A weak antiferromagnetic interaction between manganese(2+) centers through a TCNQ column: crystal structures and magnetic properties of [MnII(tpa)(TCNQ)(CH3OH)](TCNQ)2.cntdot.CH3CN, [MnII(tpa)(.mu.-O2CCH3)]2(TCNQ)2.cntdot.2CH3CN, and [MnII(tpa)(NCS)2].cntdot.CH3CN (tpa = tris(2-pyridylmethyl)amine)