Spin-crossover compounds are becoming increasingly popular for device and sensor applications, and in soft materials, that make use of their switchable colour, paramagnetism and conductivity. The de novo design of new solid spin-crossover compounds with pre-defined switching properties is desirable for application purposes. This challenging problem of crystal engineering requires an understanding of how the temperature and cooperativity of a spin-transition are influenced by the structure of the bulk material. Towards that end, this critical review presents a survey of molecular spin-crossover compounds with good availability of crystallographic data. A picture is emerging that changes in molecular shape between the high- and low-spin states, and the ability of a lattice to accommodate such changes, can play an important role in determining the existence and the cooperativity of a thermal spin-transition in the solid state (198 references).

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Structure:function relationships in molecular spin-crossover complexes

Photoswitchable coordination compounds

Spin-state transitions in metal complexes which are driven by a change of temperature T or pressure p are always associated with a considerable reorganization of molecular geometry. The change involves metal-ligand bond lengths R, bond angles, and a variation of ligand orientation. In particular, the elongation 4R by up to ∼ 10% occurring in the course of the LS → HS conversion produces an expansion of molecular volume ΔV ≌ 25 A3 per metal atom. The average crystal structure for given values of T and p is reproduced by the fractional occupancy of the individual structures of the high-spin (HS) and low-spin (LS) isomer. The transitions are reasonably well described by a number of theoretical models which are equivalent to the Bragg and Williams approximation of the Ising model. The dynamics of the spin-state transitions in solution, based on measurements by ultrasonic and photo-perturbation techniques, is in general rapid with rate constants between 4 × 105 and 3 × 108 s−1. Similar results are obtained for the spin conversion in solid complexes where the line shape analysis of Mossbauer spectra based on the theory of Blume and Tjon is applied. The dynamics may be rationalized employing one-dimensional cross sections through Gibbs free-energy surfaces G = G(R), an alternative being the comparison of the results with quantum-mechanical calculations for a radiationless non-adiabatic multiphonon process.

Nature and dynamics of the spin-state interconversion in metal complexes

Publisher Summary This chapter presents a discussion on the coordination chemistry of 2,2’: 6’, 2”-terpyridine and higher oligopyridines. The potentially terdentate ligand 2,2’: 6’, 2”-terpyridine presented, was first isolated by Morgan and Burstall as one of the numerous products from the reaction of pyridine with iron (III) chloride. There has been a considerable increase in the use of these ligands in recent years, prompted in part by the attractive photochemical and photophysical properties exhibited by complexes of the related ligands 2,2’-bipyridine and 1,10-phenanthroline. The chapter discusses stabilization of unusual complexes and states that low oxidation states are characterized by an excess of electron density at the metal atom; stabilization may be achieved by the use of ligands capable of reducing that electron density. One of the simplest ways to reduce the electron density is to design ligands with low-lying vacant orbitals of suitable symmetry for overlap with filled metal orbitals. This results in the transfer of electron density from the metal to the ligand (back-donation). In general, ligand nonbonding or π antibonding orbitals are of the correct symmetry for such overlap. The oligopyridines are ideally suited to such roles; they possess a filled highest occupied molecular orbital (HOMO) and a vacant lowest unoccupied molecular orbital (LUMO) of suitable energies for interaction with metal d orbitals. The chapter also discusses monodentate, bidentate, and bridging; terdentate and higher polydentate; cyclometallated, and others.

The Coordination Chemistry of 2,2′:6′,2″-Terpyridine and Higher Oligopyridines

Structural aspects of copper catalyzed atom transfer radical polymerization

High-spin-low-spin equilibriums of cobalt(2+) in the terpyridine complexes Co(terpy)2X2.nH2O

Copper(2+) in five-coordination: a case of a pseudo-Jahn-Teller effect. 1. Structure and spectroscopy of the compounds Cu(terpy)X2.nH2O

テルピリジン錯体Co(terpy) 2 X 2 ・nH 2 Oにおけるコバルト(2+)の高スピン-低スピン平衡

Etude au moyen de champ de coordinat et par spectrometrie RPE des complexes de Cu 2+ en coordinence 5 dans les composes Cu (terpy) X 2 •nH 2 O et analyse RX pour X=Cl et n=0 et 1. Les polyedres CuN 3 Cl 2 sont des pyramides tetragonales fortement allongees avec des liaisons equatoriales relativement fortes

Cu2+ in five-coordination: a case of a second-order Jahn-Teller effect. I: Structure and spectroscopy of the compounds Cu(terpy)X2-nH2O

Fluxionality in hexacoordinated copper(II) complexes with 2,2':6',2''-terpyridine (terpy) and related ligands: structural and spectroscopic investigations

W. Henke

Papers

High-spin-low-spin equilibriums of cobalt(2+) in the terpyridine complexes Co(terpy)2X2.nH2O

Copper(2+) in five-coordination: a case of a pseudo-Jahn-Teller effect. 1. Structure and spectroscopy of the compounds Cu(terpy)X2.nH2O

テルピリジン錯体Co(terpy) 2 X 2 ・nH 2 Oにおけるコバルト(2+)の高スピン-低スピン平衡

Cu2+ in five-coordination: a case of a second-order Jahn-Teller effect. I: Structure and spectroscopy of the compounds Cu(terpy)X2-nH2O

Fluxionality in hexacoordinated copper(II) complexes with 2,2':6',2''-terpyridine (terpy) and related ligands: structural and spectroscopic investigations