  We analyze and give estimates for the long-distance coupling via floating
metallic gates between different types of spin qubits in quantum dots made of
different commonly used materials. In particular, we consider the hybrid, the
singlet-triplet, and the spin-$1/2$ qubits, and the pairwise coupling between
each type of these qubits with another hybrid qubit in GaAs, InAs, Si, and
$\mathrm{Si_{0.9}Ge_{0.1}}$. We show that hybrid qubits can be capacitively
coupled strongly enough to implement two-qubit gates, as long as the distance
of the dots from the metallic gates is small enough. Thus, hybrid qubits are
good candidates for scalable implementations of quantum computing in
semiconducting nanostructures.
