  Transition metal (TM) defects in silicon carbide have favorable spin
coherence properties and are suitable as quantum memory for quantum
communication. To characterize TM defects as quantum spin-photon interfaces, we
model defects that have one active electron with spin 1/2 in the atomic $D$
shell. The spin structure, as well as the magnetic and optical resonance
properties of the active electron emerge from the interplay of the crystal
potential and spin-orbit coupling and are described by a general model derived
using group theory. We find that the spin-orbit coupling leads to additional
allowed transitions and a modification of the $g$-tensor. To describe the
dependence of the Rabi frequency on the magnitude and direction of the static
and driving fields, we derive an effective Hamiltonian. This theoretical
description can also be instrumental to perform and optimize spin control in TM
defects.
