Synthesis and characterization of trans-M2(T(i)PB)2(O2C-CH=CH-2-C4H3S)2 (M = Mo or W) and comments on the metal-to-ligand charge transfer bands in MM quadruply bonded complexes of the type trans-M2(T(i)PB)2L2, where T(i)PB = 2,4,6-triisopropylbenzoate and L = π-accepting carboxylate ligand

The preparation and characterization of the compounds trans-M(2)(T(i)PB)(2)(O(2)C-CH=CH-2-C(4)H(3)S)(2) where M = Mo or W and T(i)PB = 2,4,6-triisopropylbenzoate are reported. The optical spectra of the new compounds are compared with those of related trans-M(2)(T(i)PB)(2)L(2) compounds where L = O(2)C-C(6)H(4)-4-CN, O(2)C-α,α'-terthienyl (TTh), and O(2)C-4-C(6)H(4)N-B(C(6)F(5))(3), that show strong metal-to-ligand charge transfer bands because of M(2)δ to Lπ conjugation, and are notably temperature dependant due to the various conformations of the two trans-L groups. Upon cooling the spectral features sharpen as the planar geometry that optimizes M(2)δ-Lπ conjugation is favored. As the electronic coupling of the two trans-Lπ systems increases the (0,0) electronic transition gains intensity indicating a greater nesting of the ground state (S(0)) and excited state (S(1)) potential energy surfaces. These features are discussed in terms of the related electronic coupling of [M(2)]-[M(2)] complexes.