High-spin complexes magnetic moments, 701

Pentadentate macrocyclic ligands L13-L15 accommodate Ni(II) ion with simultaneous dissociation of the two amide protons to form the square-pyramidal high-spin complex of [NiH 2L]°, 27. These complexes are air-sensitive and show a very low Ni(II)/Ni(III) oxidation potential of +0.24 V vs SCE (72, 73). They form 1 1 dioxygen adducts, 27-02, at room temperature in aqueous solution, which are formulated as Nilu-02 based on the EPR spectral data. The magnetic moment of the 1 1 02 adduct is 2.83 BM, which is interpreted in terms of weak interactions of Ni(III) with the superoxide where the spin coupling is weak. The oxygen uptake reaction is first-order with respect to both [02] and [NiH 2L]° in aqueous solutions and yields a second-order rate constant... 

Two distinct gronps of complexes can be distingnished on the basis of experimental paramagnetic susceptibilities. High-spin complexes are those for which the effective magnetic moment is very close to the spin-only valne for the free (gaseous) transition ion. Low-spin complexes have mnch lower moments than would be predicted for the free metal ion and can even be diamagnetic. 

State of the free ion has no orbital angnlar momentnm and no effective conpling to excited states. Thus the magnetic moment of their high-spin complexes should be very close to the spin-only value. Comment on the spin type and ligand-field strength of each complex studied. 

The observed effective magnetic moments (43-46) for the 5- and 6-coordinated complexes found to have sextet ground states are all in the range of 5.9-6 i typical of high-spin complexes. The calculated AEqS for the high-spin state of these complexes are also in good agreement with the experimental values known for three of them (44-46, 50). 

Effective magnetic moment measurements of [Fe(TPP) (CIO ) ] ( ) have yielded values in the range of 4.5-5.3 at 77-300°K. This temperature dependence and range of values is consistent with contributions from sextet and quartet states with the quartet lower in energy. ESR data ( ) for this complex yielded values for g and g of 4.75 and 2.03, respectively. These results are atypical for a high-spin complex and lend further support to the conclusion that the ground state is a S = 3/2 or a 3/2,5/2 mixture with predominant S = 3/2 character. 

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