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This approach was based on J values associated with com- parable Cu¨CN¨CN¨CCu torsional angles in related diazine bridged dinuclear complexes, and published J values observed for similar triangular groups. A fit could not be obtained with any sensible J value, and what was puzzling was that the calculated c M values were much higher than those observed for typical J values which would place the maximum in roughly the right temperature range. This suggested that perhaps the data were faulty, but repeated variable temperature measurements gave the same results. While exchange contributions for the N¨CN bridge are well understood, 17 less appears to be known about NO bridges and their contribution within the central triangle. An examination of a simpler planar dinuclear system with just two m-NO in- plane bridges [Cu 2 (PAHOX) 2 ](ClO 4 ) 2 ] (Scheme 1) revealed that perhaps the values observed for some published oxime bridged triangles exhibiting spin frustration, may not be representative of the case where spin frustration is absent (vide infra). In the PAHOX case very strong antiferromagnetic exchange was observed (J = -550 cm -1 ) associated with the two in-plane NO bridges. This implies that the NO bridge propagates very strong exchange when aligned appropriately with the copper magnetic (d x2 - y2 ) orbitals through efficient magnetic orbital overlap, and suggests that with three such bridges arranged in a similar way, in conjunction with a central m 3 -O bridge, exchange could be very strong within the triangles. |
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