Medium basis sets based upon contractions of Gaussian primitives are developed for the third-row elements K through Zn. The basis functions generalize the 6-31G and 6-31G* sets commonly used for atoms up to Ar. They use six primitive Gaussians for Is, 2s, 2p, 3s, and 3p orbitals, and a split-valence pair of three and one primitives for valence orbitals, which are 4s and 5p for atoms K and Ca, and 4s, 4p, and 3d for atoms Sc through Zn. A 6-31G* set is formed by adding a single set of Gaussian polarization functions to the 6-31G set. They are Cartesian d-functions for atoms K and Ca, and Cartesian f-functions for atoms Sc through Zn. Comparison with experimental data shows relatively good agreement with bond lengths and angles for representative vapor-phase metal complexes.