Two important physical evidences supporting the synergic bonding in non-classical complexes are 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 a spring and obeys Hook's law.
`overline(v)=(1)/(2pic)sqrt((k)/(mu))cm^(-1)`
where, k=force force constant of the bond which is directly proportional to bonnd strength of CO `mu=`reduced mass of ligand
`overline(v)`=stretching frequency of the CO bond
c=velocity of light
Q. In which of the following ligand, `sigma`-bond order does not change during synergic bonding in their respective complexes:

Options

A. CO
B. `N_(2)`
C. `CH_(2)=CH_(2)`
D. `PE t_(3)`

Answer

D

Explanation

As `PE t_(3) ` accepts `M overset(pi)toL` back bonding into non-axial d-orbital of phosphorus annd it forms `Moverset(pi)toL` bond using its non-bonding molecular orbital.