Yz Orbitals Research Articles

The mixed sandwich compounds C5H5MC7H7 of the first row transition metals: variable hapticity of the seven-membered ring

The mixed sandwich compounds (η5-C5H5)M(η7-C7H7) (M = Ti, V, Cr) have been known for approximately 50 years. The vanadium derivative (‘trovacene’) has recently been shown to be a useful precursor for the preparation of a variety of paramagnetic sandwich compounds. In this connection the structures of the complete series of the first row transition metal mixed sandwich compounds C5H5MC7H7 (M = Ti, V, Cr, Mn, Fe, Co, Ni) have been investigated by density functional theory. The experimentally known parallel ring sandwich structures (η5-C5H5)M(η7-C7H7) are found for the Ti, V, and Cr derivatives. Analysis of the frontier molecular orbitals of these complexes indicates that, for the metal d-orbitals, the z 2 orbital is essentially non-bonding, the x 2–y 2 and xy orbitals are δ bonding to the seven-membered ring, and the xz and yz orbitals are π bonding to the five-membered ring. For the later transition metals, (η5-C5H5)M(η5-C7H7) structures with pentahapto seven-membered rings are preferred for iron and manganese and (η5-C5H5)M(η3-C7H7) structures with trihapto seven-membered rings are preferred for cobalt and nickel, leading to either 17- or 18-electron complexes in all cases. A higher energy parallel sandwich structure (η5-C5H5)Co(η7-C7H7) with a quartet spin state and a formal 21-electron cobalt configuration is also found for the cobalt derivative.

Comment on “Nonmagnetic Impurity Resonances as a Signature of Sign-Reversal Pairing in FeAs-Based Superconductors”

In a recent Letter [PRL 103, 186402 (2009)], the energy band structure of Fe-As-based superconductors is fitted with a tight-binding model with two Fe ions per unit cell and two degenerate xz and yz orbitals per Fe ion. The author claims that the proposed model, which differs markedly from a model previously used by other authors for the same two orbitals in the same compounds, possesses the symmetry required to describe the Fe-As planes in iron-pnictide superconductors. In this comment we argue that this is not the case.

Electronic Structure of a Paramagnetic {MNO}6 Complex: MnNO 5,5-Tropocoronand

Using density functional theory (OLYP/STO-TZP) calculations, we have investigated the electronic structure of [Mn(5,5-tropocoronand)(NO)], a rare paramagnetic {MNO}(6) complex. Experimental methods, including magnetic susceptibility measurements and high-field electron paramagnetic resonance spectroscopy, have not provided an unambiguous spin state assignment for this complex. In other respects, however, the compound was fully characterized, including by means of single-crystal X-ray structure determination. The optimized S = 1 OLYP geometry reproduced all key aspects of the trigonal-bipyramidal molecular structure, including a short Mn-N(O) distance (approximately 1.7 A) and an essentially linear MnNO angle. In contrast, the S = 0 and S = 2 optimized structures disagreed with the crystal structure in critical respects. Moreover, three different exchange-correlation functionals (OLYP, B3LYP, and B3LYP*) indicated an S = 1 ground state by a clear margin of energy. An examination of the Kohn-Sham MOs of this state indicated a primarily d(xz)(2)d(yz)(2)d(xy)(1)d(x(2)-z(2))(1) electronic configuration, where the z axis is identified with the nearly linear MnNO axis. The d(y(2)) orbital is formally unoccupied in this state, interacting, as it does, head-on with two tropocoronand nitrogens lying along the y axis, the pseudo-3-fold axis of the trigonal bipyramid. The doubly occupied d(xz) and d(yz) orbitals are in actuality d(pi)(Fe)-pi*(NO)-based pi-bonding molecular orbitals, the alpha and beta "components" of which are significantly offset spatially. This offset results in excess minority spin density on the NO unit. Thus, the OLYP/TZP atomic spin populations are Mn, 2.85; N(O), -0.52; and O, -0.35.

Exact diagonalization studies on two-band minimal model for iron-based superconductors

In order to explore a superconducting mechanism on iron-based superconductors, we numerically study a two-band minimal model considering two degenerate d xz and d yz orbitals on Fe atom. We perform exact diagonalization on a two-band and two-leg square ladder totally composed of 10 lattice sites, which is computationally equivalent to 4-leg 20-sites square-Hubbard-ladder. Consequently, we find that a robust pairing occurs in a wide parameter range when the intra-orbital repulsive interaction becomes smaller than the inter-orbital one. Moreover, the obtained binding energy can grow into much larger value than that obtained in the single band Hubbard model depending on the parameter range.

Theoretical investigation of the molecular and electronic structures and excitation spectra of iron phthalocyanine and its derivatives, FePc and FePcLn (L = Py, CN−; n = 1, 2)

The effects of axial ligands on the ground-state geometries, electronic structures and the characteristic optical properties of iron phthalocyanine and its derivatives, FePc and FePcL(n) (L = pyridine (Py) and cyanide (CN(-)); n = 1, 2), were investigated using the density functional theory (DFT) method. The geometries of FePc with a triplet spin state and of FePc(Py), FePc(Py)(2), FePc(CN(-)) and FePc(CN(-))(2) with singlet spin states were optimized under D(4h), C(2v), D(2h), C(4v), and D(4h) molecular symmetries, respectively. The highest occupied molecular orbitals (HOMOs) of FePc, FePc(Py), FePc(Py)(2), and FePc(CN(-)) are pi-type orbitals, which have no contribution from the p(z) atomic orbitals of all nitrogen atoms, whereas the HOMO of FePc(CN(-))(2) is the 7e(g) orbital, which has contributions from the d(xz) and the d(yz) orbitals of the Fe atom mixing with the pi-orbitals of the axial CN(-) ligands. The time-dependent (TD) DFT method gives many optically allowed excitations for FePc, FePc(Py), FePc(Py)(2), FePc(CN(-)), and FePc(CN(-))(2) in the UV-VIS region. Our calculated bands corresponded well with the experimental results. In FePc(Py)(2), the metal-ligand charge transfer (MLCT) transitions from the metal d to the axial-ligand pi*-type orbitals contributed to the B band region. In FePc(CN(-))(2), the MLCT transitions from the metal d to the Pc-ring pi*-type orbitals contributed mainly to the first B band region, but those from the metal d to the axial-ligand pi*-type orbitals did not appear in the energy regions of the Q and B bands. Thus, the axial ligands caused a spectral change in FePc through orbital mixing.

Solvent dependence of the g-anisotropy in the ESR of cyanide-bridged mixed-valence complexes

The low temperature (approximately 5 K) X-band ESR spectra are reported of the cyanide-bridged mixed-valence complexes [(OC)5Cr(mu-CN)M(NH3)5]X2 (M = Ru, Os; X = PF6(-)) in frozen matrices formed from nitromethane, acetonitrile and dimethylformamide with toluene. The anisotropy (g paralell-g perpendicular) is greater for the ruthenium than for the osmium complex. It is positive in all cases and is strongly dependent on the hydrogen-bonding interaction between the solvent matrix and the metal-ammine fragment, decreasing in the order nitromethane > acetonitrile > dimethylformamide. The axial ligand field parameter, Delta, is quite insensitive to the ammine metal (M) and is mainly determined by the solvent matrix. Density functional calculations, together with a simplified MO model, show that: (a) The value of Delta is dominated by the interaction between the filled cyanide pi-orbitals and the ammine-metal d(xz,yz) orbitals, (b) Delta decreases with increasing solvent donicity because the resulting positive shift of the d-orbital energies reduces this interaction, (c) the insensitivity of Delta to the ammine-metal arises because an increase in the energy mismatch between the cyanide pi-orbitals and the d-orbitals in osmium compound is offset by an increase in the 5d resonance integrals relative to those in the 4d shell. Semi-quantitative values are obtained for the pi and pi* resonance integrals. We point out that g paralell determines that portion of the ammine-metal spin population that interacts with the cyanide bridge, and should therefore be correlated with the degree of metal-metal charge transfer in low-spin d6-d5 intervalence compounds. X-band ESR spectra of the polycrystalline powders (M = Ru, Os; X = CF3SO3(-)) are rhombic with similar axial and rhombic ligand field parameters. The rhombicity is interpreted as resulting from asymmetric cation-anion hydrogen-bonding that is apparent in the crystal structures of these isomorphous compounds.

Oxo-rhenium(V) complexes containing bis(diphenylphosphino)ethane and catecholate ligands

Bis(diphenylphosphino)ethane (dppe) complexes of the type ReOCl(dppe)(O–O), where O–O = catecholate or tetrachlorocatecholate dianion, were prepared by reacting ReOCl 3(dppe) with the catechol ligand in the presence of NEt 3. X-ray diffraction on the tetrachlorocatecholate compound showed that the molecule adopts a distorted octahedral structure, in which the dppe ligand and the bidentate catecholate lie in the equatorial plane, perpendicular to the O Re–Cl unit. In contrast, for ReOCl(PPh 3) 2(O–O), the position trans to the Re O bond is occupied by a catecholate oxygen, whereas the two PPh 3 ligands are trans to one another in the equatorial plane. The UV–Vis absorption spectrum of ReO(OMe)(dppe)(oxalate) is similar to those of ReO(OR)X 2(dppe) compounds, showing two weak bands for the spin-allowed d–d transitions from the filled interaxial d orbital in the xy plane into the inequivalent metal d xz and d yz orbitals, respectively. For the catecholate complexes, the spectra are dominated by charge-transfer transitions from the HOMO π orbital of the catecholate ligand into the d xz and d yz orbitals. Both the singlet–singlet and the singlet–triplet transitions are generally observed. No information could be obtained on the weaker d–d transitions for the catecholate compounds.

Oxo-rhenium(V) complexes with analogs of bis(diphenylphosphino)ethane

Compounds of the types ReOCl 3(L–L) and ReOCl 2(OEt)(L–L) with bis(diphenylphosphino)ethene (dppen) and bis(diphenylphosphino)ferrocene (dppf) were prepared by reacting the diphosphine with ReOCl 3(OPPh 3)(Me 2S). The ReOCl 3(L–L) compound with bis(diphenylarsino)ethane (dpae) was prepared similarly. Crystallographic work on ReOCl 3(dppen) and ReOCl 3(dpae) confirmed the approximate octahedral environment of the metal and the presence of a Cl ligand trans to the Re O bond. The ReOCl 2(OEt)(L–L) compounds contain a transO Re-OEt unit. The visible spectra of both types of compounds include a two-component d–d absorption pattern originating from the spin-allowed excitation of a d electron from the interaxial d orbital in the equatorial plane to the empty d xz and d yz orbitals. For ReOCl 2(OEt)(dppf), these bands are masked by strong transitions taking place in the ferrocene moiety. The absorptions of the other systems are not greatly displaced compared to the similar dppe compounds, a small decrease in the transition energy being noted for the dpae complex.

Mössbauer, Electron Paramagnetic Resonance, and Crystallographic Characterization of a High-Spin Fe(I) Diketiminate Complex with Orbital Degeneracy

The synthesis and X-ray structure of the low-coordinate, high-spin Fe(I) compound LFe(HCCPh) (L = HC(C[tBu]N[2,6-diisopropylphenyl])2]-), 1, are reported. Low-temperature Mössbauer and electron paramagnetic resonance (EPR) spectroscopies reveal that the electronic ground state is a Kramers doublet with uniaxial magnetic properties (effective g values g(x) = 8.9, 0 < g(y), g(z) < 0.3) that is well isolated from the excited states. The observation of a large and positive magnetic hyperfine field, B(int) = +68.8(3) T, demonstrates that the orbital angular moment is essentially unquenched along one spatial direction. Relaxation rates obtained from variable-temperature Mössbauer spectra were fit to an Orbach process, yielding delta = 130-190 cm(-1) for the energy gap ("zero-field splitting") between the two Kramers doublets of the S = 3/2 multiplet. Density functional theory (DFT) and time-dependent DFT calculations give insight into the electronic structures of the ground and excited states. The oxidation state of the iron and the bond order of the phenylacetylene ligand in complex 1 are analyzed using DFT, showing a substantial back-bonding interaction. Spin-orbit coupling acting in the subspace of quasi-degenerate z2 and yz orbitals provides a consistent description of both the zero-field splitting and magnetic hyperfine parameters that fits the EPR and Mössbauer data for 1. Interestingly, the spin-orbit coupling involves the same two orbitals (z2, yz) as in the analogous three-coordinate Fe(II) compounds, because back-bonding significantly lowers the energy of the xy orbital, making it the lowest doubly occupied d orbital. Despite the different oxidation state and different number of atoms in the first coordination sphere, the electronic structure of LFe(I)(HCCPh) can be interpreted similarly to that of three-coordinate Fe(II) diketiminate complexes, but with a substantial effect of back-bonding. To our knowledge, this is the first detailed Mössbauer and EPR study of a structurally characterized high-spin Fe(I) complex.

Electronic structure and ionization energies of palladium and platinum N-heterocyclic carbene complexes

Density functional methods have been used to calculate the geometries, electronic structure and ionization energies (IE) of N-heterocyclic carbene complexes of palladium and platinum, [M(CN2R2C2H2)2](M = Pd, Pt; R = H, Me, Bu t). Agreement with X-ray structures (R = Bu t) was good. Calculated IE agreed well with the photoelectron (PE) spectra (R = Bu t); metal bands were calculated to be within 0.25 eV of the experimental values, whereas the higher lying ligand bands deviated by up to 0.9 eV. Spin-orbit methods were needed to achieve this level of agreement for the Pt complex, but the calculations were found to underestimate the spin-orbit splitting somewhat. The principal metal-ligand bonding is between the carbene lone pair HOMO and a (d(z2)+ s) hybrid on the metal. The metal p(z) orbital contributes very little to the bonding. The metal d(xz,yz) orbitals mix primarily with the filled pi3 orbitals on the ligands and secondarily with the empty pi5 orbitals. Consequently they are little stabilized in comparison to the metal d(xy,x2- y2) orbitals, which are non-bonding in the complex. The first PE band for both the Pd and Pt complexes is from ionization of a (s - d(z2)) hybrid orbital. The IE is greater for Pt than for Pd on account of the post-lanthanide relativistic stabilization of the Pt 6s orbital.

A comparison of the influences of alkoxide and thiolate ligands on the electronic structure and reactivity of molybdenum(3+) and tungsten(3+) complexes. preparation and structures of M(2)(O(T)Bu)(2)(S(t)Bu)(4), [Mo(S(t)Bu)(3)(NO)](2), and W(S(t)Bu)(3)(NO)(py).

M(2)(O(t)Bu)(6) compounds (M = Mo, W) react in hydrocarbon solvents with an excess of (t)BuSH to give M(2)(O(t)Bu)(2)(S(t)Bu)(4), red, air- and temperature-sensitive compounds. (1)H NMR studies reveal the equilibrium M(2)(O(t)Bu)(6) + 4(t)BuSH <==> M(2)(O(t)Bu)(2)(S(t)Bu)(4) + 4(t)BuOH proceeds to the right slowly at 22 degrees C. The intermediates M(2)(O(t)Bu)(4)(S(t)Bu)(2), M(2)(O(t)Bu)(3)(S(t)Bu)(3), and M(2)(O(t)Bu)(5)(S(t)Bu) have been detected. The equilibrium constants show the M-O(t)Bu bonds to be enthalpically favored over the M-S(t)Bu bonds. In contrast to the M(2)(O(t)Bu)(6) compounds, M(2)(O(t)Bu)(2)(S(t)Bu)(4) compounds are inert with respect to the addition of CO, CO(2), ethyne, (t)BuC triple bond CH, MeC triple bond N, and PhC triple bond N. Addition of an excess of (t)BuSH to a hydrocarbon solution of W(2)(O(t)Bu)(6)(mu-CO) leads to the rapid expulsion of CO and subsequent formation of W(2)(O(t)Bu)(2)(S(t)Bu)(4). Addition of an excess of (t)BuSH to hydrocarbon solutions of [Mo(O(t)Bu)(3)(NO)](2) and W(O(t)Bu)(3)(NO)(py) gives the structurally related compounds [Mo(S(t)Bu)(3)(NO)](2) and W(S(t)Bu)(3)(NO)(py), with linear M-N-O moieties and five-coordinate metal atoms. The values of nu(NO) are higher in the related thiolate compounds than in their alkoxide counterparts. The bonding in the model compounds M(2)(EH)(6), M(2)(OH)(2)(EH)(4), (HE)(3)M triple bond CMe, and W(EH)(3)(NO)(NH(3)) and the fragments M(EH)(3), where M = Mo or W and E = O or S, has been examined by DFT B3LYP calculations employing various basis sets including polarization functions for O and S and two different core potentials, LANL2 and relativistic CEP. BLYP calculations were done with ZORA relativistic terms using ADF 2000. The calculations, irrespective of the method used, indicate that the M-O bonds are more ionic than the M-S bonds and that E ppi to M dpi bonding is more important for E = O. The latter raises the M-M pi orbital energies by ca. 1 eV for M(2)(OH)(6) relative to M(2)(SH)(6). For M(EH)(3) fragments, the metal d(xz)(),d(yz)() orbitals are destabilized by OH ppi bonding, and in W(EH)(3)(NO)(NH(3)) the O ppi to M dpi donation enhances W dpi to NO pi* back-bonding. Estimates of the bond strengths for the M triple bond M in M(2)(EH)(6) compounds and M triple bond C in (EH)(3)M triple bond CMe have been obtained. The stronger pi donation of the alkoxide ligands is proposed to enhance back-bonding to the pi* orbitals of alkynes and nitriles and facilitate their reductive cleavage, a reaction that is not observed for their thiolate counterpart.

NMR probe of magnetism and superconductivity in ruthenate oxides

We review recent NMR investigations on superconducting and magnetic properties in layered perovskite Sr 2RuO 4, isostructural to a high- T c cuprate. Measurements of the Knight shift and the nuclear spin-lattice relaxation rate 1/ T 1 have identified its Cooper-pair spin state as a triplet and uncovered pronounced anisotropy in spin-fluctuation spectrum in Sr 2RuO 4. The in-plane low-frequency components of the dynamical susceptibility are exchange-enhanced with a weak q → dependence associated with the two-dimensional (2D) character in electronic structure. By contrast, its out-of-plane component is unexpectedly enhanced by AF spin fluctuations upon cooling below T*∼130 K where the c-axis resistivity shows metallic behavior. We have found that the slightly-distorted cubic perovskite CaRuO 3 is a nearly ferromagnetic metal dominated by ferromagnetic spin fluctuations with a Stoner factor α=0.98. Contrasting behavior of spin fluctuations between Sr 2RuO 4 and CaRuO 3 is due to their different dimensionality in electronic structure with the closeness to ferromagnetism. We remark that a simple ferromagnetic-spin-fluctuation-mediated mechanism with analogy to 3He is not appropriate for an onset of spin-triplet p-wave superconductivity in Sr 2RuO 4. Alternately, we propose possible scenario that the in-plane local exchange interaction, which originates presumably from the Hund’s coupling between the 4 d xy and 4 d xz, yz orbitals, may play a role for the occurrence of spin-triplet p-wave superconductivity.

Ground and Excited State Spectroscopic Probes of [(TMP)Fe(IV)O]+: The First Magnetic Circular Dichroism Study of a Model Peroxidase Compound I Intermediate

MCD spectroscopy has been used to probe the ground and excited state electronic structure of [(TMP)Fe(IV)O]+, in order to obtain additional insight into the electronic structure of enzymic compound I catalytic intermediates. Differences between the MCD of HRP I and the model parallel differences in the exchange interaction. Variable-temperature MCD has determined the magnitude of the ground state zero-field splitting (D3/2 = +5.3 cm-1), which reflects a small energy separation (Δ) between the Fe dxy and dxz,yz orbitals due to Fe dxz,yz → porphyrin pπ* backbonding interactions.

Mechanisms of spin-gap formation due to orbital order for CaV 4O 9

The roles of orbital order in magnetic properties of two-dimensional spin-gap system for CaV 4O 9 are investigated theoretically. We assume that one of the doubly degenerated d xz and d yz orbitals is occupied in d 1 state of V. We consider several models with possible orbital orders. The strengths of the spin-exchange couplings for each model are determined to reproduce the temperature dependence of the uniform magnetic susceptibility in experiments by using the exact diagonalization. By the perturbation expansion, we calculate the energy dispersion of low-lying excitations and the integrated intensity of the neutron inelastic scattering. These quantities sensitively depend on the types of the orbital order. The origin of the spin gap for these models is ascribed to a generalized four-spin singlet. Although none of the models is complete to reproduce all of the reported experimental results, we point out basic mechanisms to reproduce each experimental result.

Mononuclear Molybdenum(IV) Complexes with Two Multiply Bonded Chalcogen Ligands in Trans Configuration and Chelating Biphosphine Ligands.

A series of molybdenum(IV) complexes of the type trans-Mo(Q)(Q')(P&arcraise;P)(2) has been prepared where Q and Q' are chalcogen ligands (O, S, Se, Te) and P&arcraise;P is either cis-1,2-bis(diphenylphosphino)ethylene (dppee) or 1,2-bis(diphenylphosphino)ethane (dppe). X-ray crystallographic studies were carried out to investigate how the Mo-Q distance is influenced by the mutually competitive d(pi)-p(pi) interactions between the p(x)() and p(y)() orbitals of the chalcogen ligands and the d(xz)() and d(yz)() orbitals of the molybdenum center. trans-Mo(O)(2)(dppee)(2) (1) has been prepared by the hydrolysis and deprotonation reaction of [Mo(O)(Cl)(dppee)(2)]Cl with NaOH in methanol. The compounds where Q represents the heavier chalcogens (S, Se, Te) have been prepared by the reaction between trans-Mo(N(2))(2)(P&arcraise;P)(2) and a chalcogen source: trans-Mo(S)(2)(dppee)(2) (2), BzS(3)Bz (dibenzyl trisulfide); trans-Mo(Se)(2)(dppee)(2) (3), elemental Se; trans-Mo(Te)(2)(dppee)(2) (4), TePEt(3) (Et = ethyl); trans-Mo(S)(2)(dppe)(2) (5), BzS(3)Bz; trans-Mo(Se)(2)(dppe)(2) (6), Se. trans-Mo(O)(S)(dppee)(2) (7) was obtained from the reaction of MoCl(3)(THF)(3) (THF = tetrahydrofuran), dppee, and NaHS in a mixture of THF and methanol. The attempted preparation of 7 by the reaction of SO(2) and trans-Mo(N(2))(2)(dppee)(2) yielded Mo(SO(2))(2)(dppee)(2) (8). 1 crystallizes in the monoclinic space group P2(1)/c (No. 14, Z = 2) with a = 11.1340(15) Å, b = 18.435(2) Å, c = 12.515(2) Å, and beta = 110.999(9) degrees; 2 crystallizes in the triclinic space group P&onemacr; (No. 2, Z = 1) with a = 10.102(4) Å, b = 10.722(4) Å, c = 12.195(3) Å, alpha = 100.95(3) degrees, beta = 95.04(4) degrees, and gamma = 117.81(2) degrees; 3 crystallizes in the monoclinic space group P2(1)/c (No. 14, Z = 2) with a = 11.186(5) Å, b = 18.005(8) Å, c = 12.761(9) Å, and beta = 110.35(4) degrees; 4 crystallizes in the triclinic space group P&onemacr; (No. 2, Z = 2) with a = 12.681(4) Å, b = 19.280(5) Å, c = 10.454(3) Å, alpha = 104.60(2) degrees, beta = 111.61(2) degrees, and gamma = 75.12(2) degrees; 5 crystallizes in the monoclinic space group C2/c (No. 15, Z = 8) with a = 49.515(7) Å, b = 10.9286(12) Å, c = 18.203(3) Å, and beta = 98.306(12) degrees; 6 crystallizes in the monoclinic space group C2/c (No. 15, Z = 8) with a = 49.566(9) Å, b = 10.9765(15) Å, c = 18.282(3) Å, and beta = 98.541(13) degrees; 7 crystallizes in the triclinic space group P&onemacr; (No. 2, Z = 1) with a = 10.040(1) Å, b = 10.563(1) Å, c = 12.162(2) Å, alpha = 75.30(1) degrees, beta = 85.93(1) degrees, and gamma = 63.21(1) degrees; 8 crystallizes in the monoclinic space group C2/c (No. 15, Z = 4) with a = 21.534(6) Å, b = 12.4271(13) Å, c = 19.550(5) Å, and beta = 118.480(14) degrees. The UV/vis and the (31)P{(1)H} NMR data for compounds 1-7 are also reported and discussed.

Bonding properties of copper(II)-imidazole chromophores: structures and electronic properties of tetrakis imidazole copper(II) complexes. Molecular structures of Cu( N-methylimidazole) 4(ClO 4) 2 and [Cu( N-methylimidazole) 4(H 2O) 2]Cl 2(H 2O)

The crystal and molecular structures of [Cu(NMIm) 4(ClO 4) 2] ({Bd1}) and [Cu(NMIm) 4(H 2O) 2]Cl 2·(H 2O) ({Bd2}) (NMIm = N-methylimidazole) have been determined from three-dimensional X-ray diffraction data. The structure of {Bd1} consists of discrete centrosymmetric [Cu(NMIm) 4(ClO 4) 2] units having a CuN 4 plane with a set of two trans N-methylimidazole ligands nearly perpendicular to the CuN 4 plane (dihedral angle 87.8°)) and another set approximately parallel (dihedral angle 4.4°), and two perchlorates on the long z-axis with CuO length of 2.718(13) Å. The structure of {Bd2} comprises two discrete structurally similar centrosymmetric [Cu(NMIm) 4(H 2O) 2] cations interconnected with Cl − ions and hydrate molecules by hydrogen bonds. The dihedral angles between the imidazole and the CuN 4 planes are 98.8° and 57.6° for one unit and 97.0° and 50.6° for another. Electronic and EPR spectra are reported for {Bd1} and {Bd2} and also for tetrakis complexes of imidazole, 4-methylimidazole, 4-phenylimidazole and benzimidazole. Analysis of the electronic spectra of {Bd1} and {Bd2} reveals that the former has D 2 h symmetry with non-degenerate d xz and d yz orbitals, and the latter has D 4 h symmetry with degenerate d xz and d yz . This is ascribed to the difference in donor abilities of the axial ClO 4 − and H 2O ligands. In solution, all tetrakis species exhibit different structures depending on the donor abilities of the solvent molecules.

The role of the axial ligand in meso-tetraarylmetalloporphyrin models of the P-450 cytochromes

With iodosobenzene as an oxygen source, MnTPP(Cl), [MnTPP(H 2O) 2]ClO 4, MnTPPNO 3, FeTPP(Cl), [FeTPP(H 2O) 2]ClO 4 and (FeTPP) 2O were used as model P-450 catalysts for the oxidation of cyclohexene in methylene chloride, benzene, benzonitrile and acetonitrile. The effect of added methanol and 4-methylpyridine was also investigated. A surprisingly high (80%) yield of cyclohexene oxide returned by [MnTPP(H 2O) 2]ClO 4 in benzonitrile required a reconsideration of the accepted role of the trans axial ligand in model P-450 reactions. Performance differences between the MnTPP + and FeTPP + derivatives are attributed to differences in metal—porphyrin orbital mixing. It is proposed that D 4 h metal e g (d xz , d yz ) and porphyrin e g (π*) orbital mixing in six-coordinate manganese porphyrins prevents electron loss from the porphyrin a 2 u orbital. Porphyrin ligand oxidation in pentacoordinate manganese complexes is attributed to an unsymmetrical metal—porphyrin interaction. The metal e g (d xz , d yz ) orbitals of six-coordinate iron porphyrin complexes are thought to lie below the porphyrin e g (π*) orbitals, thus preventing a protective orbital interaction. The effects of σ and π charge donation from the trans axial ligand of the model P-450 catalysts are differentiated. It is proposed that σ donation can accelerate oxene transfer from the catalyst to the substrate, while π charge donation can additionally alter the energy of the acceptor orbital on the active catalyst that is responsible for initiating substrate oxidation.

Ruthenium(III) Schiff base chloro and carbonyl complexes: Synthesis, characterization and EPR studies

The synthesis and characterization of stable ruthenium(III) Schiff base chloro and carbonyl complexes with the axial ligands chloro (Cl), imidazole (Im) and 2-methylimidazole (2-MeIm) are reported. The Schiff bases were synthesized by the interaction of naphthaldehyde with amines such as o-phenylenediamine, ethylenediamine, propylenediamine and diethylenetriamine. The geometry around ruthenium in these complexes is psuedooctahedral. The complexes are low-spin 4 d 5 (S = 1 2 ) and display characteristic EPR spectra in the powder samples and in frozen solutions at 298 and 77 K. The EPR data and EHMO calculations on model complexes indicate that the d xy orbital lies above d xz and d yz orbitals.

A theoretical model of the intensity of the near-infrared porphyrin-to-iron charge-transfer transitions in low-spin iron(III) haemoproteins. A correlation between the intensity of the magnetic circular dichroism bands and the rhombic distortion parameter of iron

A theoretical model is presented of the source of the intensity of the porphyrin-to-iron charge-transfer (c.t. ) band of low-spin iron(III) haems in proteins and model complexes. The intensity mechanism involves intensity borrowing via the ‘transfer term’ from the intense visible and Soret bands of the haem spectrum. The experimental evidence suggests that about 5% of the intensity is borrowed. The intensity of the magnetic circular dichroism (m.c.d) spectrum of the c.t. bands at 4.2 K depends upon the ground-state gz value and upon the product ab where a and b are the coefficients of the iron (III) ion d-orbitals 3dxz and 3dyz′ respectively, in the ground-state Kramers doublet. This leads to a dependence of the m.c.d. C-term intensity upon the rhombic distortion, V, experienced by the iron (III) ion. IfV is zero then a=b, and the hole in the t2g5d subshell is equally divided between the two orbitals. The c.t. band is x,y- polarized and the m.c.d, intensity a maximum value. As V tends to a large value either a or b drops to zero, and the hole becomes localized in one of the dxz,yz orbitals. This results in a c.t. transition which is predominantly x- or y-polarized. The m.c.d. intensity drops to zero since a pair of perpendicularly polarized transitions is needed for the absorption of circularly polarized light. Experimental evidence drawn from measurements on a wide range of low-spin iron (III) haemoproteins has been used to substantiate the model. This study shows that the intensity of the near-i.r m.c.d. transition can be a useful qualitative indicator of the magnitude of the rhombic crystal-field component at the iron(III) ion. This in turn allows some conclusions to be drawn about the relative orientation of the axial ligands to iron(III) in bis (histidine) co-ordinated haems.

The band structure of Ni(H 5C 3B 2). An example for energetic stabilization due to dimerization

The band structure of Ni(H 5C 3B 2) the first experimentally verified one-dimensional (1D) polydecker system, has been investigated by means of a semi-empirical crystal orbital (CO) formalism based on an improved INDO (intermediate neglect of differential overlap) hamiltonian. In order to allow for an analysis of the electronic structure of a 1D arrangement composed by unit cells with an ungerade number of electrons a grand canonical (GC) averaging scheme has been used for the definition of the crystal hamiltonian. The synthesized 1D material with 13 valence electrons per simplest stoichiometric unit is a semiconductor and shows nuclear distortions into the direction of the stacking axis (i.e. formation of non-equivalent metal-ligand contacts). The tight-binding calculations lead to a transparent theoretical explanation of this formal dimerization. The 1D arrangement built up by one Ni(H 5C 3B 2) half-sandwich per unit cell reproducing the full translational symmetry is unstable towards a dimerization that leads to a unit which is formed by two half-sandwiches. The energy of the 1D column with the doubled cell is stabilized due to a symmetry-violation of the spatial wavefunction (i.e. symmetry-breaking for a symmetry atomic arrangement). A nuclear distortion (formation of non-equivalent metal-ligand distances) causes an additional energy lowering of the 1D system. The band structure properties in the outer valence region are analyzed. The calculated band gap is in line with he experimentally observed semiconducting properties of the 1D chain. The microstates of the valence band contain both admixtures from the cyclic organic π ligand (leading contribution) and from the transition metal centers (3d xz or 3d yz orbitals).