The Tungsten-Tungsten Triple Bond. 14. Factors Influencing Triple Bonds between Tungsten Atoms Having Valence Molecular Orbital Configurations of ?2?4 and ?4δ2 and Preparation and Characterization of Bis(neopentyl)bis(acetato)bis(diethyldithiocarbamato)ditungsten

Article abstract of DOI:10.1021/ja00256a039

Compounds having the general formula W2R2(O2CX)4 (R = an alkyl lacking β hydrogens; X = alkyl, aryl, NR2) adopt one of two basic structures with W-W triple bonds of approximate valence MO configurations of either ?⁴δ² (I) or ?²?⁴ (II).The most common structure for X = alkyl or aryl is one in which the alkyl ligands, R, are aligned along the W-W axis and four O2CX ligands span a W<*>W bond of ?⁴δ² configuration (I).The other structure, found exclusively for X = NR2, is one in which there are mutually syn-alkyl ligands, and each W atom forms five W-L ? bonds that all lie roughly in a pentagonal plane with a resultant W<*>W bond of ?²?⁴ configuration (II).Both steric and electronic factors are important in determining the choice of structure I or II.Bulky supporting ligands favor I and appear to show a discriminating factor R > X, but electronic factors can override these steric influences, as in the case of X = NR2.The reaction between W2(np)2(O2CMe)4 (np = neopentyl) and NaS2CNEt2 (2 equiv) in hexane in the presence of MeCOOH yields W2(np)2(O2CMe)2(S2CNEt2)2 with the transformation of the W<*>W bond from ?⁴δ² to ?²?⁴.The reaction between W2(np)2(O2CMe)4 and thioacetic O-acid (MeCOSH) yields W2(np)2(O2CMe)3(SOCMe) with retention of the axial W-np ligands and the ?⁴δ² configuration of the W<*>W bond.With the aid of Fenske-Hall calculations we show that there is essentially no electronic energy barrier to the interconversion of ?⁴δ² and ?²?⁴ triple bonds.Conjugation between carboxylate and ? lone-pair orbitals of carbamate ligands has a destabilizing influence on the δ orbital in the ?⁴δ² manifold, rendering it unstable with respect to the ?²?⁴ configuration.A similar effect is invoked in the conversion of W2(np)2(O2CMe)4 to W2(np)2(O2CMe)2(S2CNEt)2.Pertinent distances (Angstroem) for W2(np)2(O2CMe)2(S2CNEt2)2 are W-W = 2.2909 (10) Angstroem, W-O = 2.15 (1) Angstroem (averaged), W-S = 2.52 (1) Angstroem (averaged), and W-C = 2.19 (1) Angstroem.Crystal data (-160 deg C) are as follows: a = 17.092 (12) Angstroem, b = 11.023 (7) Angstroem, c = 17.576 (12) Angstroem, β = 98.19 (4)deg, d_calcd = 1.825 g cm^-3, Z = 4, and space group P2₁/a.