In which of the following back - bonding is possible

In which of the following ppi-dpi bonding is possible ?

In which of the following ppi-dpi bonding is possible ?

Metal 'M' forms a carbonly compound in which it is present in its lower valance state. Which of the following bonding is possible in this metal carbonly?

Metal 'M' forms a carbonyl compound in which it is present in its lower valance state. Which of the following bonding is possible in this metal carbonyl?

In which of the following compounds , back bonding is possible