AN X-RAY CRYSTALLOGRAPHIC AND SINGLE-CRYSTAL EPR INVESTIGATION OF THE CATIONIC, IRON-CENTERED RADICAL TRICARBONYLBIS(TRIPHENYLPHOSPHINE)IRON(I), (FE(CO)3(PPH3)2+) - A THEORETICAL-EXAMINATION OF THE STRUCTURAL PREFERENCES OF 5-COORDINATED 17-ELECTRON COMPLEXES

The X-ray crystal structure of the 17-electron complex [Fe(CO)3(PPh3)2][PF6].1/2CH2Cl2 has been determined; the compound crystallizes in the space group P1BAR, and the cation assumes essentially a distorted square-pyramidal structure with trans basal phosphines, a P-Fe-P bond angle of 163.4 (1)-degrees, a (basal CO)-Fe-(basal CO) bond angle of 162.0 (6)-degrees, and (axial CO)-Fe-(basal CO) bond angles of 101.4 (6)-degrees and 96.6 (5)-degrees. The EPR spectrum of the cation {Fe(CO)3(PPh3)2+} has been measured in single crystals of [Co(CO)3(PPh3)2][PF6].CH2Cl2 at 77 K, the g and P-31 hyperfine interaction matrices being established in an orthogonal axis system of the monoclinic host. X-ray diffraction was used to determine the host crystal structure (space group P2(1)/n) and to establish the geometry of the diamagnetic cobalt cation as trigonal bipyramidal with axial phosphines (P-Co-P bond angle 176.5 (1)-degrees) and equatorial carbonyl groups (OC-Co-CO bond angles 117.8 (4)-degrees. 119.5 (4)-degrees, and 122.7 (5)-degrees). The radical {Fe(CO)3(PPh3)2+} has a 2A1 ground state, g (2.0091, 2.0898, 2.0795) being nearly axial with a unique component that lies almost in the equatorial plane of the host cation and having a value close to that of a free spin. The two P-31 hyperfine interaction tensors are coparallel and axially symmetric with unique components directed along the host P-Co bonds. The similarity of the principal values of g to those of {Fe(CO)5+} and their disposition with respect to the host cation show beyond any reasonable doubt that {Fe(CO)3(PPh3)2+} has a geometry approaching square pyramidal, as in crystalline [Fe(CO)3(PPh3)2]PF6.1/2CH2Cl2, and a 2A1 ground state (in C2-nu symmetry) with the unpaired electron principally located in the iron 3d(z)2 orbital. The phosphine ligands, axial in the host cation structure, occupy trans equatorial positions in the square-pyramidal structure of the cation radical. LCAO-HFS calculations for {M(CO)3L2} (M = Mn, Fe+; L = CO, PH3) are consistent with the finding that {Fe(CO)3(PPh3)2+} assumes a square-pyramidal (C2-nu structure and a 2A1 ground state rather than a trigonal-pyramidal (D3h) structure; other bipyramidal isomers are proximate in energy, but have not been observed.