The identity of boron clusters and boron interstitial clusters in Si, produced by radiation or implantation, has been studied theoretically. Local vibration modes of the single-boron interstitial and the diboron split interstitial $({\mathrm{B}}_{2}\mathrm{I})$ and its metastable isomer as well as substitutional dimers are found to be in good agreement with observations. The modes of a diboron defect that has trapped three interstitials ${\mathrm{B}}_{2}{\mathrm{I}}_{3}$ are close to those observed for the boron-related $I2$ luminescent center. The annealing of these centers around $400\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ coincides with the main recovery of the electrical activity of boron, but the formation of defects which are metastable casts doubt on previous modeling strategies.