Theoretical Study of the Structure and Properties of [(η(5)-C5Me4H)2Zr]2(μ(2),η(2),η(2)-N2). 
Recently Pool et al. [ Pool, J. A.; Lobkovsky, E.; Chirik, P. J. Nature 2004, 427, 527.]  showed that the [(η(5)-Cp')2Zr]2(μ(2),η(2),η(2)-N2), Cp' = η(5)-C5Me4H, complex is promising for dinitrogen hydrogenation.  In the present study we examine computationally the structure and relative energies of different possible positional isomers of this dimer complex as well as different isomers of the monomer complex (Cp'2Zr)(η(2)-N2).  The relative stability of isomers of the monomer is determined by the electrostatic repulsion between the negatively charged N atoms of the N2 molecule and the negatively charged C atoms of the Cp' ring that are bound to H. Substitution of H atoms by methyl groups significantly changes the charge distribution in Cp rings, increases the negative charge of CH atom, and affects the relative stability of the isomers.  On the other hand, competition between the electrostatic effects and the steric repulsion determines the relative energy of the positional isomers of the dimer (Cp'2Zr)2(μ(2),η(2),η(2)-N2).