Metal carbonyls - class-XII

1. Synergic bonding is the description of the bonding of $\pi$ - configuration ligands to the transition metal which involves donation of electrons through back bonding.
   2. Mainly carbonyl ligands in complex compounds involves in this type of bonding.

Dicobalt octacarbonyl, Co2(CO)8, in its solid state exists as a bridged structure containing one metal-metal bond (Co-Co), six terminal CO groups, and two bridging CO groups.

In metal carbonyls, as the negative charge on the complex increases, back-bonding from metal to CO increases, strengthening the M-C bond and weakening the C-O bond. Thus, the C-O bond is weakest in the anion and strongest in the cation.

Using the 18-electron rule, Cr(CO)6 has 18 electrons. Substituting CO (2e) with NO (3e) changes the count. Cr(CO)3(NO)2 would have 6 + 6 + 6 = 18 electrons. However, Cr(NO)4 is not a stable neutral carbonyl-like complex following this substitution pattern.

In V(CO)6 (neutral), the V-C bond is longer due to less back-bonding compared to the [V(CO)6]- anion, where increased back-bonding strengthens the M-C bond and shortens the distance.

In Fe3(CO)12, bridging CO ligands have a lower bond order than terminal CO ligands because they participate in more extensive back-bonding, resulting in a longer C-O bond length for bridging ligands.

Fe(CO)5 is a 18-electron complex with a trigonal bipyramidal structure. All electrons are paired in the d-orbitals, making it diamagnetic.

Al(OC2H5)3 is an aluminum alkoxide. It contains Al-O bonds, not Al-C bonds. The other options are organometallic compounds containing direct metal-carbon bonds.

According to the 18-electron rule, chromium (atomic number 24) has 6 valence electrons, and each CO ligand contributes 2 electrons. To achieve a stable noble gas configuration of 18 electrons, chromium needs 6 carbonyl ligands, making the formula Cr(CO)6.

$[PtCl _{3}(\eta ^{2}-C _{2}H _{4})]$ and $[Fe(\eta ^{5}-C _{5}H _{5}) _{2}]$ are pi bonded organometallic compound.

Manganese forms poly nuclear carbonyl.  

Cobalt with odd atomic number $(=27)$ does not form a monometallic carbonyl compound. Thus, $[Co(CO) _6]^{3+}$ is least stable among the given.

Chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal.

(A) Metal carbonyls involve synergic bonding $(M\underset {\sigma}{\overset {\pi}\rightleftharpoons } L)$. Filled metal d orbital overlaps with antibonding pi orbital of CO. Thus the bond order and bond length in CO decreases.
Thus option A is correct.
(B) $[CrCl _3(H _2O) _3]\cdot 3H _2O$ is not a hydrate isomer of $[Cr(H _2O) _6]Cl _3$.
Thus option B is incorrect.
(C) The d-orbitals involved in s$sp^3d^2  and  d^2sp^3$  hybridisation of the central metal ion are $d _{x^2-y^2}$  and  $d _{z^2}$ respectively.
Thus option C is incorrect.
(D) Both fac and mer isomers of $[Ma _3b _3]^{n\pm}$ are optically inactive as they contain plane of symmetry.
Thus option D is correct.

As phenyl group increases, resonance increases and bond length increases so order is
$[(Ph _3P) _3Mo(CO) _3] > [(Ph _2PCl) _3Mo(CO) _3] < (PhPCl _2) _3 Mo (CO) _3]$

This type of bonding where a metal is bonded with the carbon of a carbonyl compound is known as synergic bonding. In a metal carbonyl, the M-C bond has both σ and π character. The ligand donates eΘ pair to the central metal atom, and simultaneously accepts eΘ pair from central metal atom through synergic bonding.
Here, all the options are examples of organometallic compounds.
Due to synergic bonding, the metal carbon bond strength increases and bond length decreases due to the partial double bond character.
Thus, the C-O bond strength decreases and bond length increases and bond order of CO decreases as the number of antibonding electrons increase. Out of the four options, the option D has Mn+, implying that the electron donating nature among the options is minimum. Therefore, tendency of synergic bonding is minimum, and thus M-C bond length is maximum implying that C-O bond strength is maximum and C-O bond length is minimum.

The M-C bond order is highest when there is the most back-bonding. This occurs in the anionic complex [V(CO)6]-, which has the highest electron density on the metal to donate into the CO pi-star orbitals.

There are vacant orbitals in central atom in $[V(CO) _6]-$ which results in synergic bonding. Due to synergic bonding between V and CO the bond length between C and O increases and Bond order is inversely proportional to bond length.

There are vacant orbitals in central atom in $[V(CO) _6]-$ which results in synergic bonding. Due to synergic bonding between V and CO the bond length between C and O increases.

Stronger M-C bonds (higher bond order from more back-bonding) result in shorter bond lengths. [V(CO)6]- has the strongest back-bonding (most electron-rich metal), so it has the shortest M-C bond length. Option D is an error.

Option (A),(B),(C) are correct.Ferrocene is example of organometallic compound but is not $\sigma$-bonded. Its formula is $Fe(C _5H _5) _2$. It is the prototypical metallocene, a type of organometallic chemical compound consisting of two cyclopentadienyl rings bound on opposite sides of a central metal atom. Such organometallic compounds are also known as sandwich compounds. Each cyclopentadienyl (Cp) ring is then allocated a single negative charge, bringing the number of -electrons on each ring to six, and thus making them aromatic. These twelve electrons (six from each ring) are then shared with the metal via covalent bonding. 

Metal carbonyls involve synergistic bonding where the metal donates electrons into the CO pi-antibonding orbital (pi-backbonding) and CO donates electrons into the metal d-orbitals (sigma-bonding). Therefore, they are not examples of only sigma or only pi bonded complexes, making statements A, B, and D incorrect.

The C-O bond order decreases as the extent of metal-to-ligand pi-backbonding increases. This backbonding is enhanced by a higher negative charge on the metal complex, which increases electron density on the metal center.

In metal carbonyl complexes like Fe(CO)5, back-bonding from the metal d-orbitals to the pi* antibonding orbitals of CO weakens the C-O bond and increases its bond length compared to free carbon monoxide.

The metal (Fe) makes the back bonding to CO. The metal to ligand bonding creates a synergic effect which strengthens the bond between CO and the metal (Fe).Which results in the contraction of CO bond length.

The $C-O$ bond order would be lowest at $[Mn(CO) _6]^+$. 

If $PF _3$ is a better $\pi$-acceptor ligand than CO, then
extent of back donation from $M \overset{\pi}{\rightarrow} C \equiv O$ decreases, and hence bond order of CO will less decrease.
Stretching frequency of C - O bond $\propto$ bond order of C $\equiv$ O.
Therefore, stretching frequency of $[Ni(PF _3)(CO) _3] >  [Ni(CO) _4]$.