  We extend our previous theoretical study of Mn-based orthorhombic metamagnets
to those that possess large nearest neighbour Mn-Mn separations (d1>3.22A).
Based on our calculations, we design and synthesize a series of alloys,
CoMnGe_{1-x}P_{x}, to experimentally demonstrate the validity of the model.
Unusually, we predict and prepare several metamagnets from two ferromagnetic
end-members, thus demonstrating a new example of how to vary crystal structure,
within the Pnma symmetry group, to provide highly tunable metamagnetism.
