E. König

Nature and dynamics of the spin-state interconversion in metal complexes

Spin-state transitions in metal complexes which are driven by a change of temperature T or pressure p are always associated with a considerable reorganization of molecular geometry. The change involves metal-ligand bond lengths R, bond angles, and a variation of ligand orientation. In particular, the elongation 4R by up to ∼ 10% occurring in the course of the LS → HS conversion produces an expansion of molecular volume ΔV ≌ 25 A3 per metal atom. The average crystal structure for given values of T and p is reproduced by the fractional occupancy of the individual structures of the high-spin (HS) and low-spin (LS) isomer. The transitions are reasonably well described by a number of theoretical models which are equivalent to the Bragg and Williams approximation of the Ising model. The dynamics of the spin-state transitions in solution, based on measurements by ultrasonic and photo-perturbation techniques, is in general rapid with rate constants between 4 × 105 and 3 × 108 s−1. Similar results are obtained for the spin conversion in solid complexes where the line shape analysis of Mossbauer spectra based on the theory of Blume and Tjon is applied. The dynamics may be rationalized employing one-dimensional cross sections through Gibbs free-energy surfaces G = G(R), an alternative being the comparison of the results with quantum-mechanical calculations for a radiationless non-adiabatic multiphonon process.

The FeN bond lengths, the “ionic radii” of iron (II), and the crystal field parameters (10Dq) in a high-spin and low-spin [FeII-N6] complex

Preliminary but statistically significant results on the different Feue5f8N bond lengths R in a high-spin and low-spin form of Fe(bipy)2(NCS)2 are reported (bipy = 2, 2′-bipyridyl). The “inoic radius” of the low-spin Fe2+ ion is estimated. The relation between 10Dq and R−5 is satisfied within 2.6%.

The lower excited electronic states of singly and doubly reduced 2,2′-bipyridine

Abstract The electronic spectra of bipy− and bipy2− anions (bipy = 2,2′-bipyridine) have been measured between 5000 and 50 000 cm−1. The observed bands which have intensities between f = 6.30 × 10−2 and f = 0.52 are assigned on the basis of HMO and Pariser-Parr calculations.

Optical spectra and magnetism between 4.2 and 300 K for some alkali metal and alkaline earth metal uranates(V), neptunates(VI) and a plutonate(VII)

Abstract Optical spectra and the magnetism between 4.2 and 300 K of the uranates(V) Li7UO6, Li3UO4, LiUO3, NaUO3, KUO3, the neptunates(VI) Ba3NpO6, Li4NpO5, Na2NpO4 and the plutanate(VII) Li5PuO6 are reported. The spectra were assigned to transitions between the ground state Γ7(a2) and the excited states Γ8(t2), Γ7′(t2), Γ8′(t1) and Γ6(t1) of the (Sf)1 configuration in a cubic or tetragonal symmetry. The magnetic susceptibility as a function of temperature was calculated, in conjunction with the spectra, on the basis of a ligand field of cubic or tetragonal symmetry. For Li7UO6, e.g., the physical data are well reproduced, assuming a slight tetragonal distortion, with the parameter values Δ = 4604, Θ = 7099, δ = 145, ξ = 1870 cm−, k = 1.0, k′ = 0.85 in combination with a correction term TIP* = 10 × 10−6 cgs mol−1 for the magnetic susceptibility. Similar results were obtained for the other uranates(V) as well as for Ba3NpO6. For Li4NpO5, the parameter values Δ = 9141, Θ = 3527, δ = 800, ξ = 1800 cm−1, k = 0.80, k = 1.0 were employed in conjunction with TIP* = 125 × 10−6 egs mol−1. An analogous approach was followed for Na2NpO4 and Li5PuO6. A magnetic transition to long-range order of the ferromagnetic type has been observed for LiUO3 at Tc = 16.9 K and for NaUO3 at Tc = 31.1 K. Anomalous magnetic behaviour at low temperature is also indicated for KUO3, Li4NpO5 and Na2NpO4.

The effect of pressure on the thermal hysteresis of the first‐order spin transition in bis(1,10‐phenanthroline‐2‐carbaldehyde phenylhydrazone) iron (II) complexes

The effect of pressure on the thermal hysteresis of the high‐spin (5T2)u2009∏u2009low‐spin (1A1) transition in [Fe(phy)2](BF4)2 and [Fe(phy)2](ClO4)2 has been studied (phy=1, 10‐phenanthroline‐2‐carbaldehyde phenylhydrazone). An increase of the hysteresis width ΔTc as well as of the residual fractions nres5T2 and (1−n5T2)res with pressure is observed. The quantity (ΔTc)1/2 shows a linear dependence on pressure p as expected from the Landau theory. The increase of the residual fractions is explained by the assumption of decreasing size of the cooperative region with pressure.

The high-spin(5T2)⇌low-spin(1A1) transition in solid dithiocyanatobis(2,2′-bi-2,thiazoline)iron(II). Hysteresis effects, debye-waller factors and crystallographic changes

Abstract The almost discontinuous high-spin (5T2) ⇌ low-spin(1A1) transition in solid [Fe(bt)2(NCS)2] (bt = 2,2′-bi-2-thiazoline) has been studied in detail by 57Fe Mossbauer spectroscopy and X-ray diffraction. At the transition temperature Tc, the ground states involved are characterized by ΔEQ(5T2) = 3.00 mm s−1, δIS(5T2) = +0.97 mm s−1 and ΔEQ(1A1) = 0.54 mm s−1, δIS(1A1) = +0.30 mm s−1. A pronounced hysteresis of ΔTc = 9.5 K has been observed, the transition being centered on T>c = 181.86 K for rising and on T>c = 172.33 K for falling temperatures. The observations are compared with a Bragg and Williams tyupe theory and with the independent domain model. Debye-Waller factors show at Tc a discontinuity of Δftotal∼35.4% corresponding to the change between −ln fsT2 and −ln f1A1. The X-ray diffraction patterns for the 25T2 and 1A1 phases show characteristics differences. The temperature dependence for tsT2/ttotal of the Mossbauer effect and for the relative intensities tsT2/Itotal of individual peak profiles of X-ray diffraction is equivalent. The observations provide evidence for a crystallographic phase change which is associated with the (S = 2) ⇌ (S = 0 spin transformation. The existence of crystal domains of finite dimension is required for both 5T2 and 1A1 ground state molecules.

Magnetism between 0.90 and 300 k and complete ligand field theory for chromocene and cobaltocene

Abstract The magnetic behaviour of chromocene, Cr(C 5 H 5 ),and cobaltocene, Co(C 5 H 5 ), has been studied between O.90 and 300 K with particular emphasis on the cryogenic temperature range. The data are reproduced employing complete ligand field theory in slightly distorted C 8V symmetry. For chromocene, the resulting parameters are Dt = 1125, Ds = −4214, Δ 2 = 280, ζ = 67, B = 553 cm −1 , C/B = 4.0 and k = 0.37. For cobaltocene, Dt = 2210, Ds = −4165, Δ 1 = 127, ζ = 73, B = 446, C = 2340 cm −1 and k = 0.34 have been obtained. Here Δ 1 and Δ 2 are parameters responsible for the C 2v distortion. Optical spectra and g values are equally well accounted for. The analogy of both systems is discussed.

Thermally induced incomplete high-spin (5T2) ⇌ low-spin (1A1) transition in solid bis[2-(2-pyridylamino)-4-(2-pyridyl) thiazolato]iron (II)

Abstract The 57Fe Mossbauer effect in two samples (A and B) of [Fe(papt)2] and in its solvates with CHCl3 and C6H6 has been studied between 4.2 and 343 K and clearly indicates a temperature induced high-spin (5T2) ⇌ low-spin (1A1) transition in these compounds [paptH = 2-(2-pyridylamino)-4-(2-pyridyl) thiazole]. At 343 K, sample B shows a doublet with ΔEQ = 2.03 mm s−1 and δIS = +0.87 mm s−1, characteristic of a 5T2 ground state. At 257 K, a second doublet, typical for a 1A1 ground state, is observed and its intensity increases as the transition progresses but levels off below ∼ 100 K. At 4.2 K, 83% of the intensity is due to the 1A1 state, and ΔEQ(1A1) = 1.56 mm s−1 and δIS(1A1 = +0.32 mm s−1. In an applied magnetic field, Vzz(1A1) 0, η ≈ 0.75, and an internal hyperfine field Hn ≈ −13 kG have been observed. Similar results have been obtained with the other samples. Debye-Waller factors f5T2 and f1A1 were determined from the saturation corrected areas in the Mossbauer spectra, assuming Curie-Weiss dependence of the magnetic susceptibility for the 5T2 and constant υcff for the 1A1 ground state. The temperature dependence of −In f1A1 closely follows the Debye model with Θ1A1 = 165 K, whereas the same applies to −ln f5T2 only above ∼ 210 K and Θ5T2 = 134 K. The nature of the observed transition is discussed and the data presented are shown to be incompatible with a model based on a Boltzmann distribution between the two states.

Interpretation der Absorptionsspektren der Komplexionen [Mo(CN)8]4−, [Mo(CN)8]3−, [W(CN)8]4− und [W(CN)8]3−

ZusammenfassungDie Absorptions- und Reflexionsspektren der Oktocyanokomplexe desMo(IV) undW(IV) sowie die Absorptionsspektren der Oktocyanotomplexe desMo(V) undW(V) werden mitgeteilt. Die Spektren werden unter Zugrundelegung der durch Raman- und IR-spektroskopische Untersuchungen gefordertenD4d-Symmetrie dieser Verbindungen interpretiert. Die beobachteten Banden niedriger Intensität (logε<3) werden Übergängen in einem Termsystem zugeordnet, das für die Konfigurationend2 undd1 und die SymmetrieD4d berechnet worden ist. Banden hoher Intensität (logε>3) werden auf Übergänge in antibindende Zustände zurückgeführt, an denen höherep-Zustände des Zentralions sowie Ligandenzustände beteiligt sind. Die erhaltenen Werte des Feldparameters stimmen mit ligandenfeldtheoretischen Erwartungen überein.AbstractAbsorption and reflection spectra of the octacyanides ofMo(IV) andW(IV) and the absorption spectra of the octacyanides ofMo(V) andW(V) are presented. The spectra are interpreted in terms of theD4d symmetry of the compounds supported by investigations of Raman and infrared spectra. Bands of low intensity (logε<3) correspond to transitions between levels obtained in the case of the configurationsd2 andd1 respectively, in a field ofD4d symmetry. Bands of high intensity (logε>3) are attributed to transitions into antibonding levels in which p-orbitals of the central ion and ligand orbitals participate. The values of the field parameter obtained are in accord with ligand field theory.RésuméLes spectres dabsorption et de réflexion des complexes octocyanurés duMo(IV) et duW(IV) ainsi que les spectres dabsorption des mêmes complexes deMo(V) et de W(V) sont présentés. Les spectres sont interprétés en supposant la symétrieD4d des molécules indiquée par des analyses des spectres Raman et infrarouges. Les bandes de faible intensité (logε<3) sont attribuées à des transitions dans un système de niveaux, calculé pour les configurationsd2 etd1, respectivement, en symétrieD4d. Des bandes de forte intensité (logε>3) sont attribuées à des transitions vers des niveaux antiliants auxquels participent des fonctions élevéesp de lion central et des fonctions des groupes liés. Les valeurs obtenues pour le paramètre de champ sont en accord avec les prévisions de la théorie.

Magnetic properties of the quasi one-dimensional Heisenberg linear chain antiferromagnet: manganocene

Abstract Magnetic susceptibility measurements on the rhombic form of manganocene, Mn(C5H5)2, are reported between 0.94 and 297.5 K. A pronouced minimum in the 1/xcorrm versus T curve is located at Tmin = 141 ± 1 K. The results are well reproduced by the solution of Fisher to the quasi one-dimensional Heisenberg linear chain modified such as to include interchain interaction for J/k = −14 K, g = 2x = −5 K. A less accurate approximation to the experimental results has been achieved in terms of the pair model (J/k = −22.5 K, g = 2.0). The results are in excellent agreement with the zigzag chain structure of C5H5ue5f8Mn units reported for the compound.