Dimerization of Paramagnetic Trinuclear Complexes by Coordination Geometry Changes Showing Mixed Valency and Significant Antiferromagnetic Coupling through -Pt···Pt- Bonds.

Paramagnetic trinuclear complexes, trans -[Pt 2 M(piam) 4 (NH 3 ) 4 ](ClO 4 ) x ( t -M ; piam = pivalamidate, M = Mn, Fe, Co, Ni, and Cu, x = 2 or 3), aligned as Pt-M-Pt were successfully synthesized and characterized. The dihedral angles between the Pt and M coordination planes in t -M are approximately parallel, showing straight metal-metal bonds with distances of approximately 2.6 Å. Except for t -Fe , the trinuclear complexes are dimerized with close contact (approximately 3.9 Å) between the end Pt atoms to form Pt-M-Pt···Pt-M-Pt alignments with high-spin M(+2) containing five ( t -Mn ), three ( t -Co ), two ( t -Ni ), and one ( t -Cu ) unpaired electrons localized on M atoms. Several physical measurements and calculations revealed that the dimerized structures were maintained in MeCN, where cyclic voltammograms for t -M exhibited two-step oxidation and reduction attributed to Pt-M(+2)-Pt···Pt-M(+2)-Pt ↔ Pt-M(+3)-Pt···Pt-M(+2)-Pt ↔ Pt-M(+3)-Pt···Pt-M(+3)-Pt via mixed-valent states. Magnetic susceptibility measurements for t -M showed antiferromagnetic interaction, t -Mn : J = -0.9 cm -1 , t -Co : J = -3.5 cm -1 , t -Ni : J = -7.3 cm -1 , and t -Cu : J = 0.0 cm -1 , between the two M centers with distances of 9.0 Å through Pt···Pt bonds.