New conception of spin-tautomery was developed based on the results of quantum chemical investigation of the endohedral Y@C60 complex. The embedding energies of yttrium into the fullerene cavity, dipole moments, polarizabilities, and IR spectra in the doublet and quartet spin states were calculated by the density-functional theory quantum chemical methods. The degeneracy degree of the state of the complex is retained upon doublet–quartet excitation, since a twofold increase in the number of electron spin projections is compensated by a twofold decrease in the number of equivalent potential energy minima. The non-conservation of spin by an endohedral complex with a heavy atom makes it possible to interpret the interconversion of doublet and quartet states as a non-degenerate spin-tautomery.