Syntheses, properties and crystal structures of a series of mixtures of dmit and dmise metal complexes (DMIT1,3-DITHIOLE-2-THIONE-4,5-DITHIOLATE, DMISE1,3-DITHIOLE-2-SELONE-4,5-DITHIOLATE). Single-crystal esr study of [Bu4N]2[Cu/Ni(C3S4.4Se0.6)2]