Page 284 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

P1: GNH Final Pages
Encyclopedia of Physical Science and Technology EN009M-428 July 18, 2001 1:6

Metal Particles and Cluster Compounds 541

from the highest occupied molecular orbital which calcu- different perspectives. Carbon monoxide contains two de-
∗
∗
lations confirm to be primarily a carbon lone pair orbital. generate, orthogonal π bonds. The M π → CO π and the
∗
The sigma donation of a carbon lone pair should have M δ → CO π ∗ donation involve the same π orbital on CO.
little effect upon the strong C O bond. The fact that sig- Although they are symmetry allowed the interactions are
nificant changes in the C O bond do occur upon coordi- very weak due to poor overlap and energy match. The
nation to a metal center suggest that some other bonding strongest M d → CO π ∗ donation involves the other CO π ∗
interaction is involved. This additional bonding interac- orbital and an M M antibonding orbital. Donation from
tion is the π-backbonding briefly mentioned earlier. π- a metal–metal antibonding orbital should strengthen the
Backbonding involves a π-type overlap of occupied metal M M bond. Recall M M bonds are generally shortened
∗
d orbitals with the vacant π orbitals of CO. The metal to when bridged by CO. Note that there are now two metal
∗
ligand donation populates an orbital which is antibonding atoms supplying electron density into the CO π orbital;
between carbon and oxygen, the results being a reduction hence, an increased effect on the CO bond is observed.
of the C O bond order. The reduction in C O bond order The CO stretching frequencies for µ 2 -carbonyls typically
−1
is experimentally observed as a longer C O bond distance occur between 1700–1850 cm .
and a reduced C O stretching frequency. Free CO has a For triply bridging (µ 3 ) carbonyls the same situation
˚
−1
C O bond distance of 1.128 A and a ν CO of 2143 cm . exists, a sigma donation by the carbon lone pair (Fig. 29a)
The effects of coordination become apparent upon bond- and d−π backbonding occurs. Again, only the CO lobes
∗
ing to just one metal atom (i.e., terminal CO). The typical located on carbon are shown interacting with the M 3 trian-
˚
C O bond distance increases to about 1.15 A in metal car- gle. As is apparent by the C O stretching frequencies for
−1
bonyls and typical stretching frequencies are in the range µ 3 -carbonyls (as low as 1625 cm ) the amount of d−π ∗
−1
of 1800–2150 cm . backbonding is even greater than it is for µ 2 -carbonyls.
Doubly bridging or µ 2 -carbonyls participate in very Upon progressing from an M 2 to an M 3 species the number
similar bonding, that is a sigma donation of the lone pair of electrons available for d → π backbonding increases.
∗
∗
on carbon to vacant metal orbitals as well as d −π back- This additional electron supply certainly contributes to the
bonding from the metal to CO. The symmetry-allowed in- increase in backbonding which occurs. More importantly,
teractions are depicted in Fig. 28 which depicts the sigma however, there are now significant interactions involving
∗
donation of the carbon lone pair. Figure 28b–e show back- both of the orthogonal, π orbitals. It is the increase in π ∗
∗
bonding interactions. In Fig. 28c–e only the CO π lobes orbital involvement to which the further reduction in ν CO
located on carbon are shown interacting with the metal may be attributed.
atoms. Figure 28b and c show the same interaction from All of the bonding modes for CO thus far described
are ones which result in a two-electron donation of the
carbon lone pair. Additional sources of available electron
density include the C O π bond and the oxygen lone

FIGURE 28 M 2 –CO orbital interactions. (a) Sigma donation FIGURE 29 M 3 –CO orbital interactions. (a) Sigma donation of
of carbon lone pair, (b) and (c) backbonding interaction of carbon lone pair, (b) and (c) degenerate M 3 interactions back-
M π ∗ → CO π ∗ donation, (d) M π → CO π ∗ , and (e) M δ → CO π ∗ . bonding to orthogonal CO π ∗ orbitals.