A new ligand-driven light-induced spin-state switching mechanism has been designed based on B3LYP*/6-311++G(d,p) DFT calculations of a series of Co II , Ni II , and Cu II complexes with photochromic 2H-chromene (2H-1benzopyran) ligands 4-8 functionalized with N donor groups. The photoinduced electrocyclic rearrangements of the bischelate four-coordinate Ni II complexes with the ring-closed forms of the ligands, all with high-spin ground electronic states, are predicted to lead to the formation of a mixture of approximately equal amounts of the diamagnetic squareplanar and paramagnetic pseudo-tetrahedral isomers of these complexes containing ring-opened o-quinonoid forms of the ligands. Of the Co II complexes, only that with the 2Hchromen-8-amine ligand 4 exhibits the properties required for manifestation of the photoinitiated spin-state switching. No local minima corresponding to four-coordinate copper
[a]4203 complexes with ring-closed isomers of the functionalized 2Hchromenes were located on the respective potential energy surfaces. In their ring-closed forms, 2H-chromenes react with Cu II ions to give two-coordinate Cu I complexes, whereas ring-opened o-quinonoid isomers form four-coordinate bischelate Cu II complexes. In the Ni II complexes with isomeric forms of 2H-pyrano[3,2-h]quinoline ligand 8, the counterions Cland even BF 4 enter into the coordination sphere of the central atom to form stable six-coordinate metal complexes with high-spin ground states. The predicted capacity of the Ni II complexes for light-induced spin-state switching is retained with the bulkier BPh 4 counterion. The low-spin state of the ion-pair formed by the complex with the ring-opened form of ligand 8 is 10.5 kcal mol -1 more energetically favorable than the high-spin form.complexes leading to the light-induced excited spin-state trapping (LIESST), [3] which is an essentially cooperative effect dependent on the crystal packing and counterions. [4] The intramolecular mechanisms of magnetic bistability, which are represented by valence tautomeric rearrangements between redox isomers of the transition metal complexes formed by noninnocent ligands and are driven by electron transfer between a metal ion and the ligand [5] and various ramifications of spin-crossover processes in solution, provide for switching between the low-spin (LS) and high-spin (HS) states of the metal complexes. Among the latter are ligand-driven light-induced spin change (LD-LISC) [6] rearrangements and the recently discovered lightdriven coordination-induced spin-state switching (LD-CISSS) rearrangements [7] caused by reversible changes of the coordination number of a metal center. Whereas the LIESST effect is based on the trapping of a metastable excited spin-state of a metal complex, the LD-LISC and LD-CISSS mechanisms provide for the efficient spin-state switching through light-induced isomerizations of photochromic ligands that affect the ligand field of a complex (LD-LISC) or modify steric conditions to allow the binding of an additional donor center to th...