On the Transition Metal Complexation (Fischer-Type) of Phosphanylcarbenes

Abstract

The formation of a Fischer-type transition metal complex with a W(CO) 5 fragment is evaluated for the phosphanylcarbenes, Arduengo-type carbenes, and Bertrand-type carbenes by means of quantum chemical investigations at a density functional level with effective core potential methods. Accordingly, the stabilities of the complexes depend strongly on the substitution pattern of the carbenic unit. Amino-substituted carbenes as well as the Arduengo-type carbene form stable transition metal complexes. The stabilities of the complexes decrease for the phosphanylcarbenes and are at a minimum for the hitherto unknown transition metal complexes of the push-pull-type carbene of Bertrand. The matter is analyzed in terms of distortion energies required to bring the carbene units into the geometrical standard state for complexation with the transition metal fragment. The arguments evaluated for the phosphanylcarbenes should hold equally well for other carbenes substituted with electropositive ligands. For the mono-phosphanylcarbenes, η 1 as well as η 2 structures are investigated. For the diphosphanylcarbenes, a new structural type of metal complexes is predicted in which the transition metal fragment is strongly bound to a cyclic structural valence isomer of the carbene.

On the nature of the dative bond: Coordination to metals and beyond. The carbon case

“…In all cases, the heterolytic dissociation remained favored, ranging from 0. character of the carbenic center ((R 2 N) 2 P + =C(:) --SiMe 3 ↔ (R 2 N) 2 P + =C= -SiMe 3 ), thus inducing a higher distorsion energy to reach the trigonal (sp 2 ) coordination geometry [90].…”

On the d6-Transition Metal Complex Formation of Electron-Rich Methylenephosphanes, a Quantum Chemical Investigation

“…The bonding situation for transition-metal complexation contrasts that of the phosphanylcarbene complexes, where both quantities are comparable in magnitude. [44] Overall the free energies (∆G) are smaller in magnitude than the electronic energies (∆E ϩ zero-point vibrational energy corrections).…”

“…In more detail, on a qualitative basis this extends the common view on bonding in Fischer-type complexes [43] to the diagram in Scheme 4, with II referring to donation and I to back-donation of electron density. One expects that the amount of back-donation π(ML 5 )Ǟπ*(PC) is less compared to (phosphanyl)carbenes, [44] due to the energetically higher (antibonding) π*-orbital in methylenephosphane as compared with the corresponding empty (nonbonding) porbital in methylene.…”

“…It is in contrast to the phosphanylcarbene complexes in which complexation requires substantial changes in the geometries of the free carbenes. [44] The situation for η 1 -coordination is more deeply analyzed by an inspection of the energies for bonding of the transition metal fragments (Table 1). They are defined by the reactions according to Equations (1) and (2).…”

Stable non-N-heterocyclic carbenes (non-NHC): recent progress

“…The apparent reluctance of carbenes D to act as ligands has recently been tentatively rationalized by theoretical study [36]. It was concluded that in contrast with NHCs, carbenes D tend to adopt widened valence angles at the central carbon, due to the electropositive character of the phosphanyl group.…”