A detailed study of the two-step spin-state conversion in 4,7,10))] is presented with special emphasis on the structural changes evidenced by a multitemperature X-ray crystal and molecular structure study (292, 153, 103 K). The ligand (5-NO2-sal-N(l,4,7,10)) results from the Schiff base condensation of 5-nitrosalicylaldehyde with tetraazadecane in a 2:1 ratio. The

X-ray and neutron diffraction measurements at 1 I O K have been used to determine the deformation electron density for hydrogen peroxide (H202). The oxygen atom has been observed to be in a sp3 hybridization state with one of the lone pairs involved in hydrogen bonding. No charge accumulation has been found in the oxygen-oxygen bond. This is explained as a result of the difference technique used. but does indicate that the transfer of electrons to the binding region is relatively small during bond formation.

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