**Explanation:**

Tetrahedral geometry refers to a molecular shape where four ligands surround a central metal atom/ion, forming a tetrahedron. In transition metal complexes, the tetrahedral geometry is commonly found in compounds of transition metals with d^5 electron configuration such as Mn(II), Fe(II), Co(II), and Ni(II).

The term "high spin" refers to the electronic configuration of the metal ion in the complex. In high spin complexes, the electrons occupy higher energy orbitals before pairing up, resulting in unpaired electrons. On the other hand, "low spin" complexes have the electrons paired up in lower energy orbitals.

**Reasons for high spin in tetrahedral transition metal complexes:**

1. **Crystal Field Theory:** According to Crystal Field Theory, the ligands surrounding the metal ion create a crystal field that splits the d-orbitals into two sets of different energy levels. In tetrahedral complexes, the splitting pattern is called the "Tetrahedral Crystal Field Splitting." This splitting results in three lower-energy orbitals (t2) and two higher-energy orbitals (e). The energy difference between the t2 and e orbitals is relatively small, allowing the electrons to occupy the higher energy orbitals and remain unpaired.

2. **Hund's Rule:** Hund's rule states that electrons occupy different orbitals of the same energy level before they start pairing up. In tetrahedral complexes, the t2 orbitals have the same energy level, and the e orbitals have the same energy level. According to Hund's rule, the electrons will occupy the e orbitals first, resulting in unpaired electrons.

3. **Weak Ligand Field:** Tetrahedral complexes are usually formed with weak-field ligands. Weak-field ligands have a small splitting effect on the d-orbitals, resulting in a smaller energy difference between the t2 and e orbitals. This smaller energy difference favors the high spin configuration.

**Conclusion:**

The first row transition metal complexes with tetrahedral geometry are high spin due to the combination of Tetrahedral Crystal Field Splitting, Hund's rule, and the presence of weak-field ligands. This allows the electrons to occupy the higher energy orbitals and remain unpaired, resulting in a high spin configuration.