The main product of nucleophillic attack of H⁻ on the complex ion given below is

Concept:

→ In organometallic chemistry, the Green–Davies–Mingos rules predict the regiochemistry for nucleophilic addition to 18-electron metal complexes containing multiple unsaturated ligands.

The given molecule is a polyene system. Reactivity of polyenese varies as:

1.Polyene having even heptacity >  odd heptacity

2. acyclic polyenes > cyclic polyenes

Explanation:

Given molecule is 18 electron complex 

η⁶-C₆H₆ contributes 6 electron, η³ contributes 3 electron, η4 contributes 4 electron.

Molybdenum contributes 6 electrons.

⇒ 6+3+4+6-1(due to one positive charge) = 18 electrons.

→ Among the given centres we will neglect the reactivity for the centre having odd heptacity.

→ Then, centres having even heptacity, we will neglect cyclic ring because it is less reactive as compared to acyclic ring.

→ Thus, the only reactive centre left is acyclic group with even heptacity.

Conclusion:
The correct answer is option 2.