Which orbitals are considered frontier orbitals in coordination chemistry?

In coordination chemistry, frontier orbitals refer to the highest energy molecular orbitals that are involved in the formation of chemical bonds. The terminology typically aligns with the theory of crystal field splitting, particularly in octahedral complexes where the d-orbitals split into two sets: t2g and eg.

The t2g set consists of three orbitals (dxy, dxz, dyz), while the eg set includes two orbitals (dz2 and dx2-y2). In transition metal complexes, the frontier orbitals are relevant because they play a crucial role in bonding interactions with ligands. In octahedral coordination, the ligands will interact with these t2g and eg orbitals, influencing properties such as the stability and color of the complexes.

When considering bond formation and electronic transitions, it is specifically the t2g and eg orbitals that participate directly in coordination with ligands and in processes like ligand-to-metal charge transfer and metal-to-ligand charge transfer, making them the frontier orbitals of interest in this context. This is why the choice referencing t2g and eg is recognized as correct.