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[EN] A multistable spin crossover (SCO) molecular alloy system [Fe1-xMx(nBu-im)(3)(tren)](P1-yAsyF6)(2) (M = Zn-II, Ni-II; (nBu-im)(3)(tren) = tris(n-butyl-imidazol(2-ethylamino))amine) has been synthesized and characterized. By controlling the composition of this isomorphous series, two cooperative thermally induced SCO events featuring distinct critical temperatures (T-c) and hysteresis widths (Delta T-c, memory) can be selected at will. The pristine derivative 100As (x = 0, y = 1) displays a strong cooperative two-step SCO and two reversible structural phase transitions (PTs). The low temperature PTLT and the SCO occur synchronously involving conformational changes of the ligand's n-butyl arms and two different arrangements of the AsF6- anions [T-c(1) = 174 K (Delta T-c(1) = 17 K), T-c(2) = 191 K (Delta T-c(2) = 23 K) (scan rate 2 K min(-1))]. The high-temperature PTHT takes place in the high-spin state domain and essentially involves rearrangement of the AsF6- anions [T-c(PT) = 275 K (Delta T-c(PT) = 16 K)]. This behavior strongly contrasts with that of the homologous 100P [x = 0, y = 0] derivative where two separate cooperative one-step SCO can be selected by controlling the kinetics of the coupled PTLT at ambient pressure: (i) one at low temperatures, T-c = 122 K (Delta T-c = 9 K), for temperature scan rates (>1 K min(-1)) (memory channel A) where the structural modifications associated with PTLS are inhibited; (ii) the other centered at T-c = 155 K (Delta T-c = 41 K) for slower temperature scan rates PF6- substitution, and hence of the PTLT kinetics, selectively selects the memory channel B of 100P when x = 0 and y approximate to 0.7. Meanwhile, substitution of Fe-II with Zn-II or Ni-II [x approximate to 0.2, y = 0] favors the low temperature memory channel A at any scan rate. This intriguing interplay between PT, SCO and isomorphous substitution was monitored by single crystal and powder X-ray diffractometries, and magnetic and calorimetric measurements., This work was supported by the Spanish Ministerio de Economia y Competitividad (MINECO), FEDER (CTQ2016-78341-P), Unidad de Excelencia Maria de Maeztu (MDM-2015-0538), and the Generalitat Valenciana through PROMETEO/2016/147 and an EU Framework Program for Research and Innovation (RISE project number 734322). F. J. V. M. and M. M. S. thank MINECO for a predoctoral (FPI) grant.