A COMPARATIVE THEORETICAL-ANALYSIS OF THE PHYSICOCHEMICALLY DISSIMILAR TETRATHIOLATE-BRIDGED AND OXALATE-BRIDGED DITITANIUM SERIES, [(CP2TI)2(MU-C2X4)]N (WHERE X = S, N = 0, 1-, 2-, X = O, N = 0, 2+) - AN EXPLANATION OF ELECTRON DELOCALIZATION FROM THE METAL CENTERS UPON REPLACEMENT OF THE OXALATE LIGAND WITH THE TETRATHIOOXALATE LIGAND

被引:27
作者
HARRIS, HA [1 ]
KANIS, DR [1 ]
DAHL, LF [1 ]
机构
[1] UNIV WISCONSIN, DEPT CHEM, MADISON, WI 53706 USA
关键词
D O I
10.1021/ja00023a004
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
Although electronically equivalent with the oxalate-bridge (Cp2Ti)2(mu-C2O4) (2) and the corresponding nitrogen-containing (Cp2Ti)2(mu-C2(NR)4) (3), the tetrathiolate-bridged (CP2Ti)2(mu-C2S4) (1) (where Cp denotes eta-5-C5H5) exhibits sharply contrasting magnetic and electrochemical behavior at room temperature. 1 is diamagnetic and displays two quasi-reversible one-electron reductions, whereas 2 and 3 are paramagnetic (with two unpaired electrons) and only undergo single two-electron oxidations. The geometry of the TiX2C2X2Ti core in 1 (X = S) also differs markedly from those in 2 (X = O) and 3 (X = NR) in that its C2X4 bridging ligand possesses a significantly shorter C-C bond (viz., 1.41 angstrom in 1 vs 1.523 (11) angstrom in 2 and 1.504 (6) angstrom in 3) and the folding angles (alpha) of the two TiX2 planes out of the X2C2X2 plane are much larger (viz., 46.5-degrees (trans) in 1 vs 1 1.4-degrees (cis) in 2 and 11.1-degrees (trans) in 3), Molecular orbital calculations were carried out with the parameter-free Fenske-Hall model on the tetrathiolate-bridged 1, the oxalate-bridged 2, and their hypothetical ions. In order to assess the effects of change of geometry upon electronic structure, calculations were also performed on several alternative configurations of 1 and 2 (viz., C2-v cis and C2h trans isomers of 2 with alpha = 11.4-degrees; C2h trans isomers of 1 and 2 with alpha = 0.0-degrees, 11.4-degrees, and 46.5-degrees). These calculations provide a quantitative electronic basis for the stereophysical differences arising from the formal replacement of the first-row oxygen atoms in the bridging C2X4 ligand in 2 with less electronegative sulfur atoms in 1. A low-energy tau-acceptor C2S4 orbital (corresponding to the b3u LUMO for "free" C2S42- under D2h symmetry) is the main contributor (60%) to the filled HOMO and is therefore responsible for the extensive electron delocalization from the metal centers in 1. The strong bonding interactions of this tau-acceptor C2S4 orbital with the antisymmetrical dititanium symmetry combination of the valence d(x2-y2)/d(z2) Ti AOs produces a nondegenerate HOMO that accounts for the spin pairing of the unpaired electron on each Ti in 1. In contrast, the corresponding higher energy tau-acceptor C2042- orbital in 2 is energetically inaccessible such that the symmetrical and antisymmetrical dititanium combinations of the 3d(x2-y2)/3d(z2) Ti AOs form an accidentally degenerate set of half-occupied HOMOs. This MO representation is completely consistent with the valence-bond description in which 2 possess an electron-localized configuration involving the interaction of a C204(2-) dianion with two d1 Ti(III) CP2Ti+ fragments. Atomic charge calculations obtained by the natural bond orbital method gave a significantly greater total negative charge for the C2S4 ligand in 1 than for the C204 ligand in 2. This negative charge difference is compatible with the C2S4 ligand formally possessing a nonintegral oxidation state between a dianion (2-) and a tetraanion (4-); this result is also in agreement with the determined C-C distance (1.41 angstrom) in 1, which is intermediate between single-bond and double-bond distances. The geometry of the TiS2C2S2Ti core in 1 is strongly dependent on metal orbital overlap with the C2S4-tau-orbitals (especially the HOMO); the large folding angles (alpha = 46. 5-degrees) represent an optimized balance of angular-dependent Ti-S-sigma- and tau-type interactions. The near-planar geometry of the TiO2C202Ti core in 2 is due to the Ti-O interactions being dominated by metal orbital overlap with the C204 oxygen lone-pair orbitals (which favor alpha = 0.0-degrees); the relatively weak Ti-O interactions involving the C204-tau orbitals presumably account for the small folding angles (alpha = 11.4-degrees). Both the cis and trans isomers of 2 have virtually identical MO correlation diagrams and similar MO compositions; thus, the Fenske-Hall model does not account for the solid-state occurrence of the cis isomer. The different redox properties of 1 and 2 were analyzed from considerations of the HOMO/LUMO compositions.
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页码:8602 / 8611
页数:10
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