Antiferromagnetic complexes with a metal—metal bond: XII. Synthesis, molecular structures, and magnetic properties of the clusters (MeC5H4)2Cr2(μ-SCMe3)(μ3-S)2Co(CO)2 and (C5H5)2Cr2(μ-SCMe3)(μ3-S)2Mn(CO)3 with triangular Cr2M metal frames☆