In octahedral [MA2B4]: cis (A-A at 90 degrees) and trans (A-A at 180 degrees). In [MA3B3]: fac (three A on one face) and mer (three A on a meridian). Critical concept: cis-[M(en)2Cl2]+ is optically active (no symmetry plane) while trans is optically inactive (has symmetry plane). [M(en)3]n+ with three bidentate ligands is always optically active — it has D3 symmetry with no mirror plane. Number of optical isomers: chiral complexes have exactly 2 (d and l forms). Total isomers of [Co(en)2Cl2]+: 3 (cis-d, cis-l, trans).