m-Terphenyl- and biphenyl-2-diiodophosphines, TerphPl(2) and BiphPl(2), have been obtained by halide exchange from the chloro derivatives TerphPCl(2) and BiphPCl(2) and excess Lil in a benzene solution at room temperature. Whereas BiphPl(2) compounds are stable, the TerphPl(2) species undergo intramolecular C-H activation at room temperature and cyclize to form unsymmetrical 9-iodo-9-phosphafluorenes 1-(3,5-dimethylphenyl)-6,8-dimethyl-9-iodo-9-phosphafluorene, 4; 1-(4-t-butylphenyl)-7-t-butyl-9-iodo-9-phosphafluorene, 5; and 1-(2-methylphenyl)-5-methyl-9-iodo-9-phosphafluorene, 6, albeit the latter reaction is rather slow due to unfavorable steric interactions. Cyclization of the alkyl-substituted 4,4'-di-tert-butyl-biphenyl-2-diiodophosphine, 11, is slow in refluxing benzene solution, but faster than that for the parent biphenyl-2-diiodophosphine. Ab initio and density functional theory calculations are in agreement with an electrophilic aromatic substitution mechanism that is facilitated by steric strains in the terphenyl compounds 2,6-(3,5-Me2C6H3)(2)C(6)H(3)Pl(2), 1; 2,6-(4-t-BuC6H4)(2)C(6)H(3)Pl(2), 2; and 2,6-(2-MeC6H4)(2)C(6)H(3)Pl(2), 3. All new compounds have been characterized by multinuclear NMR spectroscopy and direct analysis in real time mass spectrometry. 9-Iodo-9-phosphafluorene, 12, was also analyzed by X-ray diffraction.