Low-spin 1,1′-diphosphametallocenates of chromium and iron

We report two anionic diphosphametallocenates, [K(2.2.2-crypt)][M(PC 4 Me 4 ) 2 ] (M = Cr, 2-Cr ; Fe, 2-Fe ). Both are low-spin ( S = ½) by EPR spectroscopy and SQUID magnetometry. This contrasts the high-spin ( S = &z.frac32;) ferrocenate, [K(2.2.2-crypt)][Fe(C 5 H 2 -1,2,4- t Bu) 2 ] ( 4-Fe ). Quantum chemical calculations suggest this is due to significant differences in ligand field splitting of the d-orbitals which also explain structural features in the 2-M complexes. We report two low-spin diphosphametallocenates [K(2.2.2-crypt)][M(PC 4 Me 4 ) 2 ] (M = Cr, Fe) and propose structural features that allow spin-control in 3d metallocenates.