定义Definition
三角双锥形是中心原子周围有五个成键电子对、没有孤电子对时的构型，如气态 PCl₅。三个赤道位置的原子在同一平面内，彼此夹角 120°；两个轴向原子分别在平面上下，与赤道键成 90°，两个轴向键之间为 180°。
Trigonal bipyramidal geometry arises when the central atom has five bonding pairs and no lone pairs, as in gaseous PCl₅. The three equatorial atoms lie in a plane 120° apart; the two axial atoms sit above and below the plane at 90° to the equatorial bonds and 180° to each other.
轴向与赤道位置不等价Axial and equatorial positions differ
轴向键与三个赤道键成 90°，受到的排斥更大，因此 PCl₅ 的轴向 P–Cl 键（约 214 pm）比赤道键（约 202 pm）长。
An axial bond is at 90° to three equatorial bonds and feels more repulsion, so in PCl₅ the axial P–Cl bonds (about 214 pm) are longer than the equatorial ones (about 202 pm).
含孤电子对的衍生构型Shapes with lone pairs
孤电子对优先占据赤道位置（那里的 90° 排斥较少）：一对孤电子对得到跷跷板形（SF₄），两对得到 T 形（ClF₃），三对得到直线形（XeF₂）。
Lone pairs go into equatorial positions, where they suffer fewer 90° repulsions: one lone pair gives a seesaw (SF₄), two give a T shape (ClF₃) and three give a linear molecule (XeF₂).