The structural characteristics and physical properties of the 3,5-bis(2-pyridyl)-1,2,4-triazolate (L1) bridged dinuclear iron(II) spin-crossover complex [{Fe(NCBH3)(py)}2(μ-L1)2] (1) in both powder (1p) and single crystal (1c) forms have been investigated. Both forms of [{Fe(NCBH3)(py)}2(μ-L1)2] display a thermally induced spin transition; however, the transitions have different T1/2 values and different degrees of spin conversion. Both forms display the photomagnetic light-induced excited spin-state trapping (LIESST) effect as well as reverse LIESST and have been compared by Raman spectral and powder X-ray diffraction methods, which indicate that they are polymorphs. The single crystal form 1c shows a "half" spin transition and has been further characterised at temperatures above and below the spin transition by low temperature crystallographic methods including single crystal LIESST experiments (at 40 K) and by Mössbauer spectroscopy; thus, the nature of the [HS-LS] form and the different spin isomers were revealed. To complement the experimental results, compound 1 and several other related FeII dinuclear spin-crossover compounds have been evaluated by quantum-chemical DFT calculations. Additionally, the susceptibilities for the powder form 1p, which displays a complete two-step spin-crossover, were also fitted to a phenomenological model for dinuclear spin-crossover complexes.Read less <