Twelve additional moles of SsO? a1.e needed to react with the 12 eenerated from HCN. reaction (4). C~nrlucwnOne m& Ri r e q k s 21fi + 12or 228 n>oleolS,0~2' to titratr the iudine liberated lnnn the rearrim zequmre. 1mole Bi = 228 mole S2OS2-
Part 2. Mg of Bi:
mg Bi = 32.46 ml X
0.100 mmole 5 ~ 0 3 ~ - 1mmole Bi ml 228 mmole S2OZ2209 mg Bi -X mmole Bi
CO Stretching in Metal Carbonyls Wai-Kee L i The Chinese University of Hong Kong Shatin, N. T., Hong Kong This question is suitable for a senior undergraduate or beginning graduate course in physical or inorganic chemistry which includes elementary group theory in the syllabus. T h e question tests students' abilities to apply the group theory techniques a s well as to make spectral assignments and structural determination by qualitative arguments. Fnrthermore, the students' understanding of bonding in metal complexes is also examined. Question 1) When two of the carbonyl groups of F e ( c 0 ) ~ (trigonal bipyramidal structure, D3h symmetry) are substituted by two L ligands, there are three possible structures for the product LzFe(C0)a: both L's are in axial positions (D~J,), both L's in equatorial positions (Czu),and one L in axial and one L in equatorial positions (C,). For each of the three possibilities, determine the symmetry and spectral activities of the CO stretching motions. 2) T h e following CO stretching bands are found in the IR spectra of (&P)zFe(C0)3and (MeNC)zFe(CO)s (these two compounds are expected to have the same symmetry), (&P)2Fe(C0)3 1887 cm-', (MeNC)zFe(CO)a: 2009 cm-' (0.5),1927cm-' (101, with the relative intensities heing given in brackets In view of these data, which of the three structures rnenrioned in part 1is most likely? Make assignments for the observed bands. Comment on why there are two observed bands in (MeNChFe(C0)3 and only one in (&P)2Fe(C0)3. 3) Given the following C-N stretch frequencies,
GO ~tret~hing motions fw the three possible Wctures of LnFe(C0h.
tensity since the non-linearity of the Me-N-C group slightly perturbs the D3h symmetry of the MeNC-Fe(CO)&NMe grouping. Cotton and Parish' also suggest another reason. 3) Resonance structures for RNC are R-fi = c: R-&=C: (1) (11) . . When the carbon atum is honded to a metal such that negative charge is drained hum C, there is a tendency for the rontrihution of rll to incrrasr, t h u i raising the NC hond order. On the other hand. hark donation from metaidr orbitals tends to lower the NC bond order. ~~~, in irppcnltim rc, rhc atbrementicmtd (inductive)rftrct.'l'he dau given suegrit thar the indurtiveeff~t dominatesin tMe,CNClFelCO~,nnd Couun and I'ari\h c~ back dunation prevmL~in (hle~.SiNC~t'c~C'O,~. a full explanation of why this is the case.'
-
~~
~
'Cotton, F. A. and Parish, R. V., J. Chem. Soc., 1440 (1960).
Free MeGN-C: 2145 cm-llA)
Note that frequency A (free) is about 40 cm-' lower than B (coordinated), while frequency C (free) is about 60 cm-' higher than D (coordinated). Comment.
Erratum
I n the August 1979 installment of "Exam Question ExAcceptable Solution change" the matrix in the solution to the question on Hy1) DZ,,: A,,(R), E Y I R ~ )C: 2 U : z ~ 1 ( ~BI(IRIR): ~ m ) , C.: ~ A Y I R ~ ) , bridization and Bond Angle should read A"(IR/R).IR denotes infrared active vibrations; R denotes Raman active vibrations. These motions are illustrated in the figure. 2) Since there is only one intense CO stretch hand in each of the spectra, theDsh structure is heavily favored.In (MesNC)nFe(CO)s, the totally symmetric A1' mode may have gained a little (IR) in-
!;)
722 1 Journal of Chemical Education
