First-Principles Modeling of Bonding and Magnetic Exchange in the Metal-TCNE Magnet Family*

The chemical bond and its role as a mediator of magnetic exchange interaction remains a crucial aspect in the study of molecular magnetism. Within the M-TCNE (M$=$3$d$ metal; TCNE$=$tetracyanoethylene) class of organic-based magnets, only V[TCNE]$_{\mathrm{x}}$ (x$\sim $2) orders magnetically above room-temperature ($T_{\mathrm{c}}\sim $400 K), while structural factors underlying this exceptional behavior remain elusive. Conversely, Mn-TCNE complexes of diverse crystal structure, e.g., 1D-chain MnTPP[TCNE] ($T_{\mathrm{c}}\sim $10 K), 2D-layer [Mn(TCNE)(NCMe)$_{2}$]SbF$_{6}$ ($T_{\mathrm{c}}\sim $75 K), and 3D-network [Mn(TCNE)$_{1.5}$](I$_{3}$)$_{0.5}(T_{\mathrm{c}}\sim $170 K) have recently become available. Using this structural data, hybrid DFT simulations has been performed and the spin-polarized electronic structures resolved. The nature of bonding and non-bonding orbital interactions crucial for understanding magnetic behavior was revealed. Orbital ordering, hybridization, and trends in spin-density transfer (bonding/backbonding) as well as the formation of exchange/superexchange pathways have been identified and interpreted in terms of the dimensionality of magnetic interaction. The role of these and additional factors in establishing high-$T_{\mathrm{c}}$ magnetism in the broader M-TCNE class will be discussed.