The controlled decomposition of the sandwich-type polyoxometalates K-12[(M(OH2)(2))(3)(A-alpha-PW9O34)(2)] (where M = Mn(II) or Co(II)) in 0.5 M NaCl yields a new family of transition metal substituted POMs of the general formula [((MOH2)M2PW9O34)(2)(PW6O26)](17-) (where M = Mn(II) (1Mn) or Co(II) (1Co)). The structure of 1Mn, determined by single-crystal X-ray diffraction (a = 17.4682(10) Angstrom, b = 22.3071(12) Angstrom, c = 35.1195(18) Angstrom, beta = 95.898(1)degrees, monoclinic, P2(1)/c(,) Z = 4, R-1 = 6.19%, based on 50264 independent reflections), consists of two B-alpha-((MnOH2)-O-II)-(Mn2PW9O343-)-P-II units joined by a B-type hexavacant PW6O2611- fragment to form a C-shaped polyoxometalate. A low resolution X-ray structure of the Co(II) analogue, 1Co, was also obtained. The UV-visible spectrum of 1Co shows the characteristic charge-transfer bands of polyoxometalates as well as a new Co-centered peak (560 nm, epsilon = 416 M-1 cm(-1)) which appears at a higher wavelength relative to that exhibited by the parent A-type sandwich, K-12[(Co(OH2)(2))(3)(A-alpha-PW9O34)(2)]. The methyltricaprylammonium salt of 1Mn is an effective catalyst for the H2O2-based epoxidation of cis-cyclooctene, cyclohexene, and 1-hexene.