Synthesis, Characterization, and Comparative Theoretical Investigation of Dinitrogen-Bridged Group 6-Gold Heterobimetallic Complexes

[Image: see text] We have prepared and characterized a series of unprecedented group 6–group 11, N(2)-bridged, heterobimetallic [ML(4)(η(1)-N(2))(μ-η(1):η(1)-N(2))Au(NHC)](+) complexes (M = Mo, W, L(2) = diphosphine) by treatment of trans-[ML(4)(N(2))(2)] with a cationic gold(I) complex [Au(NHC)](+). The adducts are very labile in solution and in the solid, especially in the case of molybdenum, and decomposition pathways are likely initiated by electron transfers from the zerovalent group 6 atom to gold. Spectroscopic and structural parameters point to the fact that the gold adducts are very similar to Lewis pairs formed out of strong main-group Lewis acids (LA) and low-valent, end-on dinitrogen complexes, with a bent M–N–N–Au motif. To verify how far the analogy goes, we computed the electronic structures of [W(depe)(2)(η(1)-N(2))(μ-η(1):η(1)-N(2))AuNHC](+) (10(W)(+)) and [W(depe)(2)(η(1)-N(2))(μ-η(1):η(1)-N(2))B(C(6)F(5))(3)] (11(W)). A careful analysis of the frontier orbitals of both compounds shows that a filled orbital resulting from the combination of the π* orbital of the bridging N(2) with a d orbital of the group 6 metal overlaps in 10(W)(+) with an empty sd hybrid orbital at gold, whereas in 11(W) with an sp(3) hybrid orbital at boron. The bent N–N–LA arrangement maximizes these interactions, providing a similar level of N(2) “push–pull” activation in the two compounds. In the gold case, the HOMO–2 orbital is further delocalized to the empty carbenic p orbital, and an NBO analysis suggests an important electrostatic component in the μ-N(2)–[Au(NHC)](+) bond.