Paramagnetic Resonance and Covalent Bonding

IN the course of a programme of investigating paramagnetic resonance in compounds containing 4 d and 5 d transition elements, we have recently found an unusual hyperfine structure in ammonium chloroiridate, which gives information about the bonding in this compound. The essential experimental result is that in a crystal of (NH 4 ) 2 [IrCl 6 ], diluted with the isomorphous diamagnetic platinum salt, the resonance spectrum at a temperature of 20° K. and wave-length of 1 cm. shows a complex hyperfine structure consisting of many lines, of which sixteen equally spaced lines can be resolved in the most favourable directions, with overall separation of about 150 gauss. At first sight one would expect only four lines corresponding to the known nuclear spins ( I = 3/2) of iridium-191 and 193. (The nuclear magnetic moments are approximately equal.) It is possible to explain the presence of the additional lines as arising from the chlorine-35, 37 ( I = 3/2) nuclei, and the detail of the spectrum by assuming that the hyperfine structure of chlorine is one-third of that of the iridium.