A synthetic sequence involving the reaction of a phosphorus or sulfur dihalide with a monoanionic boron cluster followed by an in situ dehydrohalogenation reaction initiated by proton sponge results in the clean insertion of the heteroatom into the cage framework. Using this method, a range of nido-11-vertex thia-and phosphaboranes, thia- and phosphadicarbaboranes, and the first thiaphosphaborane has been produced in high yields. Thus, the reaction of decaborane with SCl2 or RPCl2 and excess proton sponge gave the known anions, nido-7-SB10H11- (1(-)) and nido-7-RPB10H11- [2a(-) (R = Me) and 2b(-) (R = Ph)], respectively. Acidification of 1(-), 2a(-), and 2b(-) resulted in the isolation of nido-7-SB10H12 (1) and nido-7-RPB10H12 (2a and 2b), respectively, in near-quantitative yields. Reaction of nido-5,6-C2B8H12 with SCl2 or RPCl2 in the presence of proton sponge produced the new compounds, nido-7,10,11-SC2B8H10 (3) (77%) and nido-7,10,11-RPC2B8H10 [R = Me (97%) 4a, Ph (83%) 4b], respectively. Reaction of arachno-6-SB9H11 with PhPCl2 and proton sponge resulted in the formation of the new mixed heteroatom cage, nido-10-Ph-7,10-SPB8H9 (5), which is the first example of a thiaphosphaborane cluster. Consistent with their nido skeletal electron counts, all clusters were shown by spectroscopic and DFT/GIAO computational studies to have similar cage frameworks containing a five-membered open face.