Electronic Properties and Bonding in Transition Metal Complexes: Influence of Pressure

Abstract

Changes undergone by optical and magnetic properties of O_h, T_d and D_4h Transition metal (TM) complexes induced by variations of the metal-ligand distance, R, are explored in this work. In parallel, the corresponding variations experienced by the chemical bonding in the complex are analysed in detail. Particular attention is addressed to TM complexes associated with impurities in insulating lattices with halides or oxygen as ligands. Experimental and theoretical results show that charge transfer (CT) transitions experience a blue shift when R decreases. This fact, which allows one to detect R variations down to ∼0.1 pm, is shown to come mainly from the increase of electrostatic repulsion between metal electrons and anions. By means of this technique it is pointed out that the equatorial distance of an elongated D_4h CuCl₆ ^4– unit can increase when a hydrostatic pressure is applied. Electronic relaxation in the excited state is shown to play a key role for explaining the energy of CT transitions. Particular attention is also paid to the admixture of p and s ligand valence orbitals into antibonding e_g ^*(∼3z²–r²; x²–y²) and t_2g ^*(∼xy; xz; yz) orbitals of octahedral complexes. Although the s-admixture in e_g ^* is much smaller than the p-admixture the former is shown to be much more sensitive to R variations than the latter. As a salient feature it is stressed that the small s-admixture in e_g ^* is mainly responsible for the strong sensitivity of the cubic field splitting parameter,10Dq, to changes of R.