A quantum-admixture model for the d(6) configuration ferrous complex molecules with the high-spin <-> low-spin transition has been established by using the unified crystal-field-coupling (UCFC) scheme. A general study has been made on the spin transition of octahedrally coordinated d(6) complexes, and a special application has been given to an Fe(II) compound Fe-II(TRIM)(2)(PhCO2)(ClO4). The results show the following: (i) The quantum picture of the spin transition of a d6 System, such as Fe(II), is much more complex than a simple transition between the pure T-5(2), and (1)A(1)g states as usually understood. In practice, owing to spin-orbit coupling, spin is no longer a good quantum number and there is no longer a pure T-5(2), or (1)A(1)g. state. Each of them splits into substates and each substate is a linear combination of various multiplets. The high-spin -> low-spin transition of an octahedrally coordinated d6 ion is practically the crossover of the two lowest substates of 5T2, at the critical point. (ii) At the spin-transition critical point the magnetic moment mu(eff) approximate to 5.22 mu(B), which is obviously different from the simple average of they mu(eff) values of high-spin and low-spin states but near the saturation value. (iii) The calculation of the effective molecular magnetic moment mu(eff) for an octahedrally coordinated Fe(II) ion shows that the mu(eff)-T curve is in good agreement with Lemercier et al.'s experiment and both the low-spin value mu(eff) = 0.51 mu(B) and the high-spin value mu(eff) = 5.4 mu(B) are comparable with the experimental values 0.76 mu(B) and 5.4 mu(B), respectively. (iv) The T dependence of the crystal field parameter Dq in the spin-transition region is approximately linear.