A Caged Neutral 17-Valence-Electron Iron(I) Radical [Fe(CO) 2 (Cl)(P((CH 2 ) 10 ) 3 P)] • : Synthetic, Structural, Spectroscopic, Redox, and Computational Studies.

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