  We demonstrate the creation of a spin-1/2 state via the atomically controlled
generation of magnetic carbon radical ions (CRIs) in synthetic two-dimensional
transition metal dichalcogenides (TMDs). Hydrogenated carbon impurities located
at chalcogen sites introduced by chemical doping can be activated with atomic
precision by hydrogen depassivation using a scanning probe tip. In its anionic
state, the carbon impurity exhibits a magnetic moment of 1 $\mu_\text{B}$
resulting from an unpaired electron populating a spin-polarized in-gap orbital
of C$^{\bullet -}_\text{S}$. Fermi level control by the underlying graphene
substrate can charge and decharge the defect, thereby activating or quenching
the defect magnetic moment. By inelastic tunneling spectroscopy and density
functional theory calculations we show that the CRI defect states couple to a
small number of vibrational modes, including a local, breathing-type mode.
Interestingly, the electron-phonon coupling strength critically depends on the
spin state and differs for monolayer and bilayer WS$_2$. These carbon radical
ions in TMDs comprise a new class of surface-bound, single-atom spin-qubits
that can be selectively introduced, are spatially precise, feature a
well-understood vibronic spectrum, and are charge state controlled.
