Theoretical studies on metal-metal interaction, excited states, and spectroscopic properties of binuclear Au-Au, Au-Rh, and Rh-Rh complexes with diphosphine ligands: buildup of complexity from monomers to dimers.

To understand their photocatalytic activity and application in luminescent materials, a series of gold and rhodium phosphine complexes (mononuclear Au(I)(PH(3))(2) (1) and Rh(I)(CNH)(2)(PH(3))(2) (2); homobinuclear Au(I)(2)(PH(2)CH(2)PH(2))(2) (3) and Rh(I)(2)(CNH)(4)(PH(2)CH(2)PH(2))(2) (4); heterobinuclear Au(I)Rh(I)(CNH)(2)(PH(2)CH(2)PH(2))(2) (5), [Au(I)Rh(I)(CNH)(2)(PH(2)NHPH(2))(2)Cl(2)] (6), and Au(I)Rh(I)(CNH)(2)(PH(2)NHPH(2))(2) (7); and oxidized derivatives Au(II)Rh(II)(CNH)(2)(PH(2)CH(2)PH(2))(2) (8), Au(II)Rh(II)(CNH)(2)(PH(2)NHPH(2))(2)Cl(3) (9), and Au(II)Rh(II)(CNH)(2)(PH(2)NHPH(2))(2) (10)) were investigated using ab initio methods and density functional theory. With the use of the MP2 method, the M-M' distances in 3-7 were estimated to be in the range of 2.76-3.02 A, implying the existence of weak metal-metal interaction. This is further evident in the stretching frequencies and bond orders of M-M'. The two-electron oxidation from 5-7 to their respective partners 8-10 was shown to mainly occur in the gold-rhodium centers. Experimental absorption spectra were well reproduced by our time-dependent density functional theory calculations. The metal-metal interaction results in a large shift of d(z(2)) --> p(z) transition absorptions in binuclear complexes relative to mononuclear analogues and concomitantly produces a low-lying excited state that is responsible for increasing visible-light photocatalytic activities. Upon excitation, the metal-centered transition and the metal-to-metal charge transfer strengthen the metal-metal interaction in triplet excited states for 3-6, while the promotion of electrons into the sigma*(d(z(2))) orbital weakens the interaction in 9.