  We consider the electronic properties of layered molecular crystals of the
type theta-D$_2$A, where A is an anion and D is a donor molecule such as
BEDT-TTF [where BEDT-TTF is bis-(ethylenedithia-tetrathiafulvalene)] which is
arranged in the theta type pattern within the layers. We argue that the
simplest strongly correlated electron model that can describe the rich phase
diagram of these materials is the extended Hubbard model on the square lattice
at a quarter filling. In the limit where the Coulomb repulsion on a single site
is large, the nearest-neighbour Coulomb repulsion, V, plays a crucial role.
When V is much larger than the intermolecular hopping integral t the ground
state is an insulator with charge ordering. In this phase antiferromagnetism
arises due to a novel fourth-order superexchange process around a plaquette on
the square lattice. We argue that the charge ordered phase is destroyed below a
critical non-zero value V, of the order of t. Slave boson theory is used to
explicitly demonstrate this for the SU(N) generalisation of the model, in the
large N limit. We also discuss the relevance of the model to the all-organic
family beta''-(BEDT-TTF)$_2$SF$_5$YSO$_3$ where Y = CH$_2$CF$_2$, CH$_2$, CHF.
