Abstract: A symmetric 2:1 metal complex represented by the general formula: ##STR1## where R is a hydrogen atom, an alkyl group having from 1 to 10 carbon atoms, an alkoxy group having from 1 to 4 carbon atoms, an alkoxycarbonyl group having from 2 to 5 carbon atoms, an acyl group having from 2 to 5 carbon atoms, an aminocarbonyl group, an alklyaminocarbonyl group having from 2 to 5 carbon atoms, an alkylsulfonyl group having from 1 to 3 carbon atoms, an aminosulfonyl group, an acylamino group having from 2 to 5 carbon atoms, a nitro group, a cyano group or a halogen atom, m is an integer of from 1 to 4, when m is 2 or more, the plurality of R may be the same or different substituents, Q is --NHCO--Y--Z or --CONH--Y--Z (where Y is an alkylene group having from 1 to 4 carbon atoms, and Z is a quaternary ammonium group), M is a chromium atom or a cobalt atom, and X.sup..crclbar. is an anion.