| An electrochemical and magnetic moment approach to assessing the degree of electronic and antiferromagnetic exchange coupling respectively in asymmetric dinuclear complexes is outlined. First, the monomers from which these systems are constituted were synthesized by substituting trans ammine in mononuclear [Ru(NH3)5(2,3-Cl2pcyd)][ClO 4]2 with various nitrogen heterocycles of varying sigma-donor strength. Six mononuclear ruthenium(III) and corresponding ruthenium(II) complexes of 2,3-Cl2phenylcyanamide monoanion ligand, trans-[( Ln)Ru(NH3)5-n(2,3-Cl 2pcyd)]m+, were synthesized and characterized, where Ln is 3,5-dimethylpyradine, pyridine, 2,6-dimethylpyrazine, pyrazine, methylpyrazinium cation (n = 1, tetraammines), and 2.2'-bipyridine (n = 2, triammine). The results showed that the metal-ligand coupling measured, H LM increases as energetic matching between metal and ligand orbitals is increased. However, at a certain threshold, when L n is mepyz+, the calculated HLM decreases with better energetic matching between the metal and ligand orbitals. This has been accounted for in terms of an increasing degree of covalency that results in a poor estimate of dipole moment length in the model used to calculate HLM.;Three dinuclear complexes, trans-[LnRu(NH 3)5-n-dicyd-Ru(NH3)5] [PF6]4 where Ln = pyridine, pyrazine (n = 1), and 2,2'-bipyridine (n = 2) were synthesized and characterized via magnetic resonance, electrochemical and infrared and electronic absorption spectroscopy techniques. This data along with that of the analogous symmetric complex, [Ru(NH 3)5-dicyd-Ru(NH3)5][PF6] 4, allowed a test of the predictive capabilities of the CNS theoretical model of-metal-metal coupling. The free energy of resonance exchange and the antiferromagnetic exchange constants were calculated from metal-metal coupling elements derived from the CNS model and the metal-ligand coupling elements derived from Hush-Mulliken model, respectively. These calculated values were compared to the values of the symmetric pentaammine complex studied previously and from the complexes' solvent-dependent, room-temperature magnetic moments. Magnetic data showed that the asymmetric complexes had slightly enhanced coupling compared to the symmetric pentaammine complex. However, the theoretical estimates of the free energy of resonance exchange were in poor agreement with those of the symmetric pentaammine complex and this was suggested to be due to a poor estimate of the transition dipole moment length. |
