Superexchange and some molecular orbital calculations of the exchange parameter for copper(II) carboxylate dimers

Abstract

For the copper(@ carboxylate dimer C U ~( R C O ~) ~, the exchange parameter J [ = $(Esinglet-Etrjplet)] is calculated using a simplified INDO molecular orbital procedure with configuration interaction. The primary calcula?ion utilizes ten atomic orbitals, namely the metal-ion 3d,~-~2 orbitals and an sp2 hybrid orbital from each oxygen atom of the four carboxy!ate ligands. Using limited and complete configuration interaction procedures, values of -80 and -130 cin-I are calculated for the exchange parameter. Some lNDO calculations with two rather than four RCO, ligands indicate that overlap between the two oxygen orbitals of each carboxylate ligand, rather thanthecopperdx2-y2 -dx2 -y z 8-typeoverlap, is of primary importance for determining themagnitude of the exchange parameter. This 0-0 overlap becomes effective because the oxygen lone-pair electrons are calculated to delocalize appreciably into the singly-occupied Cu(n) orbitals.For the lowest-energy S = 0 and S = 1 states of the dimer with two carboxylate ligands, the weights for all valence-bond structures are calculated from the complete configuration-interactioii wave-functions. The valence-bond structure with a Cu-Cu &bond in the S = 0 state is found to have a small weight. The importance of the 0-0 overlap for antifcrromagnetism is deduced from valence-bond resonance considerations. 11-1* For valence-bond structures (2)-(4), 3, 1 and 1 other mirror-image structures exist. In fig. 2, the structural weights are for all equivalent structures taken together.

Crystal structure and magnetic properties of catena-.mu.-sulfato-[N,N'-bis(2-hydroxyethyl)dithiooxamido(2-)-N,O,S:N',O',S']bis[aquacopper(II)]: a chain of copper(II) dinuclear units with a 594 cm-1 singlet-triplet separation and a 5.61 .ANG. copper-copper distance

“…[45][46] In the last few years, several orbital models were proposed to describe the mechanism of the exchange interaction in binuclear complexes, of which the metallic centers are paramagnetic. [10][11][12][13][14][15][16][17][18]47 In most of these models, the exchange-interaction parameter J characterizing the singlettriplet separation (/ < 0 when the singlet is the ground state), whatever its sign may be, is interpreted as resulting from an antiferromagnetic component JAF and a ferromagnetic component JF. We shall focus on JAF which is largely dominant in the studied complex.…”

“…In the last few years, several attempts have been made to correlate the magnetic properties of binuclear complexes with structural data.1 (7)(8)(9)(10)(11)(12)(13)(14)(15)(16) The most striking results were obtained with Cu(II) dimers. 17,18 This work is intended to be an experimental and theoretical contribution to the study of the exchange interaction in polynuclear systems.…”