Orbital interactions in metal dimer complexes

A molecular orbital analysis shows that the antiferromagnetic contributions to magnetic coupling, favoring a lowspin ground state for a dimer containing two weakly interacting metal centers, can be analyzed in terms of pairwise interactions of dimeric molecular orbitals, with the square of the splitting in energy between the members of a pair being a measure of the stabilization of the low-spin state. The effect of geometrical distortions, electronegativity, and variation of substituents on the magnetic interaction in dimeric systems is examined in detail for singly bridged L,M-X-ML, ( n = 3 , 4 , 5); Cu~C16~and other doubly bridged species where the bridging ligands are halogens, OR, pyridine N-oxides, oxalate, squarate; and the acetate bridged dimers C u ~ ( R C 0 0 ) 4 . The emphasis is on d9 Cu(I1) dimers, but other transition metal systems are also analyzed. Transition metal complexes containing more than one metal atom with unpaired electrons can generally be categorized according to their magnetic behavior into three main groups depending on the strength of the metal-metal interaction. In the noninteracting type the magnetic properties of the dimer (or polymer) a re essentially unchanged from the paramagnetic monomer. In the strongly interacting type formation of relatively strong metal-metal bonds occurs, and the molecule will display simple diamagnetic behavior (for even numbers of electrons). In this paper the properties of weakly interacting metal ions will be investigated. In such compounds this weak coupling between the electrons of the two metal ions leads to low-lying excited states of different spin which can be populated a t thermal energies (SI000 cm-I). The resulting magnetic behavior will be antiferromagnetic or ferromagnetic, depending on whether the low spin (spins paired) or high spin (spins parallel) state is the ground state, respectively. These interactions-often termed superexchange because of the large distances involved (3-5 A) between the metal ions-have been observed in a wide variety of compounds. I 5 In experimental studies the magnetic interaction between spins SA and Sg for atoms A and B is usually written in a form suggested originally by Heisenberg, Dirac, and Van