  Since their discovery, it has been suggested that pairing in pnictides can be
mediated by spin fluctuations between hole and electron bands. In this view,
multiband superconductivity would substantially differ from other systems like
MgB2, where pairing is predominantly intraband. Indeed, interband-dominated
pairing leads to the coexistence of bonding and antibonding superconducting
channels. Here we show that this has profound consequences on the nature of the
low-energy superconducting collective modes. In particular, the so-called
Leggett mode for phase fluctuations is absent in the usual two-band description
of pnictides. On the other hand, when also the repulsion between the hole bands
is taken into account, a more general three-band description should be used,
and a Leggett mode is then allowed. Such a model, that has been proposed for
strongly hole-doped 122 compounds, can also admit a low-temperature s+is phase
which breaks the time reversal symmetry. We show that the (quantum and thermal)
transition from the ordinary superconductor to the s+is state is accompanied by
the vanishing of the mass of Leggett-like phase fluctuations, regardless the
specific values of the interaction parameters. This general result can be
obtained by means of a generalized construction of the effective action for the
collective degrees of freedom that allows us also to deal with the non-trivial
case of dominant interband pairing.
