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Preparation, characterization, and crystal, molecular, and electronic structure of (H2EDTA)99TcIV(μ-O)2 99TcIV(H2EDTA)·5H2O. A 2.33-A? Tc-Tc distance which may represent a σ2π2δ*2 bond
Bürg,Anderegg,Bl?uenstein
, p. 3829 - 3834 (2008/10/08)
The preparation, elemental and titrimetric analysis, magnetic properties, and optical spectrum of (H2EDTA)99TcIV(μ-O)2 99TcIV(H2EDTA)·5H2O are described. An X-ray structure determination (Rw = 0.047) shows an almost planar four-membered Tc(μ-O)2Tc ring. Each EDTA coordinates to one Tc by two N's and two O's, and two carboxylate groups are protonated and noncoordinating (Figure 1). Interatomic distances provide strong support for a Tc(IV) oxidation state. The Tc-Tc distance is short at 2.33 A?; the molecule is diamagnetic and shows a strong optical absorption band at 20 200 cm-1 (ε ～2000). Extended Hu?ckel calculations suggest a partly antibonding σ2π2δ*2 configuration for the six metal d electrons rather than the fully bonding σ2π2δ2 configuration expected from naive application of the 18-electron rule. None of the available evidence contradicts this assignment, but more experiments are needed to prove it.
