To determine the optimal extent to which ENI S.P.A.'s capital structure should rely on hybrid bonds, we need to evaluate the company's leverage and debt capacity. 

Hybrid bonds are essentially a mix of debt and equity. They are generally more expensive than senior debt but cheaper than equity, and rating agencies typically grant them 50% equity credit. Because of their higher cost relative to standard senior debt, companies typically only optimize by issuing hybrid bonds if they have exhausted their senior debt capacity for a given target credit rating (often to defend an Investment Grade rating during downturns or large M&A activities).

1. **EBITDA and Leverage Check:**
For the 2022 period, ENI generated an Operating Profit of €17.51 billion. Adding back D&A of €7.21 billion and impairments of €1.14 billion gives an approximate EBITDA of over €25 billion. 
The total debt (Short-term + Long-term borrowings + Lease liabilities) stands at roughly €31.8 billion. With Cash and Cash Equivalents of €10.15 billion, ENI's Net Debt is around €21.7 billion.
This puts ENI's Net Debt-to-EBITDA ratio at less than 1.0x, representing very low leverage and massive senior debt capacity. 

2. **Cash Generation and Equity:**
In 2022, ENI had a highly profitable year (Profit of €13.9 billion) and generated substantial operating cash flows (€17.46 billion). The company is overcapitalized relative to its debt, as evidenced by its aggressive share buybacks (purchasing €2.4 billion in treasury shares) and large dividend payouts.

3. **Capital Structure Optimization:**
Because ENI has ample headroom in its credit metrics to issue much cheaper senior debt, hybrid bonds are structurally sub-optimal for lowering the Weighted Average Cost of Capital (WACC) in this state. Any reliance on hybrids here is strictly dominated by issuing cheaper standard senior corporate bonds (which fully benefit from tax shields without the subordination premium). While ENI did issue perpetual subordinated bonds during the heavily disrupted 2020/2021 periods to protect its metrics, under normalized and highly cash-generative conditions like 2022, the mathematically optimal long-term reliance on hybrids for the capital structure reverts to zero.

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