Molecular orbital calculations at the ab initio level are used to study polytopal rearrangements in H
2Ru(PH
3)
4 andH
2Fe(CO)
4 as models of 18-electron, octahedral metal dihydrides. It is found that, in both cases, the transitionstate for these rearrangements is a dihydrogen species. For H
2Fe(CO)
4, this is a square pyramidal complex wherethe H
2 ligand occupies an apical position and is rotated by 45
![](/images/entities/deg.gif)
from its original orientation. This is preciselyanalogous to the transition state for Fe-olefin rotation in (olefin)Fe(CO)
4 complexes and has a very similarelectronic origin. Another transition state very close in energy is found wherein the basic coordination geometryis a trigonal bipyramid and the H
2 ligand is coordinated in the axial position. For H
2Ru(PH
3)
4, the former stationarypoint lies at a much higher energy and the latter clearly serves as the transition state for hydride exchange. Thereason for this difference is discussed along with the roles of electron correlation in the two compounds.