MOLECULAR-ORBITAL CALCULATIONS ON DINITROGEN-BRIDGED TRANSITION-METAL DIMERS
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Fenske-Hall molecular orbital calculations on [{Ru(NH3)5}2(N2)]4+, [{(Nb(PH3)2(CH2)(CH3)}2(N2)], [{Zr(C5H5)2(N2)}2(N2)], and [{Re(PH3)4Cl}|MoCl4(OH))N2] are reported. These molecules model -N2 dimers in which the N-N bond lengths vary from 1.124 to 1.298 . The calculated Mulliken populations of the -N21g orbitals decrease in the order Nb dimer > Zr dimer > Re-Mo dimer > Ru dimer, which is also the order of decreasing bond length as determined by X-ray crystallography. Our calculations suggest that more electrons are donated into the -N21g lirg system in the Nb, Zr, and Re-Mo cases because of the presence of unoccupied d1 orbitals, which are at higher energy than occupied d1 orbitals and stabilize the -N21gMO. These orbitals also allow delocalization of the -N21g lirg electrons, which alleviates charge buildup. Differences between the Nb and Zr dimer M-N and N-N bond strengths are enhanced by synerget donation in the Nb dimer. 1984, American Chemical Society. All rights reserved.