Which of the following ions has the weakest #"C-O"# bonds?

A) #"Ti(CO)"_6^"2-"#
B) #"Cr(CO)"_6#
C) #"Mn(CO)"_6^"+"#
D) #"Fe(CO)"_6^"2+"#

2 Answers
anor277
Aug 15, 2017

Would it not be #[Ti(CO)_6]^(2-)#?

Explanation:

Anyway, as a scientist, you are obliged to consult data. The #nu_(C-=O)# values should be listed in your text.

Ernest Z.
Aug 15, 2017

A) #"Ti(CO)"_6^"2-"# has the weakest #"C-O"# bond due to back-bonding.

Explanation:

In back-bonding in metal carbonyls, electrons are partially transferred from
a #"d"#-orbital of the metal to an antibonding molecular orbital of #"CO"#.

i.stack.imgur.com

This electron transfer strengthens the #"Metal–C"# bond and weakens the #"C–O"# bond.

Thus, the greater the ability of the metal to donate #"d"#-electron density, the weaker the #"C-O"# bond will be.

Since #"CO"# is a neutral ligand, the #"Ti"# atom in #"Ti(CO)"_6^"2-"# has the greatest electron density.

Thus, #"Ti(CO)"_6^"2-"# has the weakest #"C-O"# bonds due to back-bonding.

The decrease in #"C-O"# bond strength is reflected in a corresponding decrease in the infrared carbonyl stretching frequency.

Conversely, as the metal gets more positive character, the carbonyl stretching frequency increases.

#bb("Compound"color(white)(m)barν_text(CO)"/cm"^"-1")#
#color(white)(m)"Ti(CO)"_6^"2-"color(white)(mmm)1748#
#color(white)(m)"Cr(CO)"_6color(white)(mmml)2000#
#color(white)(m)"Mn(CO)"_6^"+"color(white)(mmll)2090#
#color(white)(m)"Fe(CO)"_6^"2+"color(white)(mmll)2204#

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