Question

Which of the following is not true for metal carbonyls ?

• A. The oxidation state of central metal in the carbonyl can be zero, negative or low positive
• B. $d\pi -p\pi$ back bonding from $M\to CO$ is present
• C. $d\pi -{\pi }^{\ast }MO$ back bonding is present from $M\to CO$
• D. Bond order of $CO$ decreases during $M\stackrel{\pi }{\to }CO$

Answer 1

The correct option is B $d\pi -p\pi$ back bonding from $M\to CO$ is present

(a) The oxidation state of central metal in the carbonyl can be zero, negative or low positive. For example in $\left[Ni\right(CO{)}_4]$, oxidation state of central metal i.e., Nickel is zero. In $\left[Fe\right(CO{)}_4{]}^{2-}$ Oxidation state of $Fe$ is $-2$ and in $\left[Ir\right(C{O}_6{]}^{3+}$, Ir is in positive oxidation state. This is so because for formation of backbonding metal needs to donate electron to $CO$, if metal is present in higher positive oxidation state, electrostatic attraction between metal and electrons will be high and electrons will not be easily donated to carbonyl.
(b) For the formation of $d\pi -p\pi$ bond, there should be vacant d or p subshell in metal and the surrounding carbonlys must have empty or partially filled d or p orbitals In metal carbonyl $CO$ has vacant ${\pi }^{\ast }$ ABMO. So, $d\pi -p\pi$ back bonding from $M\to CO$ is absent.
(c) $d\pi -{\pi }^{\ast }MO$ back bonding is present from $M\to CO$ because valence electrons of metals are present in d orbital and $CO$ has vacant ${\pi }^{\ast }$ ABMO.
(d) As we know from Molecular orbital theory, as electrons in antibonding orbital increases, bond order decreases. In $M\stackrel{\pi }{\to }CO$, carbonly accepts electrons from metal in its antibonding orbital. Hence, bond order of $CO$ decreases.