When trigonal-bipyramidal clusters, [PPN]-[E2Mn3(CO)(9)] (E = S, Se), were treated with Cr(CO)(6) and PPNCl in a molar ratio of 1:1:2 or 1:2:2 in 4 M KOH/MeCN/MeOH solutions, mono-Cr(CO)(5)-incorporated HE2Mn3-complexes [PPN](2)[HE2Mn3Cr(CO)(14)] (E S, [PPN](2)[1a]; Se, [PPN](2)[1b]), respectively, were formed. X-ray crystallographic analysis showed that la and 1b were isostructural and each displayed an E2Mn3 square-pyramidal core with one of the two basal E atoms externally coordinated with one Cr(CO)(5) group and one Mn-Mn bond bridged by one hydrogen atom. However, when the TMBA(+) salts for [E2Mn3(CO)(9)](-) were mixed with Cr(CO)(6) in a molar ratio of 1:1 in 4 M KOH/MeOH solutions and refluxed at 60 degrees C, mono-Cr(CO)(3)-incorporated E2Mn3Cr octahedral clusters [TMBA](3)[E2Mn3Cr(CO)(12)] (E = S, [TMBA](3)[2a]; Se, [TMBA](3)[2b]), respectively, were obtained. Clusters 2a and 2b were isostructural, and each consisted of an octahedral E2Mn3Cr core, in which each Mn-Mn or Mn-Cr bond of the Mn3Cr plane was semibridged by one carbonyl ligand. Clusters la and 1b (with [TMBA] salts) underwent metal core closure to form octahedral clusters 2a and 2b upon treatment with KOH/MeOH at 60 degrees C. In addition, la and 1b were found to undergo cluster expansion to form di-Cr(CO)(5)-incorporated HE2Mn3-clusters [HE2Mn3Cr2(CO)(19)](2-) (E = S, 3a; Se, 3b), respectively, upon the addition of 1 or 2 equiv of Cr(CO)(6) heated in refluxing CH2Cl2. Clusters 3a and 3b were structurally related to clusters la and 1b, but with the other bare E atom (E = S, 3a; Se, 3b) further externally coordinated with one Cr(CO)(5) group. The nature, cluster transformation, and electrochemical properties of the mixed manganese-chromium carbonyl sulfides and selenides were systematically discussed in terms of the chalcogen elements, the introduced chromium carbonyl group, and the metal skeleton with the aid of molecular calculations at the BP86 level of the density functional theory.