原子価結合理論

Valence bond theory explains complex geometry through metal orbital hybridization. The central metal atom hybridizes its s, p, and d orbitals to create empty hybrid orbitals of equivalent energy. These hybrid orbitals accept electron pairs from ligand orbitals, forming coordinate covalent bonds. The type and number of hybrid orbitals determine the complex's geometry: sp for linear, sp3 for tetrahedral, dsp2 for square planar, and d2sp3 or sp3d2 for octahedral structures.

Inner orbital complexes use inner d orbitals (like 3d) in hybridization, typically forming with strong field ligands that force d-electron pairing. These complexes are diamagnetic and more stable. Outer orbital complexes use outermost empty d orbitals (like 4d) in hybridization, occurring with weak field ligands. These complexes remain paramagnetic with unpaired electrons, are less stable, and more labile due to higher orbital energies.

In hexamminecobalt(III), ammonia ligands are strong field ligands that force the 3d electrons of Co3+ to rearrange and pair up. This pairing creates two vacant 3d orbitals. These vacant 3d orbitals combine with one 4s and three 4p orbitals to generate six d2sp3 hybrid orbitals. These hybrid orbitals then accept electron pairs from the six ammonia groups, forming a diamagnetic octahedral complex.

Ligand strength determines d-orbital participation. Strong field ligands force unpaired d electrons to pair up, creating vacant inner d orbitals that participate in hybridization. Weak field ligands do not cause d-electron rearrangement, so inner d orbitals remain occupied. Instead, outer empty d orbitals combine with s and p orbitals for hybridization. This difference produces inner versus outer orbital complexes with distinct magnetic and stability properties.

In tetrachloronickelate, the Ni2+ ion (3d8 configuration) uses empty 4s and 4p orbitals to form four sp3 hybrid orbitals. These sp3 orbitals point toward the corners of a tetrahedron and accept electron pairs from four chloride ligands. The resulting complex is paramagnetic because the d electrons remain unpaired. This tetrahedral geometry contrasts with octahedral structures that require six hybrid orbitals.

In square planar complexes like tetrachloroplatinate, the Pt2+ ion (d8 configuration) experiences d-electron rearrangement from chloride ligands, creating one vacant d orbital. This vacant d orbital combines with one 4s and two 4p orbitals to form four dsp2 hybrid orbitals directed toward square corners. These hybrid orbitals accept electron pairs from four chloride ligands, forming a diamagnetic complex with square planar geometry.

Although valence bond theory successfully explains complex geometry and hybridization patterns, it cannot account for electronic spectra or the variation in magnetic behavior with temperature. VBT does not explain why coordination complexes exhibit different colors or predict temperature-dependent magnetic changes. These phenomena require crystal field theory, which considers ligand field effects on d-orbital energy levels more comprehensively.