Multiple choice

Which of the following statements is/are false?

  1. Increase in the extent of back donation causes the stretching frequencies to be found at lower wavenumbers.
  2. The correct order of C-O stretching frequency is Mn(CO)6]+ < [Cr(CO)6] < [V(CO)6]-.
  3. C-O stretching frequency in Ni(CO)4 is higher than in Fe(CO)5.

  1. Only 1

  2. Only 2

  3. Only 3

  4. Only 1 and 2

  5. Only 2 and 3
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Increase in the extent of back donation causes the stretching frequencies to be found at lower wavenumbers. In [V(CO)6]-, vanadium has a -1 charge and with the addition of six C-O ligands, there is a tendency to relieve part of the negative charge by extensive back donation. The tendency is less in [Cr(CO)6] and even less when the metal has a positive charge as in [Mn(CO)6]+. Thus, the correct order of decreasing C-O stretching frequency is Mn(CO)6]+, 2090 cm-1 > [Cr(CO)6], 1981 cm-1 > [V(CO)6]-, 1859 cm-1.

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