Hexakis(carbonyl)iron(II) Undecafluorodiantimonate(V), [Fe(CO)₆][Sb₂F₁₁]₂, and -Hexafluoroantimonate(V), [Fe(CO)₆][SbF₆]₂, Their Syntheses, and Spectroscopic and Structural Characterization by Single Crystal X-ray Diffraction and Normal Coordinate Analysis

Abstract

Hexakis(carbonyl)iron(II) undecafluorodiantimonate(V), [Fe(CO)6][Sb2F11]2, is conveniently prepared by the oxidative carbonylation of Fe(CO)5 with XeF2 as external oxidizer in the conjugate Brønsted-Lewis superacid HF−SbF5. The colorless compound crystallizes from the reaction medium in high purity. The molecular structure is obtained by single-crystal X-ray diffraction. The cation is a regular octahedron, while the vertex-shared di-octahedral [Sb2F11]- anion is distorted from D 4 h symmetry by bending and rotational processes, due to significant interionic interactions, primarily of the F···C type. Washing of [Fe(CO)6][Sb2F11]2 with anhydrous HF results in an unusual elution of SbF5 and the quantitative conversion to hexakis(carbonyl)iron(II) hexafluoroantimonate(V) [Fe(CO)6][SbF6]2. The molecular structure of the salt shows octahedral ions with slight tetragonal distortions for the cation (elongation) and the anion (compression). Both salts are thermally stable up to 150 °C. The averaged bond distances and the vibrational wavenumbers of [Fe(CO)6]2+ are nearly identical in both compounds. The [Fe(CO)6]2+ cation, the first and so far only isolated and structurally characterized dipositive, superelectrophilic carbonyl cation formed by a 3d-metal, is further characterized by a normal coordinate analysis (NCA). The obtained force constants are compared to those of the isoelectronic molecule Cr(CO)6. Changes in π-back-bonding affect the F CO/CO and F CO/MC interaction force constants more strongly than the stretching force constants F CO and F MC. All 13 fundamentals of [Fe(CO)6]2+ are detected and assigned with the help of the data obtained from the normal coordinate analysis and density functional calculations published previously. The electronic ground state 1A1g of the [Fe(CO)6]2+ cation is established by magnetic susceptibility measurements of polycrystalline [Fe(CO)6][SbF6]2 and [Fe(CO)6][Sb2F11]2 between 2 and 300 K. The magnetic impurity formed during synthesis is identified as Fe[SbF6]2 which has iron(II) in high spin (5T2g) ground state. Consistent with a diamagnetic ground state are the single line in the 57Fe Mössbauer spectrum (i.s. = −0.003(8) mm s-1 relative to α-Fe), obtained on polycrystalline samples and the single sharp line in the 13C NMR spectrum in DF solution at 178 ppm with J (57Fe-13C) of 19.2 Hz.

NATURE OF Mδ+···δ+C-Oδ-INTERACTIONS IN METAL CARBONYLS. AN ELECTRONIC STUDY BASED ON THE TOPOLOGY OF THE ELECTRON CHARGE DENSITY DISTRIBUTION AND ITS LAPLACIAN FUNCTION

“…Las distancias de enlace M-C y C-O calculadas están en buen acuerdo con las distancias experimentales y teóricas informadas por otros autores. 7,37,[42][43][44] De acuerdo a Orpen y col. 45 la longitud promedio de los enlaces V-C, Cr-C, Mn-C y Fe-C en carbonilos metálicos son 1,946, 1,866, 1,808, y 1,782 Å, respectivamente. Como era de esperar, estas longitudes disminuyen al aumentar el número atómico del metal, debido a que en este sentido disminuye el radio iónico de los metales de transición.…”

The Photochemistry of [FeIIIN3(cyclam‐ac)]PF6 at 266 nm

“…[10] Only in very rare cases does the CO vibration of metal carbonyls appear above 2137 cm À1 ; the stretching frequency of free carbon monoxide [11] as back-bonding tends to weaken the CO bond and hence reduce its stretching frequency. On the contrary, if the 266 nm photolysis results in a N À N bond cleavage under N 2 formation, the resultant iron nitrido complex might attach a CO group, thereby yielding the isocyanato complex 5 (see Scheme 2), which in turn should feature a highly characteristic IR absorption around 2200 cm…”

[Co(CO)5][(CF3)3BF]: A Stable Salt of a Homoleptic Trigonal‐Bipyramidal Metal–Carbonyl Cation

“…[3][4][5][6] In particular SbF 5 , the strongest molecular Lewis acid [7,8] and the conjugate Brønsted--Lewis superacid HF-SbF 5 , [1,2] have allowed the formation of a substantial number of salts with superelectrophilic, [9] s-bonded metal-carbonyl cations in which the metal atoms are in the oxidation states + 2 and + 3. The counteranions [ [10] [M(CO) 4 ][Sb 2 F 11 ] 2 (M = Pd, Pt), [11] [M(CO) 6 [12,13] and the recently reported [Ir(CO) 6 ][SbF 6 ] 3 ·4 HF. [14] There are two general limitations to the use of SbF 5 3 ].…”