P-bound derivatives of the hypophosphorous acids can be prepared from complexed phosphinidene oxides. The anion [MoCp(CO) 2{P(O)R*}] (-)[(H-DBU) (+) salt (R* = 2,4,6-C 6H 2 (t)Bu 3; Cp = eta (5)-C 5H 5) reacts with SnCl 2Ph 2 to give the neutral oxophosphinidene complex [MoCp{P(O)R*}(CO) 2(SnPh 2Cl)] in high yield. Basic hydrolysis of the latter complex occurs readily to yield the novel Mo-Sn metallocyclic derivatives [MoSn{mu- O, P-OP(OH)R*}CpPh 2(CO) 2] and [MoSn{mu- O, O, P-O 2PR*}CpPh(CO) 2] 2. These products display respectively hydrogenhypophosphite(1-) and hypophosphite(2-) anions coordinated through all of their O (to Sn) and P (to Mo) atoms.