Oxygen three-coordinate

Metal-Oxygen Bond Lengths, M—O—C Angles, and Torsion Angles for Three-Coordinate Aluminum and Gallium Alkoxides and Aryloxides... 

Thus it has so far proved possible to isolate stable derivatives of monomeric metaphosphoric acid and of metathio- and metaselenophosphoric acid, which, understandably, generally bear tert-butyl and/or trimethylsilyl substituents u. Specifically, we know aminobisiminophosphoranes (3, Z = NR2, X = Y = NR)2,3,4), aminoiminothio (or seleno)phosphoranes (5, Z = NR2, X = NR, Y = S or Se)5), and aminoiminomethylenephosphoranes (1, R = NR2, X = NR)6>. Conspicuously, no stable phosphorus(V) three-coordinate compounds have been synthesized with oxygen as divalent ligand. 

A study of Pd(PPhs)4,4, reported recently by Roth and coworkers, reveals yet another mechanistic pathway for Pd° oxygenation [122]. Complete dissociation of one PPha ligand from Pd occurs in solution to produce a three-coordinate palladium(O) species, 26. Kinetic studies reveal that 26 reacts with dioxygen via parallel associative and dissociative pathways (Scheme 5). The latter dissociative pathway results in the formation of the two-coordinate complex 27, which undergoes very rapid reaction with dioxygen in a manner directly analogous to that of the well-defined two-coordinate NHC complexes 23 and 24. 

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