We present here two-electronic model, which describes singlet-triplet interaction (1)pi - (3)Sigma(+) in linear triatomic molecules. The analysis takes into account spin-orbital coupling terms in electronic Hamiltonian, as well as its symmetry properties. We give the symmetry operators of electronic Hamiltonian including space operators (acting on electronic coordinates) and matrix operators (acting on electronic spin). We consider only deformation pi-modes and our resulting 5 x 5 vibronic matrix describes actual relativistic pseudo-Renner effect ((1)pi - (3)Sigma) x pi. The eigenvalues of vibronic matrix (i.e. potential energy surfaces) have axial symmetry and represented by analytical expressions, include five electrostatic and three spin-orbital parameters. (C) 2017 Elsevier B.V. All rights reserved.