Tsang Bik Tsin

Tris [2-(2′-pyridyl)benzimidazole]iron(II) complexes. Some new examples of 5T2 — 1A1 spin equilibria

Abstract Mossbauer spectra and magnetic susceptibilities for three iron (II) complexes of 2-(2′-pyridyl)benzimidazole have been measured in the temperature range 80–300°K. The results are consistent with the existence of 5 T 2 — 1 A 1 spin equilibria in all three complexes.

Studies of spin crossover in some tris[2-(2′-pyridyl)benzimidazole] iron(II) complexes

Four complexes of the bidentate chelating ligand 2-(2′-pyridyl)benzimidazole (pbim) are described: [Fe(pbim)3]-Br2; [Fe(pbim)3][NO3]2·OH2; and [Fe(pbim)3][ClO4]2·xOH2(x= 1 or 2). The complexes have been characterized by Mossbauer spectra (4.2–300 K), susceptibility measurements (80–310 K), solid-state (80–300 K) and solution visible spectra (298 K), and conductance measurements and i.r. spectra (295 K). All four complexes exhibit 5T2g–1A1g spin crossover, the details of which are sensitivie to the nature of the anion and, for the perchlorate derivatives, the number of molecules of water of crystrallization. The latter effect is attributed to hydrogen bonding between water molecules and the imino-hydrogen atom of the ligand. From magnetic-perturb-action Mossbauer spectra, it is deduced that the [Fe(pbim)3]2+ cation has a mer-octahedral configuration in both spin states, indicating substantial inequivalence of the pyridine and imidazole nitrogen atoms in the pbim ligand.

Slow spin—lattice relaxation and magnetic hyperfine interactions in hexakis(pyridine-n-oxide)iron(II) perchlorate

Abstract Fe(C 5 H 5 NO) 6 (ClO 4 ) 2 has been studied by 57 Fe Mossbauer spectroscopy at 4.2 K in longitudinal applied magnetic fields ranging from 1 to 50 kG, and the existence of slow spin—lattice relaxation in the complex has been confirmed. The ground spin—orbit-split doublet is effectively isolated from all higher lying levels at 4.2 K and can be treated as a Kramers doublet, using a pseudo-spin ( S = 1/2) hamiltonian. Spectra recorded in fields of 10, 30 and 50 kG can all be fitted in the slow relaxation limit with identical values of the g - and A -tensor components, viz., g ⊥ = 0.6, g | = 9.0; A ⊥ = 1.50 mm s −1 , A | = −1.86 mm s −1 . The 2 axis of the electric field gradient is the easy axis of magnetisation, and the hyperfine field along the axis is 135 kG.

Mössbauer spectroscopic studies of carbonyl haemochromes; correlations with reactivity

Mossbauer spectra of several bis(piperidine) and carbonylpiperidine haemochromes suggest that replacement of the axial amine by carbon monoxide causes pronounced changes in the iron–porphyrin bonding, i.e. causes a substantial cis-effect.

Hyperfine interactions at 57Fe in octaethylhaemin

Abstract Mossbauer measurements on octaethylhaemin have been made between 4.2 and 300 K, and (at the lower temperature) in applied magnetic fields of up to 50 kG. From a spin hamiltonian treatment the saturation value of the hyperfine field at the 57 Fe nucleus is found to be 495 kG, and the splitting parameter for the ferric ion levels by the axial ligand field is D = 8.0 cm −1 . The electric field gradient 1/2 e 2 qQ = +0.93 mm s −1 .

Tin-119 Mössbauer study of complexes with chlorine-bridged tin–molybdenum and tin–tungsten bonds

Tin-119 Mossbauer spectra have been recorded for a number of complexes of the type [L2M(CO)3(SnR3–nCln)Cl](L2=αα′-bipyridyl, o-phenanthroline, dithiahexane; M = Mo or W; R = Me or Ph; n= 1–3), in which the M–Sn bond is bridged by a Cl atom. Measurements in applied magnetic fields of 50 kG show that the quadrupole coupling constant e2qQ is positive and the asymmetry parameter η is small when n= 2 or 3. These facts, together with trends in isomer shift and quadrupole-splitting values, indicate that in these complexes W is a better σ-donor than Mo. For [(bipy)Mo(CO)3(SnPh2Cl)Cl], e2qQ < 0 and η≃ 0·8. It is concluded that the Mo atom and the two C atoms in this complex occupy approximately equatorial positions in the distorted trigonal bipyramidal Sn environment, and the large η is attributed to geometrical constraints imposed by the bridging Cl atom. The Mossbauer spectrum of [(bipy)Mo(CO)3(Snl3)l] suggests partial dissociation of the complex to [(bipy)Mo(CO)3(l)l]and Snl2.