Thomas Schönherr

Elucidation of V(III) complex coordination numbers by electronic spectra

Abstract The ligand field transitions in the optical and near-infrared spectra of V(III) complexes with aminopolycarboxylato ligands (edta, tmdta) are analyzed based on the angular overlap model (AOM) approach. Results from both solid state and solution spectra lead to an explanation of the spectral differences between six- and seven-coordinate complexes, particularly in the near-infrared region. Structural and electronic effects responsible for the preference to a certain geometrical arrangement are discussed.

The role of the π-bonding network for trigonal level splittings of tris-bidentate Cr(acac)3 and Cr(ox) 3 3−

The varying π-bonding contributions in the title compounds caused by the different electronic and molecular structure of the chelate rings are used for explaining the large band splittings in the absorption spectra by trigonal symmetry. It is shown that usual ligand field theory and the angular overlap model are not able to account for the trigonal level splitting of Cr(acac)3 for which the coordination sphere of oxygen atoms is nearly octahedrally arranged. The experimental finding can, however, be rationalized by an extended angular overlap model which considers the phase coupling of π-orbitals in the ligands leading to non-additive contributions to the metal-ligand bond energy.

The unique spectroscopic behavior of the Fe(III)-nitroprusside: a DFT study of the vibronic coupling in the ground and in the lowest excited state

Abstract The adiabatic potential energy surface of the ground state (GS-APES) of the Fe III -nitroprusside anion [Fe(CN) 5 NO] 2− (Fe–NTP) has been studied by means of density functional theory (DFT). A decomposition of the total bonding energy into orbital interaction, electrostatic bonding and exchange (Pauli) repulsion terms (extended transition state method, ETS) has been carried out and the chemical bonding at all stationary points of the ground state potential surface (minima and saddle points) has been characterized. On this basis, the unique topology of the electronic ground state with minima for linear Fe–N–O (ground state, GS) and Fe–O–N (metastable state, MS1) and side-on Fe–(NO) bonded (metastable state, MS2) has been rationalized. The potential energy surface of the lowest excited [3d→π ∗ (NO)] electronic state (ES-APES) with minima at non-linear Fe–N–O and Fe–O–N arrangements has been calculated starting from higher symmetric ( C 4 v ) structures (linear Fe–N–O and Fe–O–N bond) utilizing a 1 E Jahn–Teller coupling mechanism. We found that after geometrical relaxation, the ES-APES crosses the GS-APES in the vicinity of the transition states, thus affecting the photo-isomerisation process. In the light of these results a combined ground and excited state configurational energy diagram is presented and used to discuss the mechanism of photo-isomerisation.

Spectroscopic properties and ligand field parameters of bis(hydrotris(1-pyrazolyl)borato) trivalent metal complexes: anomalous covalency in the metal-pyrazolyl nitrogen bond and energy level splittings due to anisotropic π bonding

Abstract On the basis of UV-Vis absorption, magnetic circular dicroism, luminescence, and multinuclear NMR spectra of bis(hydrotis(1-pyrazolyl) borato) chromium(III) and cobalt(III) complexes, the ligand field parameters were discussed in comparison with those of the corresponding hexaam(m)ine complexes. The trigonal splitting of the quartet excited 4 T 2g and 4 T 18 states and the large separation of the lower frequency doublet states ( 2 E g and 2 T 1g ) were elucidated in terms of the angular overlap model.

Spin-orbit electronic states of octahedral Pt-group d6 complexes as derived from reflectance spectra and ligand field calculations

The 90 K diffuse reflectance spectra of some octahedral Rh(III), Ir(III) and Pt(IV) complex compounds with chloride, thiocyanate and amine ligands are reported. The spectra show in the visible and near ultraviolet distinct bands due to spin-orbit components of singlet-triplet d-d transitions which are explained by ligand field calculations including spin-orbit coupling considering all possible d-electron configurations. Model parameter sets are obtained by fitting the measured band peaks to possible transitions between calculated energy levels. For some of the bands the vibrational structure could be resolved which is assigned to metal-ligand stretching vibrations of the electronically excited complex octahedra with vibrational fundamentals lower than those of the ground state.

Solvatochromism in ligand-field absorption bands and solvent-dependent deuteron nuclear magnetic resonance spectra of [CrF3(tri(2-pyridyl)amine)]: implication of trigonal splitting for the first spin-allowed d–d band and evaluation of π bonding peculiarities

Abstract The correlation of the remarkable solvatochromism and solvent-dependent deuteron NMR spectra of fac-[CrF3(tpa)] (tpa=tri(2-pyridyl)amine) with the acceptor numbers (ANs) for solvents demonstrated the sensitivity of the Crligand bond length or the angular overlap model (AOM) parameters to the embedding solvent, and also suggested a significant trigonal splitting in the first 4T2←4A2 ligand-field band. The inverse correlation of the 2H NMR shifts for 4- and 3-deuterons of the pyridyl in the tpa complex with the solvent electron AN gives direct evidence for the π interaction with the pyridyl groups in the tpa ligand that varies from strong acceptor to weak donor behavior depending upon the solvent.

Crystal structure and ligand field states of [Cr(NH3)5NCO](NO3)2

The crystal structure of [Cr(NH3)5CO](NO3)2 has been determined from three-dimensional X-ray diffractometer data. The compound crystallizes in the tetragonal space group I4mm(C4v9), Z= 2, lattice parameters a = 7.426(4) and c = 11.113(6) A. The cation has 4mm(C4v) site symmetry consistent with a linearly coordinated cyanate group. The chromium ion deviates from the equatorial plane of ammine ligands by 0.06 A toward the cyanato ligand, which was found to coordinate via its nitrogen atom. Structural information and optical spectra were combined in order to rationalize d-d transitions and to determine bonding properties of the NCO− ligand. By using the parametrization of the angular overlap model (AOM), we obtained antibonding parameters eσ=6060 cm−1 and eπ=420 cm−1, respectively, from the fitting of quartet and doublet transitions. In the calculation of energies of the highly resolved doublet states, we evaluated the influence of the second coordination sphere (NO3− anions). The large splitting of the first excited state, 2Eg(Oh), was shown to originate predominantly from a low-symmetry contribution to the interelectronic repulsion, as earlier observed for related halogenopentaammine complexes.

The ligand field potential of the amido group in transition group ion phosphinothioicamido chelates

Abstract The d-d absorption spectra of some tris-phosphinothioicamido complexes, M[et 2 P(S)NR] 3 (M = Ti, Cr and R = alkyl, cyclo-hexyl or phenyl) in toluene solution have been recorded. Low symmetry splittings of spin-allowed transitions calculated using the angular overlap model refer to the presence of meridional isomers. Comparison with the measured spectra indicates a rather strong metal-amido π bond which explains the relatively small cubic ligand field parameter attributed to this group. For band assignments to ligand field states of lower symmetry, only relative values between σ- and π-antibonding parameters are used in the model.

Study on coordination numbers six and seven in V(III) complexes with aquo, cyanide and aminopolycarboxylic ligands by means of the angular overlap model

Abstract Peculiarities due to six- and seven-coordination in V(III) complexes are investigated within the framework of the angular overlap model (AOM). Calculations of orbital schemes and d – d transitions are performed for complexes with aquo, cyanide and aminopolycarboxylic ligands. The competing influence of electronic and geometric effects on the energy level pattern is discussed in view of the derived AOM parameters. A characteristic feature of seven-coordination is the large splitting of the 3 T 1g ground state (in O h notation) that leads to typical absorptions in the near-infrared ([V(edta)H 2 O)] − ), (edta, ethylenediaminetetraacetate) or visible ([V(CN) 7 ] 4− ) spectral region which can be used in particular to distinguish between both coordination numbers in solution spectra. In the pentagonal-bipyramidal [V(CN) 7 ] 4− complex there is an unexpectedly high covalency in the σ -interactions with the axial ligands that conflicts a meaningful parametrization within the ligand field approach.

Pair states and orbital exchange coupling parameters of the dimeric diol complexes [Cr(ox)2(OH)]24− and [Cr(gly)2(OH)]2

Abstract A method for calculating exchange-induced energy-level splittings in polynuclear transition-metal complexes when orbitally unquenched single-ion states are involved is described. The method refers to Diracs permutation operator in the spin space. Using symmetry-adapted wavefunctions, the pair energies are readily derived, often directly from the diagonal matrix elements of the bilinear exchange hamiltonian The theoretical results have been applied to absorption and emission spectra from pure spin-flip transitions of two di-μ-hydroxo bridged chromium(III) complexes containing oxalate and glycine chelate ligands. Low-temperature measurements down to 1.9 K yielded highly resolved spectra, in particular in the region of the 4 A 2g 4 A 2g → 4 A 2g 2 E g transitions. Pair lines due to inequivalent molecules in the crystal lattice of Na 4 [Cr(ox) 2 (OH)] 2 were distinguished by site-selective emission spectroscopy. Complete assignments were derived using spin and symmetry selection rules, and pair energies were calculated in terms of orbital exchange coupling constants J ij , which strictly obey the Goodenough-Kanamori rules. The following parameter values were obtained for the binuclear oxalate (glycinate) complex (in cm −1 ): j 11 = 4 (55), j 12 = − 29 (−150), j 13 = 13 (30), j 33 = −6 (−10). These data show that super-exchange via the bridging OH − ligands dominates direct exchange, resulting m a relatively weak antiferromagnetic coupling for the 4 A 2g 4 A 2g ground state: J gr ab = −0.3 (−4.2) cm −1 . Magneto-structural correlations are found to arise predominantly from the position of the hydrogen atoms within the bridging unit.