Exchange Coupling in an Isostructural Series of Face-Sharing Bioctahedral Complexes [LM(II)(&mgr;-X)(3)M(II)L]BPh(4) (M = Mn, Fe, Co, Ni, Zn; X = Cl, Br; L = 1,4,7-Trimethyl-1,4,7-triazacyclononane).