The involvement of metal-to-CO charge transfer and ligand-field excited states in the spectroscopy and photochemistry of mixed-ligand metal carbonyls. A theoretical and spectroscopic study of [W(CO)(4)(1,2-ethylenediamine)] and [W(CO)(4)(N,N'-bis-alkyl-1,4-diazabutadiene)

A new interpretation of the electronic spectroscopy, photochemistry, and photophysics of group 6 metal cis-tetracarbonyls [M(CO)(4)L(2)] is proposed, that is based on an interplay between M --> L and M --> CO MLCT excited states. TD-DFT and resonance Raman spectroscopy show that the lowest allowed electronic transition of [W(CO)(4)(en)] (en = 1,2-ethylenediamine) has a W(CO(eq))(2) --> CO(ax) charge-transfer character, whereby the electron density is transferred from the equatorial W(CO(eq))(2) moiety to pi orbitals of the axial CO ligands, with a net decrease of electron density on the W atom. The lowest, emissive excited state of [W(CO)(4)(en)] was identified as a spin-triplet W(CO(eq))(2) --> CO(ax) CT excited state both computationally and by picosecond time-resolved IR spectroscopy. This state undergoes 1.5 ps vibrational relaxation/solvation and decays to the ground state with a approximately 160 ps lifetime. The nu(CO) wavenumbers and IR intensity pattern calculated by DFT for the triplet W(CO(eq))(2) --> CO(ax) CT excited state match well the experimental time-resolved spectrum. For [W(CO)(4)(R-DAB)] (R-DAB = N,N'-bis-alkyl-1,4-diazabutadiene), the W(CO(eq))(2) --> CO(ax) CT transition follows in energy the W --> DAB MLCT transition, and the emissive W(CO(eq))(2) --> CO(ax) CT triplet state occurs just above the manifold of triplet W --> DAB MLCT states. No LF electronic transitions were calculated to occur in a relevant energetic range for either complex. Molecular orbitals of both complexes are highly delocalized. The 5d(W) character is distributed over many molecular orbitals, while neither of them contains a predominant metal-ligand sigma 5d(W) component, contrary to predictions of the traditional ligand-field approach. The important spectroscopic, photochemical, and photophysical roles of M(CO(eq))(2) --> CO(ax) CT excited states and the limited validity of ligand field arguments can be generalized to other mixed-ligand carbonyl complexes.     

Replacement of 2,2'-bipyridine by 1,4-diazabutadiene acceptor ligands: why the bathochromic shift for [(N empty set N)IrCl(C5Me5)]+ complexes but the hypsochromic shift for (N empty set N)Ir(C5Me5)?

Replacement of 2,2'-bipyridine (bpy) by substituted 1,4-diazabutadiene (R-DAB) alpha-diimine ligands N empty set N leads to a substantial hypsochromic shift of about 0.8 eV for the long-wavelength absorption band in compounds (N empty set N)Ir(C(5)Me(5)) but to a bathochromic absorption shift of about 0.4 eV for the complex ions [(N empty set N)IrCl(C(5)Me(5))](+). DFT calculations on model complexes based on experimental (R-DAB compounds) and geometry-optimized structures (bpy systems) reveal that the low-energy transitions of the cationic chloro complexes are largely of ligand-to-ligand charge-transfer character L'LCT (L = alpha-diimine, L' = Cl) whereas the neutral compounds exhibit pi --> pi transitions between the considerably mixed metal d(pi) and alpha-diimine pi orbitals. The much more pronounced metal-ligand orbital interaction for the R-DAB complexes causes the qualitatively different shifts on replacing the stronger basic bpy by the better pi-acceptors R-DAB. Only the LUMO of the neutral compounds is destabilized on replacement of bpy by R-DAB whereas the LUMO of [(N empty set N)IrCl(C(5)R'(5))](+) and both HOMOs are stabilized through this change.     

Why [(eta5-C5Me(n)H5-n)2Ti]2(micro2,eta2,eta2-N2) can not add a H2 molecule to the side-on-coordinated N2 while its Zr and Hf analogues can? insights from computational studies

The potential energy surface of the reaction [(eta5-C5MenH5-n)2M]2(micro2,eta2,eta2-N2) + H2 --> [(eta5-C5MenH5-n)2M][(eta5-C5MenH5-n)2MH](micro2,eta2,eta2-NNH) at low-lying singlet and triplet electronic states of the reactants was investigated using density functional methods, for n = 0 and 4, and M = Ti, Zr, and Hf. Ground electronic states of the Ti complexes are found to be triplet states, while that for the corresponding Zr and Hf complexes are singlet states. In their singlet state, all these complexes satisfy known necessary conditions (they have a side-on-coordinated N2 molecule and appropriate frontier orbitals) for successful addition of an H2 molecule to the coordinated N2, and consequently, add of an H2 molecule with a reasonable energy barrier. Hf complexes show slightly higher reactivity than corresponding Zr complexes, and in turn, both are more reactive than their singlet-state Ti counterparts. The calculated trend in reactivity of Zr and Hf complexes is consistent with the latest experimental data (see refs 13 and 16). However, Ti complexes have the ground triplet state that lacks in appropriate frontier orbitals. As a result, H2 addition to the Ti complexes at their triplet ground states requires a larger activation barrier than the singlet state and is endothermic (lacks of driven force for reaction). On the basis of these results, we predict that the [(eta5-C5Me4H)2M]2(micro2,eta2,eta2-N2) and [(eta5-C5H5)2M]2(micro2,eta2,eta2-N2) complexes cannot react with an H2 molecule for M = Ti, while those for M = Zr and Hf can. It was shown that the difference in the B3LYP (hybrid) and PBE (nonhybrid) calculated energy gaps between the lowest closed-shell singlet and triplet states of the present complexes reduces via first- > second- > third-row transition metals; both hybrid and nonhybrid density functionals can be safely used to describe reactivity of the low-lying low-spin and high-spin states of second- and third-row transition metal complexes.     

Multifrequency EPR study and density functional g-tensor calculations of persistent organorhenium radical complexes

The dinuclear radical anion complexes [(mu-L)[Re(CO)(3)Cl](2)](*)(-), L = 2,2'-azobispyridine (abpy) and 2,2'-azobis(5-chloropyrimidine) (abcp), were investigated by EPR at 9.5, 94, 230, and 285 GHz (abpy complex) and at 9.5 and 285 GHz (abcp complex). Whereas the X-band measurements yielded only the isotropic metal hyperfine coupling of the (185,187)Re isotopes, the high-frequency EPR experiments in glassy frozen CH(2)Cl(2)/toluene solution revealed the g components. Both the a((185,187)Re) value and the g anisotropy, g(1) - g(3), are larger for the abcp complex, which contains the better pi-accepting bridging ligand. Confirmation for this comes also from IR and UV/vis spectroscopy of the new [(mu-abcp)[Re(CO)(3)Cl](2)](o/)(*)(-)(/2)(-) redox system. The g values are reproduced reasonably well by density functional calculations which confirm higher metal participation at the singly occupied MO and therefore larger contributions from the metal atoms to the g anisotropy in abcp systems compared to abpy complexes. Additional calculations for a series of systems [(mu-abcp)[M(CO)(3)X](2)](*)(-) (M = Tc or Re and X = Cl, and X = F, Cl, or Br with M = Re) provided further insight into the relationship between spin density distribution and g anisotropy.     

An EPR and ENDOR investigation of a series of diazabutadiene-group 13 complexes

Paramagnetic diazabutadienegallium(II or III) complexes, [(Ar-DAB)2Ga] and [{(Ar-DAB*)GaX}2] (X = Br or I; Ar-DAB = {N(Ar)C(H)}2, Ar = 2,6-diisopropylphenyl), have been prepared by reactions of an anionic gallium N-heterocyclic carbene analogue, [K(tmeda)][:Ga(Ar-DAB)], with either "GaI" or [MoBr2(CO)2(PPh3)2]. A related InIII complex, [(Ar-DAB*)InCl2(thf)], has also been prepared. These compounds were characterised by X-ray crystallography and EPR/ENDOR spectroscopy. The EPR spectra of all metal(III) complexes incorporating the Ar-DAB ligand, [(Ar-DAB(.))MX(2)(thf)(n)] (M = Al, Ga or In; X = Cl or I; n = 0 or 1) and [(Ar-DAB)2Ga], confirmed that the unpaired spin density is primarily ligand centred, with weak hyperfine couplings to Al (a = 2.85 G), Ga (a = 17-25 G) or In (a = 26.1 G) nuclei. Changing the N substituents of the diazabutadiene ligand to tert-butyl groups in the gallium complex, [(tBu-DAB*)GaI2] (tBu-DAB={N(tBu)C(H)}2), changes the unpaired electron spin distribution producing 1H and 14N couplings of 1.4 G and 8.62 G, while the aryl-substituted complex, [(Ar-DAB*)GaI2], produces couplings of about 5.0 G. These variations were also manifested in the gallium couplings, namely aGa approximately 1.4 G for [(tBu-DAB*)GaI2] and aGa approximately 25 G for [(Ar-DAB*)GaI2]. The EPR spectra of the gallium(II) and indium(II) diradical complexes, [{(Ar-DAB*)GaBr}2], [{(Ar-DAB*)GaI}2], [{(tBu-DAB*)GaI}2] and [{(Ar-DAB*)InCl}2], revealed doublet ground states, indicating that the Ga-Ga and In-In bonds prevent dipole-dipole coupling of the two unpaired electrons. The EPR spectrum of the previously reported complex, [(Ar-BIAN*)GaI2] (Ar-BIAN = bis(2,6-diisopropylphenylimino)acenaphthene) is also described. The hyperfine tensors for the imine protons, and the aryl and tert-butyl protons were obtained by ENDOR spectroscopy. In [(Ar-DAB*)GaI2], gallium hyperfine and quadrupolar couplings were detected for the first time.     

单硼五元杂环配体(C~4BH~5)过渡金属夹心高聚物的理论研究

用一维自洽场晶体轨道法探讨了过渡金属夹心高聚物[(C~4BH~5)M]~n(M=Cr,Mn,Fe,Co,Ni)的一维链结构和电子结构.计算表明,堆积单元(C~4BH~5)M中价电子数为奇数的高聚物结构会发生Peierls畸变,而价电子数为偶数的将取规则结构,由能带结构得到, [(C~4BH~5)Mn]~n的前线能带出现交叉,可能会发生进一步的结构畸变,但也有可能保持非满状态,成为异体.[(C~4BH~5)Ni]~n可能是不稳定的.其余三个高聚物为半导体或绝缘体,这系列高聚物电荷载流子通路主要依赖于金属d轨道重叠.     

Mono- vs. bi-metallic assembly on a bulky bis(imino)terpyridine framework: a combined experimental and theoretical study

The bis(imino)terpyridine ligands, 6,6'-{(2,6-i-Pr2C6H3)N=CR}2-2,2':6',2'-C15H9N3 (R = H L1, Me L2), have been prepared in high yield from the condensation reaction of the corresponding carbonyl compound with two equivalents of 2,6-diisopropylaniline. The molecular structure of L2 reveals a transoid relationship between the imino and pyridyl nitrogen groups throughout the ligand framework. Treatment of aldimine-containing L1 with one equivalent or an excess of MX2 in n-BuOH at 110 degrees C gives the mononuclear five-coordinate complexes, [(L1)MX2] (M = Fe, X = Cl 1a; M = Ni, X = Br 1b; M = Zn, X = Cl 1c), in which the metal centre occupies the terpyridine cavity and the imino groups pendant. Conversely, reaction of ketimine-containing L2 with excess MX2 in n-BuOH at 110 degrees C affords the binuclear complexes, [(L2)M2X4] (M = Fe, X = Cl 3a; M = Ni, X = Br 3b; M = Zn, X = Cl 3c), in which one metal centre occupies a bidentate pyridylimine cavity while the other a tridentate bipyridylimine cavity. 1H NMR studies on diamagnetic 3c suggests a fluxional process is operational at ambient temperature in which the central pyridine ring undergoes an exchange between metal coordination. Under less forcing conditions (room temperature in dichloromethane), the monometallic counterpart of 1b [(L2)NiBr2] (2b) has been isolated which can be converted to 3b by addition of one equivalent of (DME)NiBr2 (DME = 1,2-dimethoxyethane) in n-BuOH at 110 degrees C. Quantum mechanical calculations (DFT) have been performed on [(L1)ZnCl2] and [(L2)ZnCl2] for different monometallic conformations and show that 1a is the energetically preferred structure for L1 while there is evidence for dynamic behaviour in L2-containing species leading to bimetallic formation. Single-crystal X-ray diffraction studies have been performed on 1a, 1b, 1c, 2b, 3a, 3b(H2O) and 3c.     

Binding and photodissociation of CO in iron(II) complexes for application in positron emission tomography (PET) radiolabelling

(R-DAB)FeI(2) complexes containing bidentate diimide ligands (R-DAB = RN=CH-CH=NR; R = (i)Pr, c-C(6)H(11)) have been investigated for their ability to react with carbon monoxide to form iron(II) dicarbonyl complexes, (R-DAB)FeI(2)(CO)(2). Solution IR spectroscopy revealed two νCO stretches between 2000 and 2040 cm(-1) corresponding to a cis-arrangement of the carbonyl ligands around the iron. Photochemical decarbonylation was achieved by UV irradiation (365 nm), which occurred within 5 min as evidenced by solution IR spectroscopy. (c-C(6)H(11)-DAB)FeI(2) has been characterised by X-ray crystallography. Reactions using (11)C-labelled carbon monoxide were investigated and revealed that both (R-DAB)FeI(2) species were not effective as trapping complexes due to the low concentrations of [(11)C]CO used in these experiments. A Fe(TPP)(THF)(x) (TPP = tetraphenylporphyrin) complex was investigated with unlabelled CO and the monocarbonyl adduct Fe(TPP)(THF)CO was formed in situ as identified by IR spectroscopy (νCO = 1966 cm(-1)) yet was stable to CO loss upon UV irradiation. Carbonylation reactions of in situ-generated Fe(TPP)(THF)(x) using [(11)C]CO revealed that 97% of the [(11)C]CO stream could be trapped in one pass of the gas at room temperature and at atmospheric pressure.     

Inorganic supramolecular host architectures: [(M@18c6)2][TlI4].2H2O, M=0.5 Tl, (NH4,NH3), (H3O,H2O)

The compounds [(M@18c6)2][TlI4].2H2O, M=Tl, (NH4,NH3), (H3O,H2O) (cubic, Fd; a=1481.00 pm, for M=0.5 Tl, a=1304.65 pm for M=(NH4,NH3), a=1313.67 pm for M=(H3O,H2O)) can be obtained from solution in the presence of traces of transition metal halides (like copper and mercury halides). Apparently the transition metal cations work as a template in the form of tetrahedral [MX4] units during the synthesis of the supramolecular host architecture. That the compounds are versatile host lattices for tetrahedrally coordinated transition metal units becomes obvious by the large group of known host-guest complex compounds, [(MIIX4)(MI@18c6)4][TlX4]2.nH2O (MII=Cu, Co, Zn, Mn; MI=NH4+, Rb, Tl; X=Cl, Br).     

Metal Complexes of a Boron‐Dipyrromethene (BODIPY)‐Tagged N‐Heterocyclic Carbene (NHC) as Luminescent Carbon Monoxide Chemodosimeters

Several metal complexes with a boron dipyrromethene (BODIPY)‐functionalized N‐heterocyclic carbene (NHC) ligand 4 were synthesized. The fluorescence in [( 4 )(SIMes)RuCl₂(ind)] complex is quenched (Φ=0.003), it is weak in [( 4 )PdI₂(Clpy)] (Φ=0.033), and strong in [( 4 )AuI] (Φ=0.70). The BODIPY‐tagged complexes can experience pronounced changes in the brightness of the fluorophore upon ligand‐exchange and ligand‐dissociation reactions. Complexes [( 4 )MX(1,5‐cyclooctadiene)] (M=Rh, Ir; X=Cl, I; Φ=0.008–0.016) are converted into strongly fluorescent complexes [( 4 )MX(CO)₂] (Φ=0.53–0.70) upon reaction with carbon monoxide. The unquenching of the Rh and Ir complexes appears to be a consequence of the decreased electron density at Rh or Ir in the carbonyl complexes. In contrast, the substitution of an iodo ligand in [( 4 )AuI] by an electron‐rich thiolate decreases the brightness of the BODIPY fluorophore, rendering the BODIPY as a highly sensitive probe for changes in the coordination sphere of the transition metal.     

Theoretical investigation of paramagnetic diazabutadiene gallium(III)-pnictogen complexes: insights into the interpretation and simulation of electron paramagnetic resonance spectra

The electronic structures and the spin density distributions of the paramagnetic gallium 1,4-diaza(1,3)butadiene (DAB) model systems [((t)Bu-DAB)Ga(I)[Pn(SiH3)2]]* and the related dipnictogen species [((t)Bu-DAB)Ga[Pn(SiH3)2]2]* (Pn = N, P, As) were studied using density functional theory. The calculations demonstrate that all systems share a qualitatively similar electronic structure and are primarily ligand-centered pi-radicals. The calculated electron paramagnetic resonance (EPR) hyperfine coupling constants (HFCCs) for these model systems were optimized using iterative methods and were used to create accurate spectral simulations of the parent radicals [((t)Bu-DAB)Ga(I)[Pn(SiMe3)2]]* (Pn = N, P, or As) and [((t)Bu-DAB)Ga[Pn(SiMe3)2]2]* (Pn = P or As), the EPR spectra of which had not been simulated previously due to their complexity. Excellent agreement was observed between the calculated HFCCs and the optimum values, which can be considered the actual HFCCs for these systems. The computational results also revealed inconsistencies in the published EPR data of some related paramagnetic group 13-DAB complexes.     

Synthesis and physicochemical studies of metal complexes of ferrocene Schiff base derivatives

Two new Schiff bases were prepared by condensing acetylferrocene and 1,1′-diacetylferrocene with S-methyl- dithiocarbazate. Complexes of the two ligands acetylferrocene-1-hydrazono-S-methyldithiocarbazate HmaL and diacetylferrocene-1,1′-dihydrazono-S-methyldithio-carbazate H2daL, with copper(II), nickel(II) and cobalt(II) ions were isolated. The ligands coordinate to the metal ions through either their thioketone or thioenol forms. Both mono- and bis-ligand metal complexes as well as bis-metal complexes of the general formulae: [(H2daL)M]2+, [(daL)M], [(HmaL)2M]2+, [(maL)2M], [(HmaL)2MX2], [(H2daL)M2X4] and [(daL)M2X2] were prepared. All compounds under investigation were characterized and some of their physicochemical properties are reported. This revised version was published online in June 2006 with corrections to the Cover Date.     

A structural and theoretical analysis of transition metalloporphodimethenes and their relationship with metalloporphyrins

The paper reports the synthesis of the first-row transition metal hexaethylporphodimethene derivatives [(Et6N4)M] [M=Mn, 3; M=Co, 5; M = Cu, 7] on a multigram scale, which makes them easily available for reactivity studies. After synthesis they were converted into the corresponding five-coordinate [(Et6,N4)M(L)] [M = Mn, L =THF, 8; M =Co, L = Py, 9] and six-coordinate [(Et6,N4)M(L)2] [M = Mn, L = THF, 10; M = Mn, L = Py, 11] derivatives. The compounds mentioned above and those recently reported, namely the iron and nickel derivatives 4, 6, 12, and 13, permit the presentation of the first coherent report on the structural, optical, magnetic, and electronic characteristics of the first-row transition metal porphodimethene derivatives. The experimental results, coupled with a detailed theoretical analysis (Density Functional Theory, DFT), give the appropriate background for future development of the porphodimethene skeleton, which paves the way from porphyrinogen to porphyrins. In addition, this report, encompassing the entire first row of transition metal ion porphodimethenes, allows a valuable comparison to be made with the corresponding metallated porphyrins, thus establishing the peculiar differences in terms of structural and electronic properties and potential reactivity.     

CaNiN能带结构及其磁性与构型畸变关系的密度泛函 研究

采用密度泛函方法研究了CaNiN的电子结构及其金属性和磁性,结果表明,由于弱的空间偶合作用,该化合物中一唯(1D)NiN^2-链的能带结构(Γ→X)集中反映了CaNiN的二维(2D)以及三维(3D)能带的主要特征,其能带结构揭示了CaNiN的金属性和磁性的本质,将1DNiN^2-链的结果与[MX](M=Pt,Pd,Ni;X=Cl,Br,I)对比,阐述了畸变发生的本质在于空间轨道能量的降低与核间排斥能及电子相互作用能的升高竞争的结果,发现后者占优势则不发生畸变,畸变越小越有利于材料具有顺磁性。     

Low-valent cobalt complexes with three different pi acceptor ligands: experimental and DFT studies of the reduced and the low-lying excited states of (R-DAB)Co(NO)(CO), R-DAB = substituted 1,4-diaza-1,4-butadiene

The complexes (RN=CH-CH=NR)Co(NO)(CO) with R = isopropyl, 2,6-diisopropylphenyl, or p-tolyl are chemically and electrochemically reducible to radical anions at potentials which strongly depend on R. The DFT calculated structure for the neutral compound with R = iPr agrees with the experiment, and the computed structure of the anion radical reveals changes according to a reduction of the R-DAB ligand. EPR results confirm an (R-DAB)-based singly occupied molecular orbital in [(RNCHCHNR)Co(NO)(CO)](.-), with minor but detectable contributions from NO as supported by IR spectroelectrochemistry and as quantified by DFT spin density calculations. The calculations indicate increasingly stabilized CO, NO, and RNCHCHNR pi* acceptor orbitals, in that order. On the basis of TD-DFT (time-dependent density functional theory) calculations, the lowest-lying excited states are assigned to metal-to-(R-DAB) charge transfer transitions while bands due to the metal-to-nitrosyl charge transfer occur at higher energies but still in the visible region. Resonance Raman studies were used to probe these assignments.     

A combined theoretical and experimental study of simple terminal group 6 nitride and phosphide N[triple bond]MX3 and P[triple bond]MX3 molecules

Organometallic complexes containing terminal metal nitrides and phosphides are important synthetic reagents. Laser-ablated group 6 metal atoms react with NF 3, PF 3, and PCl 3 to form the simple lowest energy N[triple bond]MF 3, and P[triple bond]MX 3 products following insertion and halogen transfer, with the exception of P[triple bond]CrF3, which is a higher energy species and is not observed. The E[triple bond]MX3 pnictide metal trihalide molecules are identified from both argon and neon matrix infrared spectra and frequencies calculated by density functional theory and multiconfigurational second-order perturbation theory (CASSCF/CASPT2). These simple terminal nitrides involve strong triple bonds, which range from 2.80 to 2.77 to 2.59 natural bond order for M = W, Mo, and Cr, respectively, as computed by CASSCF/CASPT2, and the M[triple bond]N stretching frequencies also follow this order. The terminal phosphides are weaker with bond orders 2.74, 2.67, and 2.18, respectively, as the more diffuse 3p orbitals are less effective for bonding to the more compact metal valence d orbitals.     

Ligand-bound S = 1/2 FeMo-cofactor of nitrogenase: hyperfine interaction analysis and implication for the central ligand X identity

Broken symmetry density functional theory (BS-DFT) has been used to address the hyperfine parameters of the single atom ligand X, proposed to be coordinated by six iron ions in the center of the paramagnetic FeMo-cofactor (FeMoco) of nitrogenase. Using the X = N alternative, we recently found that any hyperfine signal from X would be small (calculated A(iso)(X = (14)N) = 0.3 MHz) due to both structural and electronic symmetry properties of the [Mo-7Fe-9S- X] FeMoco core in its resting S = 3/2 state. Here, we extend our BS-DFT approach to the 2e(-) reduced S = 1/2 FeMoco state. Alternative substrates coordinated to this FeMoco state effectively perturb the electronic and/or structural symmetry properties of the cofactor. Using an example of an allyl alcohol (H2C=CH-CH2-OH) product ligand, we consider three different binding modes at single iron site and three different BS-DFT spin state structures and show that this binding would enhance the key hyperfine signal A(iso)(X) by at least 1 order of magnitude (3.8 < or = A(iso)(X = (14)N) < or = 14.7 MHz), and this result should not depend strongly on the exact identity of X (nitrogen, carbon, or oxygen). The interstitial atom, when the nucleus has a nonzero magnetic moment, should therefore be observable by ESR methods for some ligand-bound FeMoco states. In addition, our results illustrate structural details and likely spin-coupling patterns for models for early intermediates in the catalytic cycle.     

2,5-Dianilinoterephthalate bridged MM quadruply bonded complexes of molybdenum and tungsten

From the reactions between 2,5-dianilinoterephthalic acid and M2(O2CBut)4 in toluene the dicarboxylate bridged complexes [(ButCO2)3M2]2{micro-1,4-(CO2)(2)-2,5-(NHPh)2C6H2}, (M=Mo) and (M=W) have been isolated. The compounds are air sensitive, sparingly soluble in aromatic hydrocarbons but appreciably soluble in tetrahydrofuran. Electronic structure calculations employing density functional theory on the model compounds [(HCO2)3M2]2{micro-1,4-(CO2)(2)-2,5-(NHPh)2C6H2}, indicate that the ground state structure contains a planar bridge and that for molybdenum the HOMO is a bridge based molecular orbital. However, the compounds show reversible oxidation waves (CV and DPV) that for both M=Mo and W are metal based oxidations. Furthermore, the cations + and + are shown to be valence trapped and fully delocalized respectively. The magnitude of the electronic coupling of the two M2 centers, Hab, can be estimated as 383 cm-1 for + and 1500 cm-1 for + based on the corresponding low energy IVCT or charge resonance bands.     

DFT calculations of EPR parameters in an ionic lattice of [M(CN)4](3-) (M = Ni, Pd, Fe, Ru, Os) complexes

The electronic g-tensor and hyperfine coupling constants were calculated for cyanide coordination complexes [M(CN)4]3- (M = Ni, Pd, Fe, Ru, Os) in KCl or NaCl host lattices through an embedded calculation approach using the Density Functional Theory and compared with previous experiments. For all tested complexes, the B3LYP functional is in good agreement with the experiments for the hyperfine coupling constants. For the electronic g-tensor calculations, performed using the coupled perturbed SCF theory, some discrepancies were found, and the best agreements with the experimental values were achieved by the B3LYP functional.     

Reduced and excited states of the intermediates (alpha-diimine)(C5R5)Rh in hydride transfer catalysis schemes: EPR and resonance Raman spectroscopy, and comparative DFT calculations of Co, Rh and Ir analogues

The electronic structures of the highly air-sensitive intermediates (N[caret]N) (C(5)Me(5))Rh, (N[caret]N = 2,2'-bipyridine (bpy), 2,2'-bipyrimidine (bpym), 2,2'-bipyrazine (bpz) and 3,3'-bipyridazine (bpdz)) of hydride transfer catalysis schemes were studied through resonance Raman (rR) spectroscopy and through EPR of the reduced forms [(N[caret]N) (C(5)Me(5))Rh](.-). The rR results are compatible with a predominant MLCT character of the lowest excited states [ (N[caret]N) (C(5)Me(5))Rh]*, and the EPR spectra of the reduced states reveal the presence of anion radical ligands, (N[caret]N) (.-), coordinated by unusually electron rich rhodium(i) centres. The experimental results, including the assignments of electronic transitions, are supported by DFT calculations for the model compounds [(N[caret]N)(C(5)H(5))Rh](o)/(.-), (N[caret]N) = bpy or bpym. The calculations confirm a significant but not complete mixing of metal and ligand orbitals in the lowest unoccupied MO which still retains about 3/4 pi* (N[caret]N) character. DFT calculations on (bpy)(C(5)H(5))M and [(bpy)(C(5)H(5))ClM](+), M = Co, Rh, Ir, agree with the experimental results such as the differences between the homologues, especially the different LUMO characters of the precursor cations in the case of Co-->d(M)) and Rh or Ir (-->pi*(bpy)).