Question

How many IR-active CO stretching modes does the molecule Mn(CO)5Cl have? Please explain with figures, calculations or anything needed.

Answer 1

For clear understanding, let's consider Fe(CO₂)₃(PPh₃)₂

These stretching modes are IR- active, and four v (CO) absorptions are expected for A. The reporte triphenylphosphine ligand in the axial site ed v (CO) IR spectrum is consistent with formation of isomer A, with the we consider the triphenylphosphine ligand as a simple L group for point group determination. The point groups for isomers A, B, and C are as follows CO OC--Fe ос-К CO CO CO PPh, CO (A) C (B) DSh (C) C, For A, the set of irreducible representations for the three CO stretching vibrational modes is 2 A1 + Bi. These modes are all IR-active in the C character table, and three v (CO) IR absorptions are expected for isomer A. For B, the set of irreducible representations for the three CO stretching vibrational modes is A+ E. Only the E mode is IR-active in the Ds point group, and one v (CO) IR absorption is expected for isomerB For C, the set of irreducible representations for the three CO stretching vibrational modes is 2 A' A". These modes are all IR-active in the Cs point group, and three v (CO) IR absorptions are expected for isomer C The single v (CO) IR absorption reported for Fe(CO)PPh3)2 supports the presence of the Dsh isomer B
Similarly, the same procedure has to be followed to Mn(CO)₅Cl. There are 5 IR-active CO stretching modes. The signs on the orbital lobes:

a. p_x

b. d_xy

c. d_x²-_y²

d. d_z²

e. f_xyz