  Analyzing the motion of low--spin $(s=1/2)$ holes in a high--spin $(S=1)$
background, we derive a sort of generalized t--J Hamiltonian for the $\rm
NiO_2$ planes of Sr--doped nickelates. In addition to the rather complex
carrier--spin and spin--spin couplings we take into account the coupling of the
doped holes to in--plane oxygen breathing modes by a Holstein--type interaction
term. Because of strong magnetic confinement effects the holes are nearly
entirely prelocalized and the electron--phonon coupling becomes much more
effective in forming polarons than in the isostructural cuprates. In the light
of recent experiments on $\rm La_{2-x}Sr_xNiO_4$ we discuss how the variety of
the observed transport and charge/spin--ordering phenomena can be qualitatively
understood in terms of our model Hamiltonian.
