7i -donor ligands

For the earlier metals these compounds usually involve 7i donor ligands like halide or alkoxide... 

The 18 electron-rule normally does not apply to transition metal complexes with 7i-donor ligands and metal atoms in normal oxidation states. In this case the metal orbitals with 7r-symmetry lie at relatively high energy and they stay empty or are only partially filled. In this way the formation of paramagnetic species can be possible. 

Formal valence state of iron L is a two-electron donor ligand Cp = 7i-cyclopentadienyl... 

In this chapter, the most efficient synthetic routes, the main stmctural features as well as reactivity patterns of odd-chain metallacumulene complexes bearing 7i-donor substituents, i.e., [M]=C(=C) =CR R ( = 1, 3, 5 R /R = NR2, OR, SR, SeR), are reviewed. In addition, the coordination chemistry of phosphonioace-tylides (R3P C=C ) and tricarbon monoxide (C3O) will also be discussed since these heteroatom-containing 77 -carbon ligands lead to closely related bonding situations, with participation of both neutral cumulenic and zwitterionic alkynyl-type mesomeric forms (Fig. 3). 

Structural parameters of selected mononuclear complexes containing heteroatomic 7i-donor-substituted allenylidene ligands are summarized in Table 1. When these data are compared to those reported for related complexes bearing conventional alkyl or aryl substituents (see Table 1 of chapter All-Carbon-Substituted Allenylidene and Related Cumulenylidene Ligands ), a lengthening of the M=C bond... 

D. Complexes with a-donor/7i-acceptor ligands 1. Alkenyl and alkynyl compounds... 

Since all the noble metal ions are soft Lewis acids, their size is adaptable according their 7i-donor or rc-acceptor capacity. Thus, irrespective of their oxidation state, all the noble metal ions fit well into the central hole of the porphyrinato ligand, and, generally, the C20N4 core of the porphyrin ring is nicely planar. 

Fischer-type complexes such as 1 were first prepared in 1964 and their chemical properties studied [1], Schrock-type nucleophilic complexes such as 2 were prepared later [2], They are formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7i-acccptcr ligand to metals of high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and the structures may be considered as ylides (eq. 8.1)... 

Computational studies by Rablen have suggested that ligands with 7i-donor abilities are much more efficient in the reduction of the B-H BDE [8]. On the basis of this finding, it has recently been demonstrated that complexes of NHCs and borane (NHC boranes) constitute a very interesting new class of radical hydrogen atom donors. Two of the first examples, 1 and 2, together with their calculated BDEs, are shown in Scheme 1. 

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