Regarding the complex $\left[\mathrm{Ni}{\left(\mathrm{CO}\right)}_4\right]$, false statement is 

It is a diamagnetic, tetrahedral complex;

Back bonding occurs due to the donation of an electron pair from metal to anti bonding pi molecular orbital of CO;

M-C bond acquires double bond character;

C-O bond length will be intermediate of 2 and 3.

Thus the C-O bond in$\hspace{0.17em}\left[\mathrm{Ni}{\left(\mathrm{CO}\right)}_4\right]$ is longer than in carbon monoxide (bond order 3);