Im Niemandsland der Sechsfach-Koordination bei d0-Übergangsmetallkomplexen: Strukturvorhersagen für [WCl5CH3], [WCl4(CH3)2] und [WCl3(CH3)3]

Abstract

Ungewöhnliche Strukturen zwischen Oktaeder und trigonalem Prisma wurden in Dichtefunktionalrechnungen an den „einfachen”︁ sechsfach koordinierten d0‐Titelkomplexen gefunden. Während für [WCl5CH3] ein verzerrtes Oktaeder noch eine etwas niedrigere Energie aufweist als ein Prisma, tauchen für [WCl4(CH3)2] intermediäre Strukturen auf (siehe Abbildung). Im Falle von [WCl3(CH3)3] sind prismatische Anordnungen bereits günstiger. Diese und viele verwandte Komplexe sollten experimentell zugänglich sein.

“Non‐VSEPR” Structures and Bonding in d0Systems

“…Thus, the π‐donor abilities of the ligands appear to be more important here than the bond ionicities. This has the interesting consequence that successive alkyl substitution in mixed compounds [W(CH 3 ) m X 6− m ] leads to a transition from octahedral to prismatic structure at lower m value for X=F than for X=Cl (see Section 4.2) 78. These results are consistent with a recent study on metal–chalcogen bonding in related d 0 complexes 181.…”

“…In this case, however, the distortion from an octahedron to a trigonal prism would be expected to require the largest energy for the most ionic systems, that is, with fluoride ligands. In disagreement with this expectation, our DFT calculations78 (see Table 2) indicate almost twice the barrier for the trigonal twist of the tungsten or molybdenum chlorides or bromides than for the respective fluorides (our results for [WF 6 ] agree well with those of previous ab initio or DFT calculations62, 187). Thus, the prismatic structure is almost twice as favorable energetically for the fluoride as for the chloride, in spite of the much larger ionicity of the former (e.g., computed NPA metal charges at the O h minima are +0.05, +0.25, +1.91, and +2.23 for [MoCl 6 ], [WCl 6 ], [MoF 6 ], and [WF 6 ], respectively).…”

“…The structures of all of these complexes are presumed to be highly fluxional. Already with three methyl ligands, there appears to be a clearer preference for the trigonal prism, with the presence of different isomers of similar energy 78. This preference is further enhanced upon substitution of the remaining halides by methyl groups, with no particularly surprising structures found along the way 206.…”

“…However, meanwhile both experiment77 and theory62, 76 agree with an octahedral structure. The conversion from octahedron into trigonal prism is computed62, 76 to require actually somewhat higher energy than, for example, in [WF 6 ] 78. Table 2 summarizes activation barriers calculated for the trigonal twist of the Group 6 hexahalides.…”

“…This leads us into the range of structures intermediate between octahedron and trigonal prism, typically along the trigonal‐twist pathway (see also Section 2.5). A recent DFT study explored a different route into this “no‐man's‐land”:78 starting from [WCl 6 ] or [WF 6 ], the halides were successively replaced by methyl ligands, to provide the heteroleptic series [WX 6− n (CH 3 ) n ] ( n =1–5). The question was, how the structures proceed from the octahedral preference in [WX 6 ] to the distorted trigonal prism of [W(CH 3 ) 6 ].…”