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The Tolman cone angle method assumes empirical bond data and defines the perimeter as the maximum possible circumscription of an idealized free-spinning substituent. The metal-ligand bond length in the Tolman model was determined empirically from crystal structures of tetrahedral nickel complexes.
The Tolman electronic parameter (TEP) is a measure of the electron donating or withdrawing ability of a ligand. It is determined by measuring the frequency of the A 1 C-O vibrational mode (ν(CO)) of a (pseudo)-C 3v symmetric complex, [LNi(CO) 3 ] by infrared spectroscopy , where L is the ligand of interest.
The Tolman cone angle and Tolman electronic parameter [3] are named after him. In 1972, he proposed the 16 and 18 electron rule , extending Irving Langmuir 's 18-Electron rule to include the many examples of stable 16 electron square planar d 8 complexes. [ 4 ]
Cone angle is a common and useful parameter for evaluating the steric properties of phosphine ligands. In contrast to tertiary phosphines, tertiary amines , especially arylamine derivatives, are reluctant to bind to metals.
As a ligand, trimethylphosphine's Tolman cone angle is 118°. [7] This angle is an indication of the amount of steric protection that this ligand provides to the metal that to which it is bound. Being a relatively compact phosphine, several can bind to a single transition metal, as illustrated by the existence of Pt(PEt 3) 4. [8]
In terms of steric bulk, PPh 3 has a Tolman cone angle of 145°, [19] which is intermediate between those of P(C 6 H 11) 3 (170°) and P(CH 3) 3 (115°). In an early application in homogeneous catalysis, NiBr 2 (PPh 3) 2 was used by Walter Reppe for the synthesis of acrylate esters from alkynes, carbon monoxide, and alcohols. [20]
Brett Tolman. December 12, 2024 at 4:38 PM. Biden wags finger at White House Christmas party (Getty Images)
Triethylphosphine is a highly basic ligand that forms coordination complexes with many metals. As a ligand, triethylphosphine's Tolman cone angle is 132°. [3] Being a relatively compact phosphine, several can bind to a single transition metal, as illustrated by the existence of Pt(PEt 3) 4. [4]