Synthesis and characterization of trans-M2(TiPB)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(TiPB)2L2, where TiPB=2,4,6-triisopropylbenzoate and L = π-accepting carboxylate ligand

The preparation and characterization of the compounds trans-M-2((TPB)-P-i)(2)(O2C-CH=CH-2-C4H3S)(2) where M = Mo or W and (TPB)-P-i = 2,4,6-triisopropylbenzoate are reported. The optical spectra of the new compounds are compared with those of related trans-M-2((TPB)-P-i)(2)L-2 compounds where L = O2C-C6H4-4-CN, O2C-alpha,alpha'-terthienyl (TTh), and O2C-4-C6H4N-B(C6F5)(3), that show strong metal-to-ligand charge transfer bands because of M-2 delta to L pi 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 delta-L pi conjugation is favored. As the electronic coupling of the two trans-L pi 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.