What is a paramagnetic complex? Why is [Fe(H2O)6]2+ paramagnetic while [Fe(CN)6]2- is diamagnetic, despite both having octahedral Fe2+ centres?

A paramagnetic complex is any complex that has at least one unpaired electron.[Fe(H2O)6]2+ is paramagnetic since H2O is a weak field ligand, this means there is a small energy gap between the t2g and eg and so promoting electrons to the eg becomes energetically favourable above putting 2 electrons in the same oribtal. Due to this the complex adopts a high spin conformation putting 3 electrons in the t2g orbital and 2 in the eg. This means the complex has 5 unpaired electrons and is paramagnetic. In the case of [Fe(CN)6]2- CN is a strong field ligand so the energy gap between the t2g and eg is high, as such the t2g orbitals are completely filled with 6 electrons and there is no unpaired electrons. The complex is, therefore, diamagnetic.