Reaction of Fe3+, M2+, and acetate ions in aqueous solution gives [Fe1I1,Ml1O( MeCO,),( H,O),]-3H,O ( M = Mg, Mn, Co, Ni, or Zn), which o n crystallisation from pyridine (py) is converted into [FeIil,Ml1O( MeCO,),(py),]. Reaction of chromium(ii) acetate with metaI(i1) acetate in pyridine in the presence of air gives [Crlll,MtlO(MeCO,),(py),] ( M = Mg, Co, or Ni), and with [Fellt,M1tO-(MeCO,),(py),] in pyridine in the absence of air gives [Cr111Fe111M110(MeC0,)6(py),] (M = Mn or Fe) and [Cr1I1,Fe1lO( MeCO,),( py),]. The compounds with co-ordinated pyridine have been examined crystallographically and magnetically. The Cr,Mg, Cr,Ni, Fe2Ni, and CrFeNi compounds crystallise in the monoclinic system, space group Cc or C2/c, Z = 4, but the other seven compounds are rhombohedra1 and isomorphous w i t h [M,O(MeCO,),(py),]-py ( M = M n or Fe), indicating that the three metal atoms are crystallographically equivalent as a result of disorder in the molecular orientation. Powder magnetic susceptibilities for the Cr,Co and Cr,Ni compounds(4.2-60 K), for the Fe,Mn and Fe,Ni compounds (4.2-295 K), and for the Fe,Ni(aquo) and Fe,Mg compounds (80-295 K) have been fitted by use of an exchange Hamiltonian t o yield values of the exchange parametersJ for the Cr-Cr, Fe-Fe, Cr-Ni, Fe-Ni, and Fe-Mn interactions, which are discussed in terms of superexchange mechanisms. The values of JFeFe in the Fe,Mg, Fe,Mn, and Fe,Ni compounds and of JCrCr in the Cr,Co and Cr,Ni compounds are approximately twice their values in the cations [M1I1,O(MeCO,),( H,O),] + ( M = Cr or Fe), indicating that the p 3 -0 atom provides the main superexchange pathway. Diffuse-reflectance and solution spectra ( 6 0 0 0 4 0 000 cm-') of the compounds have been recorded a t room temperature and are discussed in terms of the ligand-field model; a band in the spectra of the Fe1I1,Fe1l and Cr1l1Fe1I1Fel1 compounds at ca. 7 000 cm-' is assigned t o intervalence transfer. The room-temperature spectra of [ M 3 0 ( MeCO,),L,] CI (M, = Cr1I1,, Cr1It 2 ' Fell1 CrillFelll,, or FeIt1,; L = H, O or py) have also been obtained, and intense absorption bands at ca. 19 000 and 26 000 cm-' are tentatively assigned t o simultaneous (Cr3+, Fe3+) double excitations.
