An Assessment of the Parameters Relevant to the Subdivision of σ and Ω Electronic Effects in M-P Bonds

The bite angle makes the difference: a practical ligand parameter for diphosphine ligands

“…Metal NMR shifts can be a sensitive probe to electronic changes on a metal centre. 45 The 103 Rh NMR shift of a series of [Rh(hfacac)(P-P)] (hfacac = hexafluoroacetylacetonate) complexes is related to the P-Rh-P bite angles in a direct linear correlation. 46 The 103 Rh signal is shifted downfield with increasing ligand bite angle.…”

On the coordination chemistry of phosphinecarboxamide: assessing ligand basicity

“…3 Understanding their steroelectronic properties and the nature of the metal-phosphine bond has historically attracted considerable attention and no little controversy. 4 Amongst the most well-established means of quantifying the strength and character of the metal-phosphine interaction are Tolman's Electronic Parameter (TEP; w i ) and Cone Angle (TCA; y). 5 While the latter is widely accepted as a valid measure of steric bulk, the interpretation of the electronic parameter remains controversial, and several alternatives have also been proposed.…”

“…6 The problem in finding an unambiguous measure of the electronic properties of a phosphine lies in the separation of the (often) complementary effects resulting from the participation of s-donor and p-acceptor orbitals (the so-called s/p controversy). 4 Nevertheless, such methods remain commonly employed despite their limitations, and are very insightful for the interpretation of net effects such as trans-influence. So much so, that many of these concepts have since been extended to other families of supporting ligands such as N-heterocyclic carbenes.…”

Steric effects of bulky phosphines on 95Mo NMR shieldings

“…(a) Strong IT-ligands: PF,, ETPB, P(OMe),, and P(OPh), (b) Strong a -and (or) moderate IT-ligands: PEt,, P(n-Bu),, and PMe,, (c) Extra bulky ligands: P(OSiMe,),, PCy,, P(2-Me-Ph),, and P(2,6-Me,-OPh), (a) Strong m-ligands: PF3(104"), ETPB(lOlO), P(OMe)3(1070), and P(OPh),(128") These four ligands are well known as good IT-acceptors, based on the high stretching frequencies of trans CO's in transition metal complexes (6,13), shorter M-P bond distances (14), high stability of the M-P link (15,16), high reactivity toward displacement of CO in metal carbonyl moieties (1,3,4), and other electrochemical (12J 17) and spectroscopic studies (6, 8a, 18). In agreement with these observations and the expectation that the AE term in the u p term is enhanced by good ITacceptor ability, the phosphites indeed make the 9 5~o nuclei in their molybdenum carbonyl derivatives quite shielded compared with most other ligands, whether in m l or f3 derivatives (Fig.…”

“…An answer to this anomaly is more likely related to the T ability of PMe, (6,13,26). The markedly high frequency shift of 6 ( 9 5~0 ) for the f3 and c2 derivatives of PEt, and PBu, ligands can be attributed only to the steric interaction.…”