Bonding, Backbonding and Spin-Polarized Molecular Orbitals: Basis for Magnetism and Semiconducting Transport in V[TCNE]$_{x\sim 2}$*

V[TCNE]$_{x\sim 2}$ films exhibit magnetic order up to 400 K, magneto-resistance, and photo-induced magnetism. Yet the spin-polarized interactions between the TM and molecular species underlying these properties have remained elusive, in part because of its structural disorder. Using element-specific x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (MCD) at the V $L$ edges, and the C and N $K$ edges we have gained new insight into these mechanisms [1]. We find evidence for covalent bonding between the V $e_{g}$ and TCNE \textit{$\sigma $} MO states, and a weaker interaction between V $t_{2g}$ and TCNE \textit{$\pi $} MO states, consistent with a generalized bonding/backbonding model with V octahedrally coordinated by N in \textit{$\sigma $}-bridging positions between TCNE radical anions. C and N XAS and MCD reveal spin-polarized splitting of the former LUMO of neutral TCNE, indicating that a direct exchange interaction underlies these properties. This indicates an active role of TCNE$^{\bullet -}$ in the magnetic properties of extended V[TCNE]$_{x\sim 2}$ and related systems, which is distinctly different from superexchange models generally used to describe magnetic Prussian blue analogs. [1] Phys. Rev. Lett. \textbf{100}, 257204 (2008).