Transition metal dihaloboryl complexes of the types [(η 5 -C 5 R 5 )Fe(CO) 2 BX 2 ] (R ) H, Me; X ) F, Cl, Br), [(η 5 -C 5 H 4 Me)Ru(CO) 2 BX 2 ] (X ) Cl), and [(OC) 5 MnBX 2 ] (X ) Cl, Br) were prepared via salt elimination reactions. Comparison of the CO stretching frequencies indicates a higher degree of Fe-B d π -p π back-bonding in the dibromoboryl complexes [(η 5 -C 5 R 5 )Fe(CO) 2 BBr 2 ] with respect to their -BCl 2 and -BF 2 analogues. This finding is constistent with the p π -p π back-bonding abilities of the halides (F > Cl > Br). Reactions of [(η 5 -C 5 Me 5 )Fe(CO) 2 BX 2 ] (X ) Cl, Br) with 4-methylpyridine were performed, leading to the formation of the corresponding Lewis-base adducts and, with base in excess, to the first metal-substituted boronium cation [(η 5 -C 5 Me 5 )Fe(CO) 2 BBr(NC 5 H 4 -4-Me) 2 ] + .