The synthesis and characterization of a homologous series of five-coordinate rhodium(III) and iridium(III) complexes of PNP (2,6-(tBu(2)PCH(2))(2)C5H3N) and PONOP (2,6-(tBu(2)PO)(2)C5H3N) pincer ligands are described: [M(PNP)(biph)][BAr4F] (M = Rh, 1a; Ir, 1b; biph = 2,2'-biphenyl; Ar-F = 3,5-(CF3)(2)C6H3) and [M-(PONOP)(biph)][BAr4F] (M = Rh, 2a; Ir, 2b). These complexes are structurally dynamic in solution, exhibiting pseudorotation of the biph ligand on the H-1 NMR time scale (Delta G(double dagger) ca. 60 kJ mol(-1)) and, in the case of the flexible PNP complexes, undergoing interconversion between helical and puckered pincer ligand conformations (Delta G(double dagger) ca. 10 kJ mol(-1)). Remarkably, the latter is sufficiently facile that it persists in the solid state, leading to temperature-dependent disorder in the associated X-ray crystal structures. Reaction of 1 and 2 with CO occurs for the iridium congeners 1b and 2b, leading to the formation of sterically congested carbonyl derivatives.