The reaction of Cp*Ru((PPr3)-Pr-i)Cl (1) with MesBH(2) (Mes = 2,4,6-trimethylphenyl) afforded the mesitylborate complex Cp*Ru((PPr3)-Pr-i)(BH(2)MesCl) (2, 66%). Exposure of 2 to the chloride abstracting agent LiB(C6F5)(4)center dot 2.50Et(2) provided [Cp*Ru((PPr3)-Pr-i)(BH(2)Mes)]B+(C6F5)(4) (3, 54%), which features an unusual eta(2)-B-H monoborane ligand. The related borate complex Cp*Ru((PPr3)-Pr-i)(BH(3)Mes) (5, 65%) was prepared from 1 and LiH(3)BMes. Attempts to effect the insertion of unsaturated organic substrates into the B-H bonds of 3 were unsuccessful, and efforts to dehydrohalogenate 2 using (KOBu)-Bu-i instead afforded the mesitylborate complex Cp*((PPr3)-Pr-i)Ru(BH(2)MesOH) (6, 48%). Treatment of 1 with benzyl potassium generated an intermediate hydridoruthenium complex (7) resulting from dehydrogenation of a (PPr)-Pr-i fragment, which in turn was observed to react with MesBH(2) to afford the mesitylborate complex Cp*(P(Pr-i)(2)(CH3CCH2))Ru(BH(3)Mes) (8, 47%). Crystallographic characterization data are provided for 2, 3, 5, 6, and 8. A combined X-ray crystallographic and density functional theory (DFT) investigation of 3 and 5, using Natural Bond Orbital (NBO) and Atoms in Molecules (AIM) analysis, revealed that 3 and 5 are best described as donor acceptor complexes between a Cp*((PPr3)-Pr-i)Ru fragment and a bis(eta(2)-B-H) coordinating mesitylborane(borate) ligand. Significant sigma-donation from the B-H bonds into the Ru-II center exists as evidenced by the NBO populations, bond orders, and AIM delocalization indices. In the case of 3, the vacant p orbital on boron is stabilized by Ru -> B pi back-donation as well as by resonance with the mesityl group.