  The present paper is dedicated to the proof of dispersive estimates on
stratified Lie groups of step 2, for the linear Schr\"odinger equation
involving a sublaplacian. It turns out that the propagator behaves like a wave
operator on a space of the same dimension p as the center of the group, and
like a Schr\"odinger operator on a space of the same dimension k as the radical
of the canonical skew-symmetric form, which suggests a decay with exponant
-(k+p-1)/2. In this article, we identify a property of the canonical
skew-symmetric form under which we establish optimal dispersive estimates with
this rate. The relevance of this property is discussed through several
examples.
