Structural, electronic, and magnetic properties of {Fe[HC(3,5-Me(2)pz)(3)](2)}(BF4)(2) (pz = pyrazolyl): Observation of unusual spin-crossover behavior

The complex {Fe[HC(3,5-Me(2)pz)(3)](2)}(BF4)(2), shown crystallographically to contain a unique octahedral N6Fe site, shows a new type of spin-crossov er behavior. The solid is high-spin from 350 K down to a temperature of 206 K, but below 206 K it undergoes a very abrupt decrease in magnetic moment. Mossbauer studies show conclusively that the magnetic moment change observ ed at 206-200 K arises from the complex changing over a very small temperat ure range from a high-spin state at higher temperatures to a mixture of hig h-spin and low-spin at lower temperatures, a mixture that does not change c omposition as the temperature is lowered to 5 K.