A Caged Neutral\n17-Valence-Electron Iron(I) Radical\n[Fe(CO)<sub>2</sub>(Cl)(P((CH<sub>2</sub>)<sub>10</sub>)<sub>3</sub>P)]<sup>•</sup>: Synthetic, Structural, Spectroscopic, Redox,\nand C

UV irradiation of yellow CH<sub>2</sub>Cl<sub>2</sub> solutions\nof <i>trans</i>-Fe(CO)<sub>3</sub>(P((CH<sub>2</sub>)<sub>10</sub>)<sub>3</sub>P) (<b>2a</b>) and PMe<sub>3</sub> (10 equiv) gives, in addition to the previously reported\ndibridgehead diphosphine P((CH<sub>2</sub>)<sub>10</sub>)<sub>3</sub>P (46%), a green paramagnetic complex that crystallography shows\nto be the trigonal-bipyramidal iron(I) radical <i>trans</i>-[Fe(CO)<sub>2</sub>(Cl)(P((CH<sub>2</sub>)<sub>10</sub>)<sub>3</sub>P)]<sup>•</sup> (<b>1a</b><sup>•</sup>; 31% after workup). This is a rare example of an isolable species\nof the formula [Fe(CO)<sub>4–<i>n</i></sub>(L)<sub><i>n</i></sub>(X)]<sup>•</sup> (<i>n</i> = 0–3, L = two-electron-donor ligand; X = one-electron-donor\nligand). Analogous precursors with longer P(CH<sub>2</sub>)<sub><i>n</i></sub>P segments (<i>n</i> = 12, 14, 16, 18)\ngive only the demetalated diphosphines, and a rationale is proposed.\nThe magnetic susceptibility of <b>1a</b><sup>•</sup>,\nassayed by Evans’ method and SQUID measurements, indicates\na spin (<i>S</i>) of <sup>1</sup>/<sub>2</sub>. Cyclic voltammetry\nshows that <b>1a</b><sup>•</sup> undergoes a partially\nreversible one-electron oxidation, but no facile reduction. The UV–visible,\nEPR, and <sup>57</sup>Fe Mössbauer spectra are analyzed in\ndetail. Complex <b>2a</b> is similarly studied, and, despite\nthe extra valence electron, exhibits a comparable oxidation potential\n(Δ<i>E</i><sub>1/2</sub> ≤ 0.04 V). The crystal\nstructure shows a cage conformation, solvation level, disorder motif,\nand unit cell parameters essentially identical to those of <b>1a</b><sup>•</sup>. DFT calculations provide much insight regarding\nthe structural, redox, and spectroscopic properties.