Two important physical evidence support the synergic bonding in non-classical complexes-bond lengths and vibrational spectra. Vibrational spectra is based on the fact that the compression and extension of a bond may be analogous to the behaviour of spring and obeys Hooke's law.
`overset(-)v=(1)/(2pic) sqrt((K)/(mu))cm^(-1)` Where, K=force constant of the bond which is directly proportional to bond strngth
`mu="reduced mass of ligand"`
`overset(-)v="stretching frequency of the CO bond"`
`C="velocity of light"`
In which of the following complex, stretching frquency for C-O bond is least as well as bond energy of M-C bond is highest ?

Options

A. (dien) `Mo(CO)_(3)`
B. `(Et_(3)P)_(3)Mo(CO)_(3)`
C. `(F_(3)P)_(3)Mo(CO)_(3)`
D. `(Cl_(3)P)_(3)Mo(CO)_(3)`

Answer

A

Explanation

Dienn is not `pi`-acid ligannd while `PF_(3),PC l_(3),PE t_(3)` are `pi`-acid ligands, therefore extent of `Mo overset(pi) to CO` back bonding in (dien) `Mo(CO)_(3)` is maximum in order of disperse incrased electron density at Mo: while in other complexes al ligands participate in `M overset(pi) to I`. back bonding.
Extent of back bonding `alphaMo-CO` bond order `prop(1)/("stretching frequency of CO")`