Low-temperature crystal structure and ESR spectra of bis(isothiocyanato)[N-(2-(diphenylphosphino)ethyl)-N',N'-diethylethylenediamine]cobalt(II), a five-coordinate complex exhibiting spin equilibrium

The problem of the temperature-dependent spin equilibrium exhibited by the five-coordinated [ C O ( N C S ) ~ ( ~(n~n~p )=] (C2H5),NCH2CH2NHCH2CH2P(C,H,),) complex is reexamined in the light of variable-temperature ESR measurements and an X-ray diffraction study at 120 K. The crystals are triclinic, space group Pi, with the following cell dimensions: a = 15.669 (10) A, b = 8.813 (6) A, c = 9.368 (7) A, LY = 111.9 (l)', (I = 93.8 (l)', y = 91.3 (l)', Z = 2. The structure was refined by full-matrix least-squares methods to the final conventional R factor 0.076. The variable-temperature ESR spectra, in accord with previous IR data, suggest the existence of a temperature-dependent population of two geometrically different forms in two different spin states. The low-temperature X-ray diffraction data show significant variations of the structural parameters of the complex as compared with those of room temperature previously reported. The high-spin-low-spin interconversion is accompanied by an increased distortion of the structure toward an elongated square pyramid. The room-temperature thermal parameters are reexamined and discussed.