The complexes of Fe3+ have magnetic properties that depend on whether the ligands are strong or weak field. Explain why this observation supports the idea that electrons are lost from the 4s orbital before the 3d orbitals in the transition metals.

The electron configuration of transition metals involves filling the outer and penultimate shells in a unique sequence. Elements such as iron (Fe) start by filling their 4s orbital before the 3d orbitals, as per the Aufbau principle. This predictable pattern is true when the element is in its ground state.

However, the behavior changes when these transition metals form ions. The ions lose their 4s electrons before their 3d electrons, despite the 4s being filled first. This is termed the 4s subshell being of 'higher energy' despite its lower principal quantum number. When Fe loses three electrons to form Fe3+, it will lose the two 4s electrons first, followed by one from the 3d orbitals. This process highlights the energy complexities within transition metal ions and sets the stage for how they interact with other elements to form complexes.

Magnetic properties of transition metal complexes arise from the behavior of unpaired electrons within the d orbitals. In general, unpaired electrons contribute to magnetic moments resulting in paramagnetism, while paired electrons result in diamagnetism. A complex's magnetism is determined by the spin state of the central metal ion's electrons and hence its overall electronic configuration.

In Fe3+ complexes, the magnetic properties vary, signaling the presence of differing numbers of unpaired electrons. These variations are due to the nature of the ligands involved: strong field ligands tend to pair up electrons, lowering the net magnetic moment, while weak field ligands leave more electrons unpaired, causing a higher magnetic moment. The presence of these magnetic properties solidifies the understanding that the 4s orbital electrons are lost before the 3d electrons in such ions.

Ligand field theory is an advanced explanation for the bond formation in metal complexes that also helps to explain their magnetic properties. It expands upon crystal field theory by considering the covalent nature of interactions between ligands and the central metal ion. According to this theory, ligands, which are typically ions or molecules that can donate a pair of electrons to the metal ion, influence the energy of the d orbitals in the metal ion.

Strong field ligands cause a large energy separation between the d orbitals, leading to a lower tendency for the d electrons to occupy the higher energy orbitals, thus pairing up electrons and resulting in low-spin complexes. Conversely, weak field ligands result in smaller energy separation, allowing electrons to spread out over more orbitals and remain unpaired, forming high-spin complexes.

The concept of high-spin versus low-spin complexes deals with the arrangement of electrons in the metal ion's d orbitals after bonding with ligands. The difference between high-spin and low-spin configurations is significant in determining the magnetic properties and overall behavior of the complex.

High-spin complexes are formed with weak field ligands, which allows electrons to occupy as many orbitals as possible with parallel spins due to a relatively low amount of crystal field splitting. This results in a greater number of unpaired electrons and stronger magnetic properties. Low-spin complexes form with strong field ligands, creating enough crystal field splitting such that fewer orbitals are occupied and electrons are forced to pair up. This results in fewer unpaired electrons, a weaker magnetic field, and sometimes completely diamagnetic behavior if all electrons are paired.