Antiferromagnetism and magnetic frustration in the metalorganic compounds <math> <mrow> <mi>M</mi> </mrow> <msub> <mi mathvariant='"normal"'>Cl</mi> <mn>2</mn> </msub> </math> -4SC( <math> <mrow> <msub> <mi mathvariant='"normal"'>NH</mi> <mn>2</mn> </msub> </mrow></math>