Effect of the counter-anion on the spin-transition properties of a family of Fe(ii) tetrazole complexes, [Fe(i4tz)<inf>6</inf>]X<inf>2</inf> (X = ClO <inf>4</inf><sup>-</sup>, PF<inf>6</inf><sup>-</sup>, SbF<inf>6</inf> <sup>-</sup>, BF<inf>4</inf><sup>-</sup>) [DOI]