to a cleavage of chlorine from phosphorus and the thermal stability of the heterophosphazene rings with respect to the cleavage of the halogen ligand bonded to the heteroatom were studied with several model reactions. Most of the reactions are exothermic. A comparison of isodesmic reactions shows that the thionylphosphazenes molecules are the least thermally stable rings with respect to ionization and that the carbophosphazene molecules are the most thermally stable rings with respect to ionization. The energy gains during the ionization reaction of the rings correlate well with the conformational changes which occur during the reactions.

Journal Article↗
PubMed2001

Invited award contribution for ACS Award in Inorganic Chemistry. Geometric and electronic structure contributions to function in bioinorganic chemistry: active sites in non-heme iron enzymes.

Spectroscopy has played a major role in the definition of structure/function correlations in bioinorganic chemistry. The importance of spectroscopy combined with electronic structure calculations is clearly demonstrated by the non-heme iron enzymes. Many members of this large class of enzymes activate dioxygen using a ferrous active site that has generally been difficult to study with most spectroscopic methods. A new spectroscopic methodology has been developed utilizing variable temperature, variable field magnetic circular dichroism, which enables one to obtain detailed insight into the geometric and electronic structure of the non-heme ferrous active site and probe its reaction mechanism on a molecular level. This spectroscopic methodology is presented and applied to a number of key mononuclear non-heme iron enzymes leading to a general mechanistic strategy for O2 activation. These studies are then extended to consider the new features present in the binuclear non-heme iron enzymes and applied to understand (1) the mechanism of the two electron/coupled proton transfer to dioxygen binding to a single iron center in hemerythrin and (2) structure/function correlations over the oxygen-activating enzymes stearoyl-ACP Delta9-desaturase, ribonucleotide reductase, and methane monooxygenase. Electronic structure/reactivity correlations for O2 activation by non-heme relative to heme iron enzymes will also be developed.

Animals↗
PubMed2003

Noninnocence of the ligand glyoxal-bis(2-mercaptoanil). The electronic structures of [Fe(gma)]2, [Fe(gma)(py)] x py, [Fe(gma)(CN)]1-/0, [Fe(gma)I], and [Fe(gma)(PR3)(n)] (n = 1, 2). Experimental and theoretical evidence for "excited state" coordination.

The electronic structure of the known iron complexes [Fe(gma)](2) (S(t) = 0) (1)(6) and [Fe(gma)(py)].py (S(t) = 1) (2)(7) where H(2)(gma) represents glyoxal-bis(2-mercaptoanil) has been shown by X-ray crystallography, Mössbauer spectroscopy, and density functional theory calculations to be best described as ferric (S(Fe) = 3/2) complexes containing a coordinated open-shell pi radical trianion (gma(*))(3)(-) and not as previously reported(6,7) as ferrous species with a coordinated closed-shell dianion (gma)(2)(-). Compound 1 (or 2) can be oxidized by I(2) yielding [Fe(III)(gma)I] (S(t) = 1/2) (3). With cyanide anions, complex 1 forms the adduct [(n-Bu)(4)N][Fe(III)(gma(*))(CN)] (S(t) = 1) (4), which can be one-electron oxidized with iodine yielding the neutral species [Fe(III)(gma)(CN)] (S(t) = 1/2) (5). With phosphines complex 1 also forms adducts(7) of which [Fe(III)(gma(*))(P(n-propyl)(3))] (S(t) = 1) (6) has been isolated and characterized by X-ray crystallography. [Fe(II)(gma)(P(n-propyl)(3))(2)] (S(t) = 0) (7) represents the only genuine ferrous species of the series. Density functional theory (DFT) calculations at the BP86 and B3LYP levels were applied to calculate the structural as well as the EPR and Mössbauer spectroscopic parameters of the title compounds as well as of the known complexes [Zn(gma)](0/)(-) and [Ni(gma)](0/)(-). Overall, the calculations give excellent agreement with the available spectroscopic information, thus lending support to the following electronic structure descriptions: The gma ligand features an unusually low lying LUMO, which readily accepts an electron to give (gma(*))(3)(-). The one-electron reduction of [Zn(gma)] and [Ni(gma)] is strictly ligand centered and differences in the physical properties of [Zn(gma(*))](-) and [Ni(gma(*))](-) are readily accounted for in terms of a model that features enhanced back-bonding from the metal to the gma LUMO in the case of [Ni(gma(*))](-). In the case of [Fe(gma)(PH(3))], [Fe(gma)(py)], and [Fe(gma)(CN)](-) an electron transfer from the iron to the gma LUMO takes place to give strong antiferromagnetic coupling between an intermediate spin Fe(III) (S(Fe) = 3/2) and (gma(*))(3)(-) (S(gma) = 1/2), yielding a total spin S(t) = 1. Broken symmetry DFT calculations take properly account of this experimentally calibrated electronic structure description. By contrast, the complexes [Fe(gma)(PH(3))(2)] and [Fe(PhBMA)] feature closed-shell ligands with a low-spin Fe(II) (S(Fe) = S(t) = 0) and an intermediate spin central Fe(II) (S(Fe) = S(t) = 1), respectively. The most interesting case is provided by the one-electron oxidized species [Fe(gma)(py)](+), [Fe(gma)I], and [Fe(gma)(CN)]. Here the combination of theory and experiment suggests the coupling of an intermediate spin Fe(III) (S(Fe) = 3/2) to the dianionic ligand (gma)(2)(-) formally in its first excited triplet state (S(gma) = 1) to give a resulting S(t) = 1/2. All physical properties are in accord with this interpretation. It is suggested that this unique "excited state" coordination is energetically driven by the strong antiferromagnetic exchange interaction between the metal and the ligand, which cannot occur for the closed-shell form of the ligand.

Journal Article↗