We present first-principles molecular dynamics simulations of oxygen difluoride impinging upon the monohydrogenated Si{001}(2 × 1) surface. Adsorption occurred in fewer than 10% of our computed trajectories, but in each reactive case the initial step involved partial dissociation to yield an adsorbed fluorine atom and a free oxygen monofluoride radical. In one trajectory, the adsorbed fluorine atom displaced a hydrogen atom into an unusual Si-H-Si bridge position, consistent with three-centre two-electron bonding. In another, a Si-Si-F motif was created, consistent with three-centre four-electron bonding. Depending upon its recoil direction, the ubiquitous monofluoride species either migrated across the surface before itself reacting to form a Si-O-Si bridge and a second adsorbed fluorine atom, or desorbed intact as a gas-phase radical.