.......... is metal ion present at the centre of chlorophyll.

Explanation

**Step-by-Step Solution:**

1. **Understanding Chlorophyll**: Chlorophyll is a green pigment found primarily in plants, responsible for the process of photosynthesis. It captures sunlight and converts carbon dioxide and water into glucose and oxygen.

2. **Chemical Structure of Chlorophyll**: The structure of chlorophyll includes a central metal ion surrounded by a large ring structure made up of carbon and nitrogen atoms.

3. **Identifying the Metal Ion**: The metal ion located at the center of chlorophyll is crucial for its function. In chlorophyll, this metal ion is specifically magnesium (Mg).

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