Abstract
Energies and charge distributions of the lower unoccupied p MO's were detd. for the isomeric bidiazine ligands 3,3'-bipyridazine (bpdz), 4,4'-bipyrimidine (bpm), 2,2'-bipyrazine (bpz), and 2,2'-bipyrimidine (bpym) as well as for 2,2'-bipyridine (bpy) by Gueckel MO perturbation calcns. In conjunction with exptl. pKa values, the calcn. results are related to the different stabilities, redox potentials, and various spectroscopic properties of Cr, Mo, and W tetracarbonyl complexes with these ligands. W(CO)5L (L = bpdz, bpm) were isolated as intermediates in the reactions between W(THF)(CO)5 and L.. Each of the 4 isomeric ligands exhibits very characteristic features, and the potential uses of the individual systems for different purposes in coordination chem. are discussed. Whereas bpdz is the strongest base in that series, thus compensating for a rather high-lying p* level, the bpm complexes exhibit the strongest back-bonding interaction because of a very low-lying p* level and sufficient ligand basicity. The bpz system is distinguished by acquiring high amts. of addnl. charge at both sets of N centers upon redn., and finally, the otherwise less outstanding bpym ligand is unique among the bidiazine isomers by forming binuclear bischelate complexes on 2nd coordination. on SciFinder(R)
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