Electron-Spin Exchange Coupling Transmitted by^58,60Ni (<i>I</i>= 0) and⁵⁹Co (<i>I</i>= 7/2): Does the Nuclear Magnetic Moment of the Spacer Atom Show?^,¹

Abstract

The trinuclear complexes {(Me2P-η6-C6H5)2M}2Ni 7•• (M = V), 8 (M = Cr), {(Me2P-η6-C6H5)2M}2CoH 10•• (M = V), 11 (M = Cr), and {(Me2P-η6-C6H5)(η6-C6H6)M}2Ni(CO)2 14•• (M = V) and 15 (M = Cr) have been prepared and characterized by means of X-ray crystal structure analysis in two representative cases (11, 15). The “closed” complexes 7••, 8, 10••, and 11 are semirigid, and the “open” species 14 and 15 are conformationally flexible. Magnetocommunication has been studied for the diradicals 7••, 10••, 14••, and 15•+•+ by fluid solution EPR spectroscopy. Comparison of 7•• with 10•• shows that the magnetic moment of the nucleus 59Co fails to shape the 51V hyperfine structure in any discernible way. The flexibility of 14•• furnishes this complex with the largest exchange coupling constant J in the series and is responsible for a pronounced temperature dependence, as demonstrated by alternating EPR line widths upon lowering the temperature. Isoelectronic replacement of V• by Cr•+ in the terminal sandwich units yielding 15•+•+ profoundly attenuates the exchange coupling. Cyclic voltammetry reveals that the complex 10•• engages in a five-step redox chain 2−|−|0|+|2+|3+. Since the central couple 0|+ represents the oxidation of the spiro cobalt atom, the redox splitting δE1/2(0|−,−|2−) is governed by a neutral spacer and δE1/2(3+|2+,2+|+) by a cationic spacer. The positive charge on the spacer exerts a leveling effect in that δE1/2(reduction) = δE1/2(oxidation), contrary to spacers of constant charge, for which δE1/2(reduction) > δE1/2(oxidation) is commonly observed.