Chemistry of Mangana- and Rhenatricarbadecaboranyl Tricarbonyl Complexes:  Evidence for an Associative Mechanism of Ligand Substitution Involving an η⁶−η⁴ Cage-Slippage Process Analagous to η⁵−η³-Cyclopentadienyl Ring-Slippage

The reaction of the tricarbadecaboranyl anion, 6-Ph-nido-5,6,9-C₃B₇H₉^-, with M(CO)₅Br [M = Mn, Re] or [(η⁶-C₁₀H₈)Mn(CO)₃⁺]BF₄^- yielded the half-sandwich metallatricarbadecaboranyl analogues of (η⁵-C₅H₅)M(CO)₃ [M = Mn, Re]. For both 1,1,1-(CO)₃-2-Ph-closo-1,2,3,4-MC₃B₇H₉ [M = Mn (2) and Re (3)], the metal is η⁶-coordinated to the puckered six-membered open face of the tricarbadecaboranyl cage. Reactions of 2 and 3 with isocyanide at room temperature produced complexes 8-(CNBu^t)-8,8,8-(CO)₃-9-Ph-nido-8,7,9,10-MC₃B₇H₉ [M = Mn (4), Re (5)], having the cage η⁴-coordinated to the metal. Photolysis of 4 and 5 then resulted in the loss of CO and the formation of 1-(CNBu^t)-1,1-(CO)₂-2-Ph-closo-1,2,3,4-MC₃B₇H₉ [M = Mn, Re (6)], where the cage is again η⁶-coordinated to the metal. Reaction of 2 and 3 with 1 equiv of phosphine at room temperature produced the η⁶-coordinated monosubstituted complexes 1,1-(CO)₂-1-P(CH₃)₃-2-Ph-closo-1,2,3,4-MC₃B₇H₉ [M = Mn (7), Re (9)] and 1,1-(CO)₂-1-P(C₆H₅)₃-2-Ph-closo-1,2,3,4-MC₃B₇H₉ [M = Mn (8), Re (10)]. NMR studies of these reactions at −40 °C showed that substitution occurs by an associative mechanism involving the initial formation of intermediates having structures similar to those of the η⁴-complexes 4 and 5. The observed η⁶−η⁴ cage-slippage is analogous to the η⁵−η³ ring-slippage that has been proposed to take place in related substitution reactions of cyclopentadienyl−metal complexes. Reaction of 9 with an additional equivalent of P(CH₃)₃ gave 8,8-(CO)₂-8,8-(P(CH₃)₃)₂-9-Ph-nido-8,7,9,10-ReC₃B₇H₉ (11), where the cage is η⁴-coordinated to the metal. Photolysis of 11 resulted in the loss of CO and the formation of the disubstituted η⁶-complex 1-CO-1,1-(P(CH₃)₃)₂-2-Ph-closo-1,2,3,4-ReC₃B₇H₉ (12).