Which is not true about metal carbonyls?

In the synergic bond of metal carbonyls,which of the following orbitals is involved?

The correct statement$(s)$ regarding the binary transition metal carbonyl compounds is (are) (Atomic numbers: $Fe = 26, Ni = 28$)
$A$. Total number of valence shell electrons at metal centre in $Fe(CO)_5$ or $Ni(CO)_4$ is $16$
$B$. These are predominantly low spin in nature
$C$. Metal-carbon bond strengthens when the oxidation state of the metal is lowered
$D$. The carbonyl $C-O$ bond weakens when the oxidation state of the metal is increased

The number of sigma bonds in Ziese's salt,$[PtCl_3(C_2H_4)]^-$,is:

The formula of the complex potassium trichloroethyleneplatinate $(II)$ is........

In $Fe(CO)_5$,the $Fe-C$ bond possesses