The heating of the 18-electron complex [3,3-(dppb)-3-H-3-Cl-closo-3,1,2-RuC2B9H11] (3) in benzene at 80 degrees C in the presence of a small amount of CCl4 as initiator afforded paramagnetic 17-electron species [3,3-(dppb)-3-Cl-closo-3,1,2-RuC2B9H11] (4) along with minor amounts of two P-phenylene ortho-cycloboronated derivatives [3-Cl-3,3,8-{Ph2P(CH2)(4)PPh- mu-(C6H4-ortho)}-closo-3,1,2-RuC2B9H10] (5) and [3,7-Cl-2-3,3,8-{Ph2P(CH2)(4)PPh-mu-(C6H4-ortho)}-closo-3,1,2-RuC2B9H10] (6) in total yield of ca. 8096. The heating of either 3 or 4 in toluene at 95 degrees C in the absence of CCl4 led to the selective formation of 5, which was isolated in 64% and 46% yield, respectively. Thermolysis of 3 at higher temperatures (boiling toluene, 110 degrees C) gives novel paramagnetic species [3-Cl-3,3,7,8-{Ph2P(CH2)(4)P-mu-(C6H4-ortho)(2)}-closo-3,1,2-RuC2B9H9] (7) featuring bis(ortho-cycloboronation) of both P-phenyl groups at the same phosphorus atom of the ruthenium-bound dppb ligand. All new paramagnetic complexes 4-7, as well as starting diamagnetic species 3, were characterized by single-crystal X-ray diffraction and, in addition, by EPR spectroscopic studies of odd-electron complexes. Ruthenacarboranes 3-5 and 7 all display high efficiency as catalysts for the atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA). Complex 5 gave the best catalyst performance in terms of polydispersity; the PDI (M-w/M-n) of the polymer samples is as low as 1.15.