Density functional theory investigation of Ru(II) and Os(II) asymmetric transfer hydrogenation catalysts.

Transition metal ions have a unique ability to organise and control the steric and electronic effects around a substrate in the active site of a catalyst. We consider half-sandwich Ru(II) (Noyori-type) and Os(II) sulfonyldiamine 16-electron active catalysts [Ru/Os(η6-p-cymene)(TsDPEN-H2)], where TsDPEN is N-tosyl-1,2-diphenylethylenediamine containing S,S or R,R chiral centres, which catalyse the highly efficient asymmetric transfer hydrogenation of aromatic ketones to chiral alcohols using formic acid as a hydride source. We discuss the recognition of the prochiral ketone acetophenone by the catalyst, the protonation of a ligand NH and transfer of hydride from formate to the metal, subsequent transfer of hydride to one enantiotopic face of the ketone, followed by proton transfer from metal-bound NH2, and regeneration of the catalyst. Our DFT calculations illustrate the role of the two chiral carbons on the N,N-chelated sulfonyldiamine ligand, the axial chirality of the π-bonded p-cymene arene, and the chirality of the metal centre. We discuss new features of the mechanism, including how a change in metal chirality of the hydride intermediate dramatically switches p-cymene coordination from η6 to η2. Moreover, the calculations suggest a step-wise mechanism involving substrate docking to the bound amine NH2 followed by hydride transfer prior to protonation of the O-atom of acetophenone and release of the enantio-pure alcohol. This implies that formation and stability of the M-H hydride intermediate is highly dependent on the presence of the protonated amine ligand. The Os(II) catalyst is more stable than the Ru(II) analogue, and these studies illustrate the subtle differences in mechanistic behaviour between these 4d6 and 5d6 second-row and third-row transition metal congeners in group 8 of the periodic table.

Spatiotemporal Studies of the One-Dimensional Coordination Polymer [Fe(ebtz)2 (C2 H5 CN)2 ](BF4 )2 : Tug of War between the Nitrile Reorientation Versus Crystal Lattice as a Tool for Tuning the Spin Crossover Properties*.

Reaction of 1,2-di(tetrazol-2-yl)ethane (ebtz) with Fe(BF4 )2 ⋅6 H2 O in different nitriles yields one-dimensional coordination polymers [Fe(ebtz)2 (RCN)2 ](BF4 )2 ⋅nRCN (n=2 for R=CH3 (1) and n=0 for R=C2 H5 (2) C3 H7 (3), C3 H5 (4), CH2 Cl (5)) exhibiting spin crossover (SCO). SCO in 1 and 3-5 is complete and occurs above 160 K. In 2, it is shifted to lower temperatures and is accompanied by wide hysteresis (T1/2 ↓ =78 K, T1/2 ↑ =123 K) and proceeds extremely slowly. Isothermal (80 K) time-resolved single-crystal X-ray diffraction studies revealed a complex nature for the HS→LS transition in 2. An initial, slow stage is associated with shrinkage of polymeric chains and with reduction of volume at 77 % (in relation to the difference between cell volumes VHS -VLS ) whereas only 16 % of iron(II) ions change spin state. In the second stage, an abrupt SCO occurs, associated with breathing of the crystal lattice along the direction of the Fe-nitrile bonds, while the nitriles reorient. HS→LS switching triggered by light (808 nm) reveals the coupling of spin state and nitrile orientation. The importance of this coupling was confirmed by studies of [Fe(ebtz)2 (C2 H5 CN/C3 H7 CN)2 ](BF4 )2 mixed crystals (2 a, 2 b), showing a shift of T1/2 to higher values and narrowing of the hysteresis loop concomitant with an increase of the fraction of butyronitrile. This increase reduces the capability of nitrile molecules to reorient. Density functional theory (DFT) studies of models of 1-5 suggest a particular possibility of 2 to adopt a low (140-145°) value of its Fe-N-C(propionitrile) angle.

Exploring the Vibrational Side of Spin-Phonon Coupling in Single-Molecule Magnets via 161 Dy Nuclear Resonance Vibrational Spectroscopy.

Synchrotron-based nuclear resonance vibrational spectroscopy (NRVS) using the Mössbauer isotope 161 Dy has been employed for the first time to study the vibrational properties of a single-molecule magnet (SMM) incorporating DyIII , namely [Dy(Cy3 PO)2 (H2 O)5 ]Br3 ⋅2 (Cy3 PO)⋅2 H2 O ⋅2 EtOH. The experimental partial phonon density of states (pDOS), which includes all vibrational modes involving a displacement of the DyIII ion, was reproduced by means of simulations using density functional theory (DFT), enabling the assignment of all intramolecular vibrational modes. This study proves that 161 Dy NRVS is a powerful experimental tool with significant potential to help to clarify the role of phonons in SMMs.

Supramolecular Iron Metallocubanes Exhibiting Site-Selective Thermal and Light-Induced Spin-Crossover.

Treatment of Fe[BF4]2·6H2O with 4,6-di(pyrazol-1-yl)-1H-pyrimid-2-one (HL1) or 4,6-di(4-methylpyrazol-1-yl)-1H-pyrimid-2-one (HL2) affords solvated crystals of [{FeIII(OH2)6}⊂FeII8(µ-L)12][BF4]7 (1, HL = HL1; 2, HL = HL2). The centrosymmetric complexes contain a cubic arrangement of iron(II) centers, with bis-bidentate [L]- ligands bridging the edges of the cube. The encapsulated [Fe(OH2)6]3+ moiety templates the assembly through 12 O-H···O hydrogen bonds to the [L]- hydroxylate groups. All four unique iron(II) ions in the cages are crystallographically high-spin at 250 K, but they undergo a gradual high → low spin-crossover on cooling, which is predominantly centered on one iron(II) site and its symmetry-related congener. This was confirmed by magnetic susceptibility data, light-induced excited spin state trapping (LIESST) effect measurements, and, for 1, Mössbauer spectroscopy and diffuse reflectance data. The clusters are stable in MeCN solution, and 1 remains high-spin above 240 K in that solvent. The cubane assembly was not obtained from reactions using other iron(II) salts or 4,6-di(pyrazol-1-yl)pyrimidine ligands, highlighting the importance of hydrogen bonding in templating the cubane assembly.

Effect of Oxidation and Protonation States on [2Fe-2S] Cluster Nitrosylation Giving {Fe(NO)2}9 Dinitrosyl Iron Complexes (DNICs).

The nitrosylation of biological Fe/S clusters to give protein-bound dinitrosyl iron complexes (DNICs) is physiologically important. Biomimetic studies on the reaction of synthetic [2Fe-2S] clusters with NO have so far been limited to diferric model complexes. This work now compares the nitrosylation of [2Fe-2S] clusters with SN- or NN-chelating benzimidazolate/thiophenolate or bis(benzimidazolate) capping ligands in their diferric (12- and 22-) and mixed-valent (FeIIFeIII, 13-, and 23-) forms. Furthermore, the effect of protonation of the imidazole part of the SN ligand has been probed on both the nitrosylation reaction and properties of the resulting DNIC. The reaction of 12- and 22- with 4 equiv NO yields the new anionic {Fe(NO)2}9 DNICs 3- and 4-, respectively, which have been comprehensively characterized, including X-ray crystallography of their PPN+ salts. Nitrosylation of mixed-valent [2Fe-2S] clusters 13- and 23- first leads to slow oxidation to the corresponding diferric congeners, followed by core degradation and DNIC formation. In the case of 23-, a second diferric intermediate very similar to 22- is detected by UV-vis spectroscopy, but could not be further identified. Nitrosylation of 1H2 gives the neutral, N-protonated DNIC 3H, and acid/base titrations show that interconversion between 3- and 3H is reversible. Peripheral ligand protonation leads to a blue shift of the NO stretching vibrations by about 23 cm-1 and a significant shift of the reduction potential to less negative values (Δ E1/2 = 0.26 V), but no effect on 57Fe Mössbauer parameters is observed. Density functional theory calculations based on the structure of 3- indicate that the electronic ground-state properties of 3- and 3H are similar, although the NO(π*) → Fe 3d π-donation is slightly increased and π-backbonding is slightly decreased upon protonation. As a result, protonation has a significant effect on the NO stretching frequencies, but only minor effects on the Fe-(NO)2 modes. This is confirmed by nuclear inelastic scattering of 3- and 3H, which shows no clear influence of protonation on the energy of the Fe-(NO)2 bending and stretching modes occurring in the range 400-600 cm-1, but characteristic changes below 350 cm-1 that reflect perturbation of free rotary motion of the thiophenolate and benzimidazole ring systems of the capping ligand after N-protonation. These findings add to the understanding of [2Fe-2S] cluster nitrosylation and will help to identify DNICs resulting from the reaction of NO with Fe/S cofactors featuring alternative, proton-responsive histidine ligands such as the Rieske and mitoNEET [2Fe-2S] clusters.

Elucidating the Structural Composition of an Fe-N-C Catalyst by Nuclear- and Electron-Resonance Techniques.

Fe-N-C catalysts are very promising materials for fuel cells and metal-air batteries. This work gives fundamental insights into the structural composition of an Fe-N-C catalyst and highlights the importance of an in-depth characterization. By nuclear- and electron-resonance techniques, we are able to show that even after mild pyrolysis and acid leaching, the catalyst contains considerable fractions of α-iron and, surprisingly, iron oxide. Our work makes it questionable to what extent FeN4 sites can be present in Fe-N-C catalysts prepared by pyrolysis at 900 °C and above. The simulation of the iron partial density of phonon states enables the identification of three FeN4 species in our catalyst, one of them comprising a sixfold coordination with end-on bonded oxygen as one of the axial ligands.

161 Dy Time-Domain Synchrotron Mössbauer Spectroscopy for Investigating Single-Molecule Magnets Incorporating Dy Ions.

Time-domain synchrotron Mössbauer spectroscopy (SMS) based on the Mössbauer effect of 161 Dy has been used to investigate the magnetic properties of a DyIII -based single-molecule magnet (SMM). The magnetic hyperfine field of [Dy(Cy3 PO)2 (H2 O)5 ]Br3 ⋅2 (Cy3 PO)⋅2 H2 O⋅2 EtOH is with B0 =582.3(5) T significantly larger than that of the free-ion DyIII with a 6 H15/2 ground state. This difference is attributed to the influence of the coordinating ligands on the Fermi contact interaction between the s and 4f electrons of the DyIII ion. This study demonstrates that 161 Dy SMS is an effective local probe of the influence of the coordinating ligands on the magnetic structure of Dy-containing compounds.

Nuclear inelastic scattering and density functional theory studies of a one-dimensional spin crossover [Fe(1,2,4-triazole)2(1,2,4-triazolato)](BF4) molecular chain.

Nuclear inelastic scattering (NIS) experiments have been performed in order to study the vibrational dynamics of the low- and high-spin states of the polynuclear 1D spin crossover compound [Fe(1,2,4-triazole)2(1,2,4-triazolato)](BF4) (1). Density functional theory (DFT) calculations using the functional B3LYP* and the basis set CEP-31G for heptameric and nonameric models of the compound yielded the normal vibrations and electronic energies for high-spin and low-spin isomers of three models differing in the distribution of anionic trz- ligands and BF4- anions. On the basis of the obtained energies a structural model with a centrosymmetric Fe(trzH)4(trz-)2 coordination core of the mononuclear unit of the chain is proposed. The obtained distribution of the BF4- counteranions in the proposed structure is similar to that obtained on the basis of X-ray powder diffraction studies by Grossjean et al. (Eur. J. Inorg. Chem., 2013, 796). The NIS data of the system diluted to 10% Fe(ii) content in a 90% Zn(ii) matrix (compound (2)) show a characteristic change of the spectral pattern of the low-spin centres, compared to the low-spin phase of the parent Fe(ii) complex (1). DFT calculations reveal that this is caused by a change of the structure of the neighbours of the low-spin centres. The spectral pattern of the high-spin centres in (2) is within a good approximation identical to that of the high-spin Fe(ii) isomer of (1). The inspection of the molecular orbitals of the monomeric model systems of [Fe(trzH)4(trz-)2] and [Fe(trzH)6], together with calculations of spin transition energies, point towards the importance of an electrostatic effect caused by the negatively charged ligands. This results in the stabilisation of the low-spin state of the complex containing the anionic ligand and shortening of the Fe-N(trz-) compared to the Fe-N(trzH) bond in high-spin, but not in low-spin [Fe(trzH)4(trz-)2].

Two-step spin transition in a 1D Fe(II) 1,2,4-triazole chain compound.

A thermochromic 1D spin crossover coordination (SCO) polymer [Fe(ßAlatrz)3](BF4)2⋅2 H2O (1⋅2 H2O), whose precursor ßAlatrz, (1,2,4-triazol-4-yl-propionate) has been tailored from a ß-amino acid ester is investigated in detail by a set of superconducting quantum interference device (SQUID), (57)Fe Mössbauer, differential scanning calorimetry, infrared, and Raman measurements. An hysteretic abrupt two-step spin crossover (T1/2(↓) = 230 K and T1/2(↑) = 235 K, and T1/2(↓) = 172 K and T1/2(↑) = 188 K, respectively) is registered for the first time for a 1,2,4-triazole-based Fe(II) 1D coordination polymer. The two-step SCO configuration is observed in a 1:2 ratio of low-spin/high-spin in the intermediate phase for a 1D chain. The origin of the stepwise transition was attributed to a distribution of chains of different lengths in 1⋅2 H2O after First Order Reversal Curves (FORC) analyses. A detailed DFT analysis allowed us to propose the normal mode assignment of the Raman peaks in the low-spin and high-spin states of 1⋅2 H2O. Vibrational spectra of 1⋅2 H2O reveal that the BF4(-) anions and water molecules play no significant role on the vibrational properties of the [Fe(ßAlatrz)3](2+) polymeric chains, although non-coordinated water molecules have a dramatic influence on the emergence of a step in the spin transition curve. The dehydrated material [Fe(ßAlatrz)3](BF4)2 (1) reveals indeed a significantly different magnetic behavior with a one-step SCO which was also investigated.

Isoprenoid Biosynthesis in Pathogenic Bacteria: Nuclear Resonance Vibrational Spectroscopy Provides Insight into the Unusual [4Fe-4S] Cluster of the E.†coli LytB/IspH Protein.

The LytB/IspH protein catalyzes the last step of the methylerythritol phosphate (MEP) pathway which is used for the biosynthesis of essential terpenoids in most pathogenic bacteria. Therefore, the MEP pathway is a target for the development of new antimicrobial agents as it is essential for microorganisms, yet absent in humans. Substrate-free LytB has a special [4Fe-4S](2+) cluster with a yet unsolved structure. This motivated us to use synchrotron-based nuclear resonance vibrational spectroscopy (NRVS) in combination with quantum chemical-molecular mechanical (QM/MM) calculations to gain more insight into the structure of substrate-free LytB. The apical iron atom of the [4Fe-4S](2+) is clearly linked to three water molecules. We additionally present NRVS data of LytB bound to its natural substrate, (E)-4-hydroxy-3-methylbut-2-en-1-yl diphosphate (HMBPP) and to the inhibitors (E)-4-amino-3-methylbut-2-en-1-yl diphosphate and (E)-4-mercapto-3-methylbut-2-en-1-yl diphosphate.

Enhancement of spin relaxation in an FeDy2 Fe coordination cluster by magnetic fields.

Two [FeLn2 Fe(µ3 -OH)2 (teg)2 (N3 )2 (C6 H5 COO)4 ] compounds (where Ln=Y(III) and Dy(III) ; teg=triethylene glycol anion) have been synthesized and studied using SQUID and Mössbauer spectroscopy. The magnetic measurements on both compounds indicate dominant antiferromagnetic interactions between the metal centers. Analysis of the (57) Fe Mössbauer spectra complement the ac magnetic susceptibility measurements, which show how a static magnetic field can quench the slow relaxation of magnetization generated by the anisotropic Dy(III) ions.

Quantification of the thermodynamic effects of the low-spin - high-spin interaction in molecular crystals of a mononuclear iron(II) spin crossover complex.

A method is proposed to estimate the energetic and entropic effects of spins of neighbouring molecules on the spin transition of a mononuclear spin crossover (SCO) complex in a molecular crystal. Density functional theory (DFT) methods have been used to model the SCO material [FeII(Lnpdtz)2(NCS)2] (Lnpdtz = 2-naphthyl-5-pyridyl-1,2,4-thiadiazole) exhibiting numerous π-π interactions using a 2D arrangement of 15 molecules. The modelling considers only the effects in the crystallographical ac plane with a particularly pronounced stacking but paves the way for future work with 3D arrangements which are computational much more costly. It involves the optimisation and normal mode calculation of the molecules in a rigid matrix of both low-spin (LS) and high-spin (HS) neighbours. This procedure has been used to calculate the previously defined cooperativity parameter Hcoop (S. Rackwitz, W. Klopper, V. Schünemann and J. A. Wolny, Phys. Chem. Chem. Phys., 2013, 15, 15450). For [FeII(Lnpdtz)2(NCS)] we obtain Hcoop = 11 kJ mol-1, a value which is comparable to those found for 3D polynuclear spin crossover materials. A normal mode analysis of the optimised centrally located molecule indicates that the vibrational entropy of the spin transition is somewhat higher (5 J K-1 mol-1) for the LS to HS transition in the LS matrix than in the HS one. The calculations show that the interactions with the neighbours influence the low-frequency modes with wave numbers <65-70 cm-1. These cause the main difference in the vibrational entropy of the spin transition for the vicinity of high- and low-spin molecules. Furthermore, a deformation of the coordination sphere of the central molecule is observed when the spins of the surrounding centres are switched. This deformation is accompanied by a change in the equatorial Fe-N bond lengths.

Experimental and theoretical evidence for unprecedented strong interactions of gold atoms with boron on boron/sulfur-doped carbon surfaces.

The 16e square-planar bis-thiolato-Au(iii) complexes [AuIII(1,2-dicarba-closo-dodecarborane-1,2-dithiolato)2][NBu4] (Au-1) and [AuIII(4-methyl-1,2-benzenedithiolato)2][NBu4] (Au-2) have been synthesized and fully characterized. Au-1 and Au-2 were encapsulated in the symmetrical triblock copolymer poloxamer (Pluronic®) P123 containing blocks of poly(ethylene oxide) and poly(propylene oxide), giving micelles AuMs-1 and AuMs-2. High electron flux in scanning transmission electron microscopy (STEM) was used to generate single gold atoms and gold nanocrystals on B/S-doped graphitic surfaces, or S-doped amorphous carbon surfaces from AuMs-1 and AuMs-2, respectively. Electron energy loss spectroscopy (EELS) data suggested strong interactions of gold atoms/nanocrystals with boron in the B/S-doped graphitic matrix. Density-functional theory (DFT) calculations, also supported the experimental findings, pointing towards strong Au-B bonds, depending on the charge on the Au-(B-graphene) fragment and the presence of further defects in the graphene lattice.

Quantification of intramolecular cooperativity in polynuclear spin crossover Fe(II) complexes by density functional theory calculations.

The influence of the spin state of nearest neighbours on the spin transition of a given centre has been modelled with density functional theory methods for the linear oligomeric Fe(ii) complexes of 4-amino-1,2,4-triazole ligands. The calculated parameter Hcoop which is equal to the difference in the LS → HS spin transition energy for a centre with two LS and two HS spin nearest neighbours, respectively, was found to be always positive, varying from 15 to 35 kJ mol(-1) depending on the applied model and exchange-correlation functional. On the other hand Hcoop was found to be of negligible value for a model of a linear Fe(ii) complex of the more flexible alkylene-bis-tetrazole ligands. This corresponds well to the observed cooperative behaviour of the spin transition for the complexes of triazole derivatives and the gradual transition for the above bis-tetrazole ligands. The analysis of the bond distances in the optimised structures points towards elongation of the Fe-N bonds of the iron centre upon changing of the spin of the neighbours from low-spin to high-spin. This effect is related to the rigid nature of the bridging triazole ligand and is assumed to be the primary reason for the observed cooperativity. The presence of the next-to-nearest neighbour effects has been inferred. The effect of the softening of the Fe-N bonds in the LS centre distorted by the presence of the HS neighbours is also observed and its entropic effects are discussed.

Ligand strain and conformations in a family of Fe(II) spin crossover hexadentate complexes involving the 2-pyridylmethyl-amino moiety: DFT modelling.

DFT calculations of the mononuclear Fe(II) spin crossover complexes [Fe(L)](2+) (L = ({bis[N-(2-pyridylmethyl)-3-aminopropyl](2-pyridylmethyl)amine})), ({[N-(2-pyridylmethyl)-3-aminopropyl][N-(2-pyridylmethyl)-2-aminoethyl](2-pyridylmethyl)amine}) and ({bis[N-(2-pyridylmethyl)-2-aminoethyl](2-pyridylmethyl)amine}) abbreviated as (66), (56) and (55) have been performed in order to explain the observed spin transition temperature differences. The complexes differ in the size of two chelate rings, revealing two six-membered, one six-membered and one five-membered, and two five membered rings for (66), (56) and (55), respectively. Calculations of the electronic energy differences ΔEel = Eel(HS) - Eel(LS) with the use of the basis set TZVP with B3LYP*, PBE, TPSS and TPSSh functionals reproduced the experimentally observed trends. The best reproduction of bond distances is obtained using the TPSSh functional. The Continuous Shape Measure (CShM) analysis of the optimised structures of all six spin isomers revealed the most significant distortion from the trigonal prism for the low-spin (66) system, which has the lowest spin transition temperature. The corresponding trigonal twist is proposed to be the main cause of releasing strain that is induced by the size of two fused chelate rings. Different conformers of low-spin and high-spin (66) systems were modelled using the TPSSh/TZVP method, including the calculations of transition states of conformational rearrangements in both spin isomers. A normal co-ordinate analysis was performed for all six spin isomers. This allows the assignment of previously reported Raman marker bands to specific modes of the (66) system. The estimate of the vibrational contribution to the spin transition entropy revealed values of 50-60 J K(-1) mol at room temperature for all three complexes.

Heme protonation affects iron-NO binding in the NO transport protein nitrophorin.

The nitrophorins (NPs) comprise an unusual group of heme proteins with stable ferric heme iron nitric oxide (Fe-NO) complexes. They are found in the salivary glands of the blood-sucking kissing bug Rhodnius prolixus, which uses the NPs to transport the highly reactive signaling molecule NO. Nuclear resonance vibrational spectroscopy (NRVS) of both isoform NP2 and a mutant NP2(Leu132Val) show, after addition of NO, a strong structured vibrational band at around 600 cm-1, which is due to modes with significant Fe-NO bending and stretching contribution. Based on a hybrid calculation method, which uses density functional theory and molecular mechanics, it is demonstrated that protonation of the heme carboxyl groups does influence both the vibrational properties of the Fe-NO entity and its electronic ground state. Moreover, heme protonation causes a significant increase of the gap between the highest occupied and lowest unoccupied molecular orbital by almost one order of magnitude leading to a stabilization of the Fe-NO bond.

Nuclear inelastic scattering and Mössbauer spectroscopy as local probes for ligand binding modes and electronic properties in proteins: vibrational behavior of a ferriheme center inside a ß-barrel protein.

In this work, we present a study of the influence of the protein matrix on its ability to tune the binding of small ligands such as NO, cyanide (CN(-)), and histamine to the ferric heme iron center in the NO-storage and -transport protein Nitrophorin 2 (NP2) from the salivary glands of the blood-sucking insect Rhodnius prolixus. Conventional Mössbauer spectroscopy shows a diamagnetic ground state of the NP2-NO complex and Type I and II electronic ground states of the NP2-CN(-) and NP2-histamine complex, respectively. The change in the vibrational signature of the protein upon ligand binding has been monitored by Nuclear Inelastic Scattering (NIS), also called Nuclear Resonant Vibrational Spectroscopy (NRVS). The NIS data thus obtained have also been calculated by quantum mechanical (QM) density functional theory (DFT) coupled with molecular mechanics (MM) methods. The calculations presented here show that the heme ruffling in NP2 is a consequence of the interaction with the protein matrix. Structure optimizations of the heme and its ligands with DFT retain the characteristic saddling and ruffling only if the protein matrix is taken into account. Furthermore, simulations of the NIS data by QM/MM calculations suggest that the pH dependence of the binding of NO, but not of CN(-) and histamine, might be a consequence of the protonation state of the heme carboxyls.

Vibrational properties of the polymeric spin crossover (SCO) Fe(II) complexes [{Fe(4-amino-1,2,4-triazole)3}X2]n: a nuclear inelastic scattering (NIS), Raman and DFT study.

The vibrational properties of the cationic spin crossover (SCO) coordination polymers [{Fe(4-amino-1,2,4-triazole)(3)}(+2)](n) containing the anions chlorine, methanosulfonate and 1-naphthalenesulfonate have been studied via nuclear inelastic scattering of synchrotron radiation (NIS) as well as by Raman spectroscopy. Although the different anions have a strong influence on the spin crossover temperature, they have little effect on the positions of the spin marker bands in the NIS and Raman spectra. By comparing the line positions of the NIS spin marker bands with those observed by Raman spectroscopy, it has been possible to distinguish vibrations symmetry (A(u) or A(g)) because modes of A(u) and A(g) symmetries are NIS active, but only the A(g) modes are Raman active. The normal mode analysis of charge compensated cationic pentameric and hexameric model structures which have been obtained by density functional calculations reproduces the experimentally observed mode frequencies and the geometry optimization reproduces iron-ligand distances reported for these and related SCO coordination complexes. The effect of charge compensation appears to be independent of the choice of the functional and the basis set which shows that DFT calculations using B3LYP in conjunction with the basis set CEP-31G are a time effective approach in order to study vibrational properties of Fe(II) SCO compounds.