Molybdenum dioxide

Molybdenum dioxide is the chemical compound with the formula MoO$2$. It is a violet-colored solid and is a metallic conductor. The mineralogical form of this compound is called tugarinovite, and is only very rarely found.

Structure
It crystallizes in a monoclinic cell, and has a distorted rutile, (TiO2) crystal structure. In TiO$2$ the oxide anions are close packed and titanium atoms occupy half of the octahedral interstices (holes). In MoO$2$ the octahedra are distorted, the Mo atoms are off-centre, leading to alternating short and long Mo – Mo distances and Mo-Mo bonding. The short Mo – Mo distance is 251 pm which is less than the Mo – Mo distance in the metal, 272.5 pm. The bond length is shorter than would be expected for a single bond. The bonding is complex and involves a delocalisation of some of the Mo electrons in a conductance band accounting for the metallic conductivity.

Preparation
MoO$2$ can be prepared :
 * by reduction of MoO3 with Mo over the course of 70 hours at 800 C. The tungsten analogue, WO$2$, is prepared similarly.
 * 2 MoO$3$ +  Mo → 3 MoO$2$


 * by reducing MoO$3$ with H2 or NH3 below 470 C

Single crystals are obtained by chemical transport using iodine. Iodine reversibly converts MoO2 into the volatile species MoO$2$I$2$.

Uses
Molybdenum dioxide is a constituent of "technical molybdenum trioxide" produced during the industrial processing of MoS2:


 * 2 MoS$2$ + 7 O$2$ → 2 MoO$3$ + 4 SO$2$
 * MoS$2$ + 6 MoO$3$ → 7 MoO$2$ + 2 SO$2$
 * 2 MoO$2$ + O$2$ → 2 MoO$3$

MoO$2$ has been reported as catalysing the dehydrogenation of alcohols, the reformation of hydrocarbons and biodiesel. Molybdenum nano-wires have been produced by reducing MoO$2$ deposited on graphite. Molybdenum dioxide has also been suggested as possible anode material for Li-ion batteries.