The coordination properties of bisorganophosphonyl and bisorganoarsenyl derivatives of heteropolytungstates towards lanthanides and early-transition-metal cations were studied in organic medium. This new class of vacant polyoxometalate (POM) hybrids proved to conserve coordination sites after the grafting of the {RP=O}2+ or {RAs=O}2+ groups onto the trivacant A,α-[PW9O34]9– and B,α-[AsW9O33]9– platforms. The consequence of the presence of free {P=O} or {As=O} groups is an increase of the hardness of the coordination sites in these anions (according to Pearson's HSAB theory). Although soft divalent transition metal cations were hardly coordinated with these hybrid monovacant POMs, complexes with hard s group (Ca2+), lanthanide (La3+, Ce3+) and d0 early-transition-metal cations were obtained and studied, in particular by single crystal X-ray diffraction. Owing to the possibility of modification of the trivacant platform, the type of the grafted function (phosphonyl or arsenyl) or the organic function R, these systems offer a unique possibility to finely modulate the coordination sphere of the cation.