Unexpected formation of a weak metal-metal bond:: Synthesis, electronic properties, and second-order NLO responses of push-pull late-early heteronuclear bimetallic complexes with W(CO)3(1,10-phenanthroline) acting as a donor ligand

In attempts to bridge the complex [W(CO)(3)(phen)(pyz)] (phen = 1,10-phenanthroline; pyz = pyrazine) to acceptor centers, either soft centers such as cis-M(CO)(2)Cl (M = Rh(I), Ir(l)) and fac-M(CO)(3)Cl-2 (M = Ru(II), Os(II)) or hard centers such as BF3, the pyrazine ligand is lost, while the fragment W(CO)(3)(phen) behaves as a a-donor base with the unexpected formation of heteronuclear early-late bimetallic compounds with a weak metal-metal bond, as confirmed by the easy substitution of W(CO)(3)(phen) by soft ligands (PPh3, CO, pyridine). The X-ray structures of [(CO)(3)(phen)W-cis-Ir(CO)(2)Cl] and [(CO)(3)(phen)W-fac-Os(CO)(3)Cl-2] confirm a single metal-metal bond with an halogen bridging asymmetrically the two metallic moieties and with the tungsten atom achieving a distorted (6 + 1) octahedral coordination. All the heteronuclear bimetallic complexes investigated show in their electronic spectra a new solvatochromic absorption band at around 385-450 nm in addition to the MLCT (W-->pi*(phen)) absorption band typical of [W(CO)(3)(phen)L] complexes (L = CO, pyz, CH3CN) and an increased, in comparison to [W(CO)(4)(phen)], negative nonlinear (NLO) second-order emission working with the EFISH technique with an incident wavelength of 1.907 mum. The increase is due to an additional negative contribution of the new absorption band at around 385-450 nm, as shown by a solvatochromic investigation.