The alkene ligand (π-C_2 R_4) is both a ' σ ' donor and a ' π '...

The alkene ligand $\left(\pi-\mathrm{C}_2 \mathrm{R}_4\right)$ is both a ' $\sigma$ ' donor and a ' $\pi$ ' acceptor, similar to the CO ligand in metal carbonyls, and exhibits synergic bonding with metals. Correct order of $\mathrm{C}-\mathrm{C}$ bond length in $\mathrm{K}\left[\operatorname{PtCl}_3 \left(\pi-\mathrm{C}_2 \mathrm{R}_4\right)\right]$ complexes in which $\mathrm{R}=\mathrm{H}, \mathrm{F}$ or CN is