in 1, there are three further interactions <4.7 a (Fe(1)-Fe(1a) = 4.51 A, Fe(2)-Fe(3a) = Fe(2a)-Fe(3) = 4.66 A); the next shortest distance in 3 (Fig. 2 b) is between Fe(3) and Fe(4) at 4.76 A.The magnetic susceptibilities of I and 2 measured against temperature show the same behavior. The effective magnetic moment per iron atom, per,/Fe, of 1 increases from 4.02 pB at room temperature to a maximum value of 4.81 pB at 5 K.( Fig. 3) The variation of x and p are parallel in 1 and 2; the x,, values are about 6 % higher for 1. The coincidence of the data for 1 and 2 seems to rule out monomeric impurities which can cause large increases in x, and p at low temperatures in compounds that interact antiferromagnetically. Therefore the variation observed is a property of the hexameric ion. This is especially important at low temperature, where a sharp increase in p is observed. 600 7 r5.0 [*] Prof.