哈普托数（英文hapticity）是说明在配体中一群相连的原子如何和中心原子配位。配体的hapticity会用希腊字母η来表示，η上标的数字表示配体中有几个相连的原子和中心原子配位。一般来说上述的标示只适用在不只一个原子参与配位的情形。

如二茂铁中环戊二烯（Cp）有五个原子参与键结，因此其哈普托数为5。

在有机金属化学中，常用哈普托数来标示有机的配体如何和中心金属配位。

Hapticity is the coordination of a ligand to a metal center via an uninterrupted and contiguous series of atoms. The hapticity of a ligand is described with the Greek letter η ('eta'). For example, η describes a ligand that coordinates through 2 contiguous atoms. In general the η-notation only applies when multiple atoms are coordinated (otherwise the κ-notation is used). In addition, if the ligand coordinates through multiple atoms that are not contiguous then this is considered denticity (not hapticity), and the κ-notation is used once again. When naming complexes care should be taken not to confuse η with μ ('mu'), which relates to bridging ligands.