A Less Carbocentric View of Agostic Interactions: The Complexes [Rh(η4-cod){H2A(mt)2}] (A = B, C+; mt = Methimazolyl)

The reaction of [Rh 2 (μ-Cl) 2 (cod) 2 ] (cod = 1,5-cyclooctadiene) with Na[H 2 B(mt) 2 ] (mt = methimazolyl) provides the complex [Rh(cod){H 2 B(mt) 2 }] ( 1 ), which in turn reacts with CO or CNC 6 H 3 Me 2 -2,6 to provide the derivatives [RhL 2 {H 2 B(mt) 2 }] (L = CO ( 2 ), CNC 6 H 3 Me 2 -2,6 ( 3 )). In a similar manner [Rh 2 (μ-Cl) 2 (cod) 2 ] reacts with H 2 C(mt) 2 to provide the cationic complex [Rh(cod){H 2 C(mt) 2 }] + ( 4 + ), which was isolated after counteranion metathesis as either 4 ·BF 4 or 4 ·PF 6 . Carbonylation of 4 ·PF 6 provides [Rh(CO) 2 {H 2 C(mt) 2 }]PF 6 ( 5 ·PF 6 ); however, the reaction is reversible in the presence of cod, regenerating 4 ·PF 6 under reduced pressure. The reactions of 4 ·PF 6 with CNC 6 H 3 Me 2 -2,6 or CNCMe 3 provide respectively the salts [Rh(CNC 6 H 3 Me 2 -2,6) 2 {H 2 C(mt) 2 }]PF 6 ( 6 ·PF 6 ) and [Rh(CNCMe 3 ) 4 ]PF 6 . The crystal structures of 1, 4 ·BF 4, 4 ·PF 6, and 6 ·BF 4 reveal long B−H···Rh or C−H···Rh interactions; however, such an interaction is effectively absent for 5 ·PF 6 ·CHCl 3, while solution spectroscopy ( 1 H NMR and IR) indicates that none of these interactions persist in solution. The complexes 1 and 4 + represent the first structurally characterized isoelectronic pair involving, albeit weak, three-center−two-electron B−H−metal or C−H−metal interactions.