  The metal-insulator transition (MIT) of VO2 is discussed with particular
emphasis on the structural instability of the rutile compounds toward
dimerization. Ti substitution experiments reveal that the MIT is robust up to
20% Ti substitutions and occurs even in extremely thin V-rich lamellas in
spinodally decomposed TiO2-VO2 composites, indicating that the MIT is
insensitive to hole doping and essentially takes on a local character. These
observations suggest that either electron correlation in the Mott-Hubbard sense
or Peierls (Fermi-surface) instability plays a minor role on the MIT. Through a
broad perspective of crystal chemistry on the rutile-related compounds, it is
noted that VO2 and another MIT compound NbO2 in the family eventually lie just
near the borderline between the two structural groups with the regular rutile
structure and the distorted structures characterized by the formation of dimers
with direct metal-metal bonding. The MITs of VO2 and NbO2 are natural
consequences of structural transitions between the two groups, as all the d
electrons are trapped in the bonding molecular orbitals of dimers at low
temperatures. Such dimer crystals are ubiquitously found in early transition
metal compounds having chain-like structures, such as MoBr3, NbCl4, Ti4O7, and
V4O7, the latter two of which also exhibit MITs probably of the same origin. In
a broader sense, the dimer crystal is a kind of molecular orbital crystals in
which virtual molecules made of transition metal atoms with partially-filled
t2g shells, such as dimers, trimers or larger ones, are generated by
metal-metal bonding and are embedded into edge- or face-sharing octahedron
networks of various kinds. The molecular orbital crystallization opens a
natural route to stabilization of unpaired t2g electrons in crystals.
