T 2 coordination

Stokes, S. M. J. Ding, F. Smith, R L. Keane, J. M. Kopach, M. E. Jervis, R. Sabat, M. Harman, W. D. Formation of o-quinone methides from T 2-coordinated phenols and their controlled release from a transition metal to generate chromans. Organometallics 2003,22, 4170-4171. 

THFiLi moiety. (THFELi unit. t-3-Coordinated. t-2"Coordinated. Central Li- -Li vector. TMEDA nitrogen atom. 

The selective t 2 coordination of the diene to the palladium center is suggested.141... 

The insertion of t]2 coordinated styrene, by cis addition, would stepwise lead to t]1 bonding of the last incorporated monomeric unit and dissociation of the phenyl ring of the penultimate monomeric unit, thus providing the possibility of enantioselective t]2 coordination of another styrene molecule to give mirror related active species (Figure 4.2) [87]. 

In contrast to iron and cobalt, end-on superoxo-copper(II) species do not dominate the field of copper-oxygen chemistry. In 1 1 copper-dioxygen adducts, an alternative side-on, t 2 coordination mode is sometimes observed these [(L)Cu11 (t 2-(02 ")] or [(L)Cum-(ri2-(022 )] complexes are discussed below. Mononuclear copper-dioxygen complexes easily react with the second molecule of the Cu(I) complex, forming peroxo- or dioxo-bridged dinuclear species (Section 4.4). For sterically unhindered... 

T]2 coordination mode, where complexes with more BhU- ligands have shorter distances. However, differences are slight, the greater discrepancy (less than 0.1 A) being found between two Ti r 3 complexes with one BH4. ... 

The crystal structure of (T]3,Ti2.octa-2(E), 7-dien-l-yl)(triphenylphosphine)nickel(II) tetrafluoroborate has been reportedlOO. The reported X-ray crystal structure of dodeca-2( ),6( ),10(Z)-trien-l-yl)nickel(II) hexafluorophosphate exhibited almost planar T 3,Tl2,T 2-coordination of the C12 ligand around the central nickel atom. Detailed solution NMR studies of the complex have been reported 02 The X-ray crystal structure of (Tl. Tl. Tl -dodeca-2( ),6( ),10(Z)-trien-l-yl)nickel(II) tris(pentafluorophenyl)fluoroborate, [Ni(Ci2Hi9)]FB(C6F5)3, was determined OS and the compound shown to be an active catalyst for the 1,4-cM-polymerisation of butadiene. 

First 2-coordinate compound of P prepared by A. B. Burg (MeiP=PCF3). First 1-coordinate P compound (HC=P) made by T. E. Gier. 

Cyclobutadiene complexes afford a classic example of the stabilization of a ligand by coordination lo a metal and, indeed, were predicted theoretically on this basis by H. C. Longuei-Higgins and L, E, Orgel (1956) some 3y before the first examples were synthesized, In the (hypothetical) free cyclobutadiene molecule 2 of the 4 rr-electrons would occupy t /i and there would be an unpaired electron m each of the 2 degenerate oibilals 2, Coordination to a metal provides funhei interactions and avoids this unstable configuration, See also the discussion on ferra-boranes (p. 174). 

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

However, it seems that these are best viewed as platinum(II) species too, so that two-electron metal-to-ligand transfer has been effected. The structures of Pt(PPh3)2Z (Z = r]2-02, t 2-C3H4, t]2-CS2) (Figure 3.16) all involve square planar coordination as expected for platinum(II) rather than the tetrahedral 4-coordination anticipated for platinum(O). 

Fukui et /. have found that acetylcobinamide reacts faster than the cobalamin with hydroxide (t,/2 = 4 and 30 min, respectively, in 0.05 N KOH) (67). It is not known whether the cobinamide is five- or six-coordinate, but these relative rates establish the labilizing effect of the trans-axial ligand for heterolysis to give Co(I) as HjO or none > 5,6-dimethylbenziminazole (see also Section B,3). 

The molecular structures of several [TpBut]ZnX derivatives have been determined by x-ray diffraction. For example, x-ray diffraction studies confirm that the acetate ligand in [TpBut]Zn(r)1-02CMe) is bound to zinc in a unidentate mode, similar to that proposed for [TpBut]Mg(7j1-02CMe), but in contrast to the bidentate coordination proposed for the copper analogue [TpBut]Cu(T)2-02CMe) (86,87). Such a change in coordination mode for copper and zinc derivatives is to be anticipated on the basis of structural studies on the nitrate derivatives [TpBut]M(N03) (M = Co, Ni, Cu, Zn), as described in Section V,B,2,e. The thioacetate [TpPh]-Zn V-SC(0)Me (81), and cyanoacetate [Tp lZnlr -C CCH N) (88) derivatives also exhibit unidentate coordination. 

In this chapter, we have described some useful properties of coordinated QM complexes. For instance, remarkably stable T 2-o-QM complexes of Os can release the QM moiety upon oxidation, followed by controlled reaction with suitable dienophiles to form the chroman ring system.15 16 ri2-Coordinatedp-QM complexes can be easily released upon exposure to an electron-deficient alkene and trapped as the 1,6-Michael addition product.18 19... 

Here, f(xj x2) is sometimes called the correlation factor. Consequently, IT x, x2) =0 defines the completely uncorrelated case. However, note that in this case, i. e., for f(x2 x2) = 0, p2(x1 x2) is normalized to the wrong number of pairs, since JJ p2llconditional probability l(x2 x1). This is the probability of finding any electron at position 2 in coordinate-spin space if there is one already known to be at position 1... 

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