The mechanism of transition-metal tetrahydroborate dimerization was established for the first time on the example of (Ph3P)(2)Cu(eta(2)-BH4) interaction with different proton donors [MeOH, CH2FCH2OH, CF3CH2OH, (CF3)(2)CHOH, (CF3)(3)CHOH, p-NO2C6H4OH, p-NO2C6H4N=NC6H4OH, p-NO2C6H4NH2] using the combination of experimental (IR, 190-300 K) and quantum-chemical (DFT/M06) methods. The formation of dihydrogen-bonded complexes as the first reaction step was established experimentally. Their structural, electronic, energetic, and spectroscopic features were thoroughly analyzed by means of quantum-chemical calculations. Bifurcate complexes involving both bridging and terminal hydride hydrogen atoms become thermodynamically preferred for strong proton donors. Their formation was found to be a prerequisite for the subsequent proton transfer and dimerization to occur. Reaction kinetics was studied at variable temperature, showing that proton transfer is the rate-determining step. This result is in agreement with the computed potential energy profile of (Ph3P)(2)Cu(eta(2)-BH4) dimerization, yielding [{(Ph3P)(2)Cu}(2)(mu,eta(4)-BH4)](+).