Unexpected paramagnetism of mono- and polynuclear 18 valence electron metal carbonyl complexes (CnRn)(CO)2Mk(L), M = chromium, manganese; k = 1, 2, 4. Implications for photoreactivity

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Journal of the American Chemical Society, 114 (14): 5693--5697 (1992)
DOI: 10.1021/ja00040a031


Several mono- and polynuclear complexes of the organometallic 16 valence electron fragments (CuRn)(CO)2M, M = Cr (n = 6) and Mn (n = 5), with conventional two-electron nitrogen-donor ligands L (acceptor-substituted pyridines, h1,2-pyrazine) or with h1,4-tetracyanoethylene (TCNE) show sizable metal-based paramagnetism in the solid state at room temp. In contrast to paramagnetic (C6H6)(CO)2Cr(py), the corresponding complex (C5H5)(CO)2Mn(py) of unsubstituted pyridine and the dinuclear complex (C5H4Me)(CO)2Mn2(m-1,4-Me2P-C6H4-PMe2) of a basic, p-accepting diphosphine exhibit essentially diamagnetic behavior. Magnetic susceptibilities at 300 K can be correlated with ligand \dqstrengths\dq as represented by basicities of L. The susceptibilities of the paramagnetic species were found to decrease considerably at lower temps., indicating a diamagnetic ground state for all compds. The easy accessibility of low-lying magnetic states of integer spin is attributable to the low symmetry of the complexes, to the intrinsically small ligand field splitting of low-valent chromium and manganese, and to the presence of one weak ligand, L. Anal. of the tetranuclear complex (C5Me5)(CO)2Mn4(h4-TCNE) reveals magnetic coupling between the four metal centers but no direct involvement of TCNE as a radical. The measured susceptibilities correlate with the photolability of the compds. and with the small g factors of singly reduced complexes, all effects being caused by the low-lying dz2 orbital. on SciFinder(R)



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