Chemistry and Chemical Biology ETDs

Publication Date

9-17-1969

Abstract

A comprehensive experimental and theoretical study of complex compounds of two d6 transition metal ions, rhodium(III) and iridium(III), was made to characterize the electronic transitions responsible for the various bands seen in the absorption and emission spectra and to determine the dependence of the energies of the excited states of a complex ion on the nature of the ligands and the central metal ion. All the complex compounds contained organic ligands—pyridine, 2,2’-bipyridine, or l,l0-phenanthroline—in the principal coordination sphere. Most also had halide ions bonded directly to the central ion. An ancillary study was made of 2,2’,2’’-terpyridine complexes of two d10 ions, zinc(II) and cadmium(II). All complexes were prepared by standard, published techniques and characterized by chemical analysis. The purity of most of the samples was further checked with thin-layer chromatography. Standard techniques were used to record the spectra.

Room temperature absorption spectra were taken in water and, for many complexes, organic solvents. All bands were classified as arising from ligand (pi-pi*), crystal-field (d-d), or charge-transfer (pi-d or d-pi*) transitions from their energy, intensity, and solvent dependences. Low-temperature (77K) luminescence spectra and lifetimes of the emitting states were measured.

Orbital and state diagrams are derived following the ligand-field theory for octahedral and tetragonal symmetries. The ionic crystal-field and covalent molecular-orbital models are discussed.

Semiempirical calculations using the crystal-field model were undertaken and crystal-field parameters were calculated from the experimental spectra. Jφrgensen’s rules for calculating the octahedral splitting parameter 6 (or lODq) and the nephelauxetic parameter􀀧 are discussed. It was found that the d-d luminescence bands display large Stokes shifts which must be included in the calculations. The results indicate that the crystal-field model provides a usable basis for correlat­ing the experimental results on d-d transitions. Predictions of the luminescence properties of other similar complexes are made.

A series of five complexes of both ions containing from one to four pyridine ligands was also treated theoretically using the semiempirical extended-Ruckel molecular-orbital model and the non-iterative method of Hoffmann. The calculations gave relatively good estimates of the energies of ,pi-pi*transitions and of the parameter delta but the energies of charge­transfer transitions were greatly underestimated.

All the complexes studied experimentally gave a unique luminescence from the lowest triplet excited state of the ion. Whenever A was suffici­ently small so that a d-d excited state \ as lowest, broad structureless luminescence spectra from excited crystal-field states were seen for complexes of both metals. For rhodium(III) complexes in which delta values were large, the crystal-field states were raised above the lowest ligand states and structured pi-pi* phosphorescence spectra, similar to those seen for the free ligands, were obtained. One iridium(III) complex displayed a charge-transfer emission.

Spin-orbit effects on the luminescence lifetimes were investi­gated. Large decreases in the lifetime of the excited states which couple with the ground state by spin-forbidden processes were seen whenever the orbitals associated with the emitting state were localized on the central ion.

The solvent dependence of the energy of charge-transfer bands of cis- and trans-tetrachlorodi(pyridine)iridate(III) was studied. Absorptions spectra from both ions gave comparable solvent shifts and linear energy plots versus Kosower's Z-values. These results are discussed in terms of the permanent dipole moment of the ion.

Document Type

Dissertation

Degree Name

Chemistry

Level of Degree

Doctoral

Department Name

Department of Chemistry and Chemical Biology

First Committee Member (Chair)

Glenn Arthur Crosby

Second Committee Member

Guido Herman Daub

Third Committee Member

Raymond N. Castle

Fourth Committee Member

Roy Dudley Caton Jr.

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