Reaction of Phosphines and Pyridine with a Heterometal Metallaborane. Ligand Substitution on Boron vs Metal Sites, Borane Displacement, and Orthometalation of a Boron-Coordinated Pyridine

The reactions of the mixed-metal metallaborane {Cp*Ru}{Cp*Ru(CO)}{Co(CO)2}(μ-CO)B3H6, 1, Cp* = η5-C5Me5, with PPh3 and PMe2Ph as representative phosphine bases and
pyridine as a representative nitrogen base have been studied. Phosphines lead to substitution
of CO on Co and the formation of {Cp*Ru}2{Co(CO)L}(μ-CO)2B3H4, L = PPh3 (2) and PMe2Ph (3). The latter has been structurally characterized in the solid state and exhibits the fac
isomeric form of an octahedral M3B3 cluster. NMR data demonstrate that 3 is fluxional in
solution and exists in two isomeric forms:  in the major one the phosphine is adjacent to the
B3 face, and in the minor one it is adjacent to the M3 face. Reaction with pyridine leads to
three products, two of which have been characterized by X-ray structure determinations.
Formation of 4, demonstrates borane
fragment displacement plus pyridine addition and subsequent orthometalation. Formation
of {Cp*Ru}2}{Co(CO)2}(μ-CO)2B2H2B(NC5H5), 5, establishes a parallel pathway involving
H substitution on boron accompanied by hydrogen elimination.