The photochemical reaction of phenyl(tribromomethyl)mercury with nido-decaborane gives a new boron cluster expanded compound. The two borons at the 6- and 9- positions in decaborane behave as electrophilic centers for the reaction with dibromocarbene. The first step in this reaction is the addition of two molar equivalents of dibromocarbene which is produced from phenyl(tribromomethyl)mercury to nido-decaborane. In the second step the unstable intermediate generates the product, 1,2-$Br_2C_2B_{10}H_{10}$ through loss of H$_2$.