The reaction of (eta(5)-C5Me5)TaMe4 with tert-butylphosphonic acid leads to the formation of a mixture of compounds: { [(eta(5)-C5Me5)TaMe][t-BuP(O)(OH)][t-BuP(O)(OH)(2)]}(2)(t-BUPO3)(2) (1) and{[(eta(5)-C5Me5)Ta][t-BuP-(O)(OH)(2)]}(2)(t-BuPO3)(2)(mu-O)(2) (2). Compound 2 was also obtained by recrystallization of 1 from a THF/hexane mixture. Reaction of (eta(5)-C5Me5)MCl4 (M = Mo, W) with PhP(O)(OH)(2) yields the binuclear phosphonates {[(eta(5)-C5Me5)M][PhP(O)(OH)(2)]}(2)(PhPO3)(2)(mu-O)(2) (M = MO (3); M = W (4)). Compounds 2.THF and 3.2.5THF were characterized by single-crystal X-ray studies. The tantalum and molybdenum phosphonates 2.THF and 3.2.5THF have different structures as compared to those of the previously reported titanophosphonate cages.