Transition metal bonding units

Fig. 2. Significant molecular orbitals of terminal transition metal-oxo units in a six-coordinate 4 ligand environment. The d° configuration is a formal triple bond. The highest occupied molecular orbital in the d configuration is formally nonbonding (8 symmetry) so the metal-oxo bond order remains 3.0. However, d-electron counts above d populate orbitals that are antibonding between the metal and the terminal multiply bonded ligand (0x0 in this case, but alternatively, alkyl-imido, nitrido, sulfido, etc.). Note that all the equatorial ligand orbitals and the metal dx2 y2 orbital (hi in 4 symmetry) are ignored for simplicity.

The Number of Skeletal Bonding Electrons (t -f a — 12) That Transition Metal Cluster Units Can Contribute ... 

Multiple metal-metal bonds have since been found in Re and many other transition metal complexes [41]. The structure of the unsupported Re2 unit (2) is shown in Chart 1. Bonds to the ligands in the equator around each metal may be staggered or eclipsed depending on electronic structure, and solvent or unidentate ligands may bind weakly at the axial positions. In some cases, polydentate ligands may bridge the metal-metal-bonded unit (3). 

A typical example of the three dominant bonding molecular orbitals obtained from the orbital interaction diagram of the cyclopropenylium ring and group 10 (Ni, Pd, Pt) transition metal ML2 unit is shown below. 

Oxalamidinate anions represent the most simple type of bis(amidinate) ligands in which two amidinate units are directly connected via a central C-C bond. Oxalamidinate complexes of d-transition metals have recently received increasing attention for their efficient catalytic activity in olefin polymerization reactions. Almost all the oxalamidinate ligands have been synthesized by deprotonation of the corresponding oxalic amidines [pathway (a) in Scheme 190]. More recently, it was found that carbodiimides, RN = C=NR, can be reductively coupled with metallic lithium into the oxalamidinate dianions [(RN)2C-C(NR)2] [route (c)J which are clearly useful for the preparation of dinuclear oxalamidinate complexes. The lithium complex obtained this way from N,N -di(p-tolyl)carbodiimide was crystallized from pyridine/pentane and... 

The components of the Ziegler-Natta catalyst unite in such a way that there is a vacant co-ordination site on the transition metal to which a molecule of monomer can bond, a second molecule of monomer then attaches itself to another vacant coordination site, from which it is able to react with the first molecule of coordinated monomer. This causes the second coordination site to become vacant once again, thereby allowing a further molecule... 

The moiety denoted as I is the initiator group. It can be as simple as a free radical or as complicated as a transition metal atom bonded to organic ligands and located on a catalytic support. The next step in the polymerization is propagation, i.e., the repeated insertion of monomer units into the chain to create an incrementally longer chain... 

The late transition metals, either Co or Ni, bond to four apical oxygen atoms of two vertex-shared 5 units (Fig. 17.9a), two each to layers both above and below, and also bond to two bridging pyrazine ligands in a trans fashion. The al-... 

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