Metal Complexes of CO, RNC, CS, and NO

Unlike a simple alkyl, CO is an unsaturated ligand, by virtue of the C-O multiple bond. As we saw in Section 1.6, such ligands are soft because they are capable of accepting metal electrons by back bonding that is, these ligands 

The OrganometalHc Chemisny of the Transition Metals, Fourth Edition, by Robert H. Crabtree Copyright 2005 John Wiley Sons, inc. 

As we saw in Section 1.6, we look first at the frontier orbitals of M and L because ihese usually dominate the M-L bonding. The electronic structure of free CO is shown in Fig. 4.1 and 4.1b. We start with both the C and the O ip-hybridized. The singly occupied sp and orbitals on each atom ftmn a o and 

We can tell where any particular CO lies on the continuum between 4.1 and 4.3, by looking at the IR spectrum. Because 43 has a lower C—O bond order than 4.1, 

Although 4.1-4.3 repre.sent three ideal. structures in the bonding range possible for CO, no one structure can be said to perfectly represent the situation for any particular case. There is therefore considerable looseness in the way carbonyls are represented in organometallic structures. Often, M—CO or M—C=0 are used. Whatever picture is chosen for graphical repre.sentation, the bonding picture discussed above still applies. 

CARBONYLS, PHOSPHINE COMPLEXES, AND LIGAND SUBSTITUTION REACTIONS 

We can tell where any particular CO lies on the continuum between 4.1 and 4.2, by looking at the IR spectrum. Because 4.2 has a lower C=0 bond order than 4.1, the greater the contribution of 4.2 to the real structure, the lower the observed CO stretching frequency will be the normal range is 1820-2150 cm. The m.o. picture leads to a similar conclusion. As the metal 

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