2,2′‐Biphosphinines and 2,2′‐Bipyridines in Homoleptic Dianionic Group 4 Complexes and Neutral 2,2′‐Biphosphinine Group 6 d⁶ Metal Complexes: Octahedral versus Trigonal‐Prismatic Geometries

Abstract

The geometric and electronic structure of formally d(6) tris-biphosphinine [M(bp)(3)](q) and tris-bipyridine [M(bpy)(3)](q) complexes were studied by means of DFT calculations with the B3LYP functional. In agreement with the available experimental data, Group 4 dianionic [M(bp)(3)](2-) complexes (1P-3P for M=Ti, Zr, and Hf, respectively) adopt a trigonal-prismatic (TP) structure, whereas the geometry of their nitrogen analogues [M(bpy)(3)](2-) (1N-3N) is nearly octahedral (OC), although a secondary minimum was found for the TP structures (1N'-3N'). The electronic factors at work in these systems are discussed by means of an MO analysis of the minima, MO correlation diagrams, and thermodynamic cycles connecting the octahedral and trigonal-prismatic limits. In all these complexes, pronounced electron transfer from the metal center to the lowest lying pi* ligand orbitals makes the d(6) electron count purely formal. However, it is shown that the bp and bpy ligands accommodate the release of electron density from the metal in different ways because of a change in the localization of the HOMO, which is a mainly metal-centered orbital in bp complexes and a pure pi* ligand orbital in bpy complexes. The energetic evolution of the HOMO allows a simple rationalization of the progressive change from the TP to the OC structure on successive oxidation of the [Zr(bp)(3)](2-) complex, a trend in agreement with the experimental structure of the monoanionic complex. The geometry of Group 6 neutral complexes [M(bp)(3)] (4P and 5P for M=Mo and W, respectively) is found to be intermediate between the TP and OC limits, as previously shown experimentally for the tungsten complex. The electron transfer from the metal center to the lowest lying pi* ligand orbitals is found to be significantly smaller than for the Group 4 dianionic analogues. The geometrical change between [Zr(bp)(3)](2-) and [W(bp)(3)] is analyzed by means of a thermodynamic cycle and it is shown that a larger ligand-ligand repulsion plays an important role in favoring the distortion of the tungsten complex away from the TP structure.

PdII and PtII Complexes of 2‐(2′‐Pyridyl)‐4,6‐diphenylphosphinine: Synthesis, Structure, and Reactivity

“…The pyridine and aromatic phosphinine rings are essentially coplanar with respect to one another in 4 and 5. The intercyclic C5ÀC6 bonds of 1.470(7)-1.476 (7) are only slightly shorter than in free 2,2'-bipyridine (1.490(3) ), whereas the N À C6 bonds of 1.372(6)-1.386 (6) are slightly longer than the corresponding bond in 2,2'-bipyridine (1.346 (2) ). [19] The P À C bond lengths of 1.711(5)-1.723 (5) are somewhat shorter than in free 2,4,6-triarylphosphinines (1.74-1.76 ), [20] whereas the carbonÀcarbon bond distances in the aromatic phosphinine subunit are in the usual range (1.386(7)-1.413 (7) ) observed for both free and complexed phosphinines.…”

“…[4,5] Only a few studies have so far been devoted to its coordination chemistry as this particular ligand requires a multistep synthesis and is, at the same time, difficult to handle due to its high sensitivity towards nucleophilic attack at the phosphorus atom and facile protonation at the nitrogen center. [6] The few examples reported in the literature are complexes of the type [MA C H T U N G T R E N N U N G (NIPHOS)(CO) 4 ] (M= Cr, Mo, W) [7] and Rh I and Ir I dimers of the type [Ir 2 A C H T U N G T R E N N U N G (cod) 2 -A C H T U N G T R E N N U N G (NIPHOS) 2 ][X] 2 and [Rh 2 A C H T U N G T R E N N U N G (nbd) 2 A C H T U N G T R E N N U N G (NIPHOS) 2 ][X] 2 (X = SbF 6 , cod = 1,5-cyclooctadiene, nbd = norbornadiene) [8] in which the phosphinine ligands adopt a m 2 -bonding mode. Latter studies have also shown that NIPHOS binds more strongly to Ir I and Rh I than 2,2'-bipyridine.…”

The Trigonal Prism in Coordination Chemistry

“…[14] A related situation seems to explain the trigonal prismatic coordination of Ti, Zr, Hf, and W complexes with biphosphinine (the phosphorous analogue of bipyridine), [51,52] which, according to structural and theoretical studies, are best described as composed of dianionic ligands and d 0 metal ions. [52,53] Finally, two remarkable complexes of d 10 ions with trigonal-prismatic coordination spheres must be noted. One is the [ZnA C H T U N G T R E N N U N G (BH 4 ) 3 ] À ion [55] in which the tetrahydroborate ligands act as bidentate, forming a compressed trigonal prism.…”

“…A related situation seems to explain the trigonal prismatic coordination of Ti, Zr, Hf, and W complexes with biphosphinine (the phosphorous analogue of bipyridine),51, 52 which, according to structural and theoretical studies, are best described as composed of dianionic ligands and d 0 metal ions 52. 53…”

2-(2′-Pyridyl)-4,6-diphenylphosphinine versus 2-(2′-pyridyl)-4,6-diphenylpyridine: an evaluation of their coordination chemistry towards Rh(i)

“…4,5 However, only a few studies were devoted so far to its coordination chemistry as this particular ligand requires a multistep synthesis and is, at the same time, difficult to handle due to its high sensitivity towards nucleophilic attack at the phosphorus atom and facile protonation at the nitrogen center. 6 The few examples reported in the literature are complexes of the type [M(NIPHOS)(CO) 4 ] (M = Cr, Mo, W), 7 We have recently started to investigate the synthesis and coordination chemistry of functionalized 2,4,6-triarylsubstituted phosphinines, which are generally accessible via the modular pyrylium-salt route. [10][11][12] It turned out that 2-(2 0 -pyridyl)-4,6-diphenyl-phosphinine 1 (Fig.…”