MIXED POLYNUCLEAR COMPLEXES : SYNTHESIS AND SPECTROSCOPY

Ahmad  K.  SAAD

Complexes of the type [M₃ O(OOCCR)₆ L₃]ⁿ⁺ (where R=H, alkyl, or aryl ect.; and  L = monodentate base) are known for a wide variety of transition metals M. The triangular M₃ cluster can consist of identical metals M^III., mixed metals M^III ₂ M’^II,, or mixed valencies M^III ₂M^II. Until recently, the only mixed-metal complexes with oxidation states (III)on both metal atoms were the iron-chromium Fe₂ Cr and FeCr₂ complexes. We have now prepared a series of these complexes containing Cr₂^III Co^{III ,} Fe₂ ^III ,Co^III,Fe₂^III  Mn^III . Here we report the characterization and electrochemical behavior of these complexes. Comparisons of E˚ values provide evidence of stabilization of the mixed valence Fe₂^III  , Fe^II , and Mn₂ ^III, Mn^II. clusters.

The complexes were characterized by elemental analysis and by redox methods to determine the oxidation states of Co^III and Mn^III. Structures were confirmed by vibrational spectroscopy, in comparison with other, established complexes.

Particularly useful features of the IR are the vibrations of the M₃ O units. Under D_3h symmetry (i.e. when the complex is trigonally symmetric) there is only one asymmetric M₃ O stretch which is E´ (doubly degenerate). Under C_2v symmetry, the two modes in E´ separate giving B₂ (asymmetric) and A₁ (symmetric). Moreover, in mixed metal complexes with valencies III, III and II, the splitting between these modes is greater than in mixed metal complexes with valencies III, III, III.

Electrochemical studies have been used to confirm the oxidation states. Cyclic voltammograms usually show quasi-reversible one-electron couples. From the values of Eº , it was concluded that the iron (II) is more stable in the mixed-valence cluster Fe₂^III . Fe^II  than in the mixed-metal Cr₂^III Fe^II, and similarly the manganese (II) in the mixed-valence Mn₂^III Mn^II is more stable than in the mixed-metals Fe₂^III Mn^II and Cr₂^III Mn^II complexes. Studies of reversibility (peak separation versus scan rate) also indicate that it is kinetically more facile to oxidize the mixed valence compared to the mixed metal complexes. Both effects can be explained if it is assumed that there is delocalisation of an extra electron in the triangular unit of the Fe₂^III Fe^II and Mn₂^III Mn^II cluster, whereas in the mixed-metal, the electron is fully localized.

The scope of the thesis was also extended to the use of inelastic neutron scattering vibrational spectroscopy (INS). The main aim of this work was to study the vibrational spectra of some mixed-valence and mixed-metal complexes and to clarify some vibrational modes which have not been understood in the past. The data will later be used to determine the hydrogen atom vibration amplitudes, but in this thesis it has been used in a qualitative manner to obtain assignments .