Magnetic exchange interactions in cyano‐bridged MoIII binuclear complexes: Broken‐symmetry and density functional theory calculations

Molecular magnetism in cyano‐bridged Mo^III binuclear complexes [Mo₂(CN)₁₁]^5? and [(Me₃tacn)₂Mo₂(CN)₅]⁺ (Me₃tacn?N, N′, N″‐trimethyl‐1,4,7‐triazacyclononane) has been calculated using Becke's three‐parameter exchange functional and the gradient‐corrected functional of Lee, Yang, and Parr (B3LYP), a hybrid density functional theory (DFT), combined with a modified broken symmetry (BS) approach and the post–Hartree‐Fock (post‐HF) method difference‐dedicated configuration interaction (DDCI). We find B3LYP combined with broken‐symmetry approach (DFT‐BS) give the similar J values to those calculated by DDCI. So we use B3LYP combined with BS approach to investigate the magnetism above two molecules. Through calculations, we find that the absolute J values decrease with the increase of r (the Mo(2)‐C_brid and Mo(1)‐N_brid distances) and are linearly related to the differences of the squared spin populations [(ρ ? ρ)] on Mo^III atoms between the highest‐spin (HS) state and the broken symmetry (BS) state. Moreover, the absolute J values are linearly related to the sum of the square of the difference in energy of the unpaired electrons (ξ) with a limited variation of the r distance. We conclude that ξ can be used to scale the degree of the antiferromagnetic coupling interactions. At the end of the paper, the spin density distributions and the mechanisms of magnetic coupling interactions are analyzed by us. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005