A novel type of heteropolyoxoanion precursors {[Ca(H2O)](6)[P4M6O34](2)}(12-) (M = W-VI, Mo-VI) constructed by two [P4M6O34](12-) subunits via a rare hexa-calcium cluster

A novel type of heteropolyoxoanion (HPOA) precursors {[Ca (H2O)](6)[P4M6O34](2)}(12-) [M = W-VI (1), Mo-VI (2)] has been synthesized and structurally characterized, which represents the rare membership of the HPOA family constituted by two early-transition-metal clusters ([P4M6O34](12-)) and a main-group metal cluster of high nuclearity ([Ca-6](12+)).