Answer to Question #88893 in Inorganic Chemistry for yahya

Question

the reaction of cf2=cf2 with fe (co) 12 gives colour less crystalline compound a ( c : 26.5 % f : 41.7 % fe : 15.4% ) the infrared spectrum of A shows bands at 2160,2108,2088 and 2055 cm

Answer 1

Interaction with Pentacarbonyliron Perfluorobutadiene and pentacarbonyliron, in petroleum solution, interact under mercury-sensitised ultra-violet radiation to give a yellow solid, which on vacuum sublimation yields transparent greenish yellow plates. The complex melts sharply at 61-61.5⁰ and analyses as C₄F₆Fe(CO)₄. The i.r. shows four terminal carbon monoxide stretching frequencies, bands due to C-F stretch, and in addition a single sharp band at 1752 cm^-1. This latter band is correctly placed for a C=C stretch, and since it is a strong absorption it is probable that it is due to an unco-ordinated double bond.

The ¹⁹F n.m.r. spectrum shows two peaks at -4.0 and +71.9 p.p.m, relative to benzotrifluoride (a +ve sign indicating an upfield shift), the area ratios being 2:1 respectively. Because of its low chemical shift, the large peak must be due to a -CF₂- group which is directly sigma-bonded to the Fe atom [1]. Whereas the low field peak consists of a doublet (J_F-F = 6.8 c.p.s.), the high field one is more complex, and the splittings have not been completely resolved, but the chemical shift is consistent for the resonance to be assigned as being due to an olefinic fluorine atom The spectral data is adequately explained by the structure on figure, which involves rearrangement of the butadiene fragment leading to a heterocyclic s-bonded monoolefin. This structure closely resembles that of the compound formed between pentacarbonyliron and tetrafluoroethylene [2], except that in this case the heterocyclic ring consists of a fully saturated perfluoromethylene chain.

The coordination symmetry of the iron is closely octahedral having a Fe-C(F₂) bond length of 2.00 Å and Fe-C(O) length of 1.82 Å. The C₄Fe ring is planar, but it is thought that in addition to the sigma-bonds formed between the 1, 4 carbon atoms and the metal, there is some interaction between the metal and the 2, 3 - ethylenic type bond.

A sigma-bonded fluorocarbon group, with its high electronegativity, withdraws electrons from the orbitals of the transition metal and thereby denuding somewhat those available for back bonding with the carbonyl. This affects the bond order in the carbonyl, and results in a shift of the carbonyl stretch to higher wave numbers. Table 1 shows the observed positions of the carbonyl stretching frequencies of this new complex and compares them with those of similar complexes to illustrate this effect. The structure of this complex is however quite different from that of the compound formed from butadiene and pentacarbonyliron, The stoicheiometry of this complex is C₄H₆Fe(CO)₃, and although it was considered initially to have a similar sigma-bonded structure [3], an X-ray crystallographic structure determination has shown that the olefin is pi-bonded [4]. This leads to the molecule being square pyramidal, two corners of the square being occupied by carbonyl carbon atoms, and two by the 1, 4 carbon atoms of the butadiene moiety. The preparation and structure determination of this new complex well illustrates the difference in type of the compounds formed by fluoroolefins; thus whereas butadiene forms a pi-type bond and remains in a similar configuration when coordinated, perfluorobutadiene forms two sigma-type bonds with internal re-arrangement within the olefin.

                                   

                               Table 1

Complex               u_max, M-C (cm^-1)                      Ref

           C₄F₆Fe(CO)₄           2153, 2097, 2092, 2076

           C₄F₈Fe(CO)₄           2160, 2108, 2088, 2055          [2]

           C₄H₆Fe(CO)₃            2051, 1978                                [5]

 

[1] E. Pitcher, A.D. Buckingham, and F.G.A. Stone, J. Chem. Phys., 1962, 36, 124.

[2] H.H. Hoehn, L. Pratt, K.F. Watterson, and G. Wilkinson, J. Chem. Soc,, 1961, 2738.

[3] P.B. Hitchcock and R. Mason, Chem. Comm., 1967, 1, 242.

[4] M.L.H. Green, L. Pratt, and G. Wilkinson, J. Chem. Soc., 1959,3753.

[5] B.F. Hallam and P.L. Pauson, J. Chem. Soc., 1958, 642.