According to VBT the extent of overlapping depends upon types of orbitals involved in overlapping and nature of overlapping. More will be the overlapping and the bond energy will also be high.
Which of the following combination of orbitals does not form covalent bond (x-axis is inter nuclear axis) :

Options

A. `s+p_(y)`
B. `p_(y)+p_(y)`
C. `d_(yz)+d_(yz)`
D. `d_(xy)+d_(xy)`

Explanation

To determine which combination of orbitals does not form a covalent bond according to Valence Bond Theory (VBT), we need to analyze the given options based on the type of orbital overlap that occurs.

### Step-by-Step Solution:

1. **Understand the Concept of Orbital Overlap**:
- According to VBT, the formation of a covalent bond requires overlapping of atomic orbitals. The extent of overlap affects the bond strength and energy. There are different types of overlaps: sigma (σ), pi (π), and delta (δ).

2. **Identify the Given Options**:
- We have four combinations of orbitals to analyze:
- Option A: s + p_y
- Option B: p_y + p_y
- Option C: d_xz + d_yz
- Option D: d_xy + d_xy

3. **Analyze Each Option**:
- **Option A (s + p_y)**:
- The s orbital is spherical, and the p_y orbital is oriented along the y-axis. When these orbitals approach each other along the x-axis (the internuclear axis), there is no effective overlap because the p orbital does not align with the s orbital. This results in zero overlap, meaning no covalent bond is formed. **This is the correct answer.**

- **Option B (p_y + p_y)**:
- Both p_y orbitals are oriented along the y-axis. When they approach each other along the x-axis, they can overlap sideways, forming a pi bond. Therefore, a covalent bond is formed here.

- **Option C (d_xz + d_yz)**:
- The d_xz and d_yz orbitals are oriented in the xz and yz planes, respectively. When these orbitals approach along the x-axis, they can overlap effectively and form a delta bond. Thus, a covalent bond is formed.

- **Option D (d_xy + d_xy)**:
- The d_xy orbitals are oriented in the xy plane. When they approach along the x-axis, they can also overlap effectively to form a pi bond. Hence, a covalent bond is formed.

4. **Conclusion**:
- After analyzing all options, we conclude that **Option A (s + p_y)** does not form a covalent bond due to zero overlap.

### Final Answer:
**Option A: s + p_y does not form a covalent bond.**

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