Cr-donor orbitals

In order to determine the shapes of the filled MOs, we must first determine the shapes of the six SALCs for the ligands. Using the O sub-rotational group and the projection operator method, the mathematical forms of the six SALCs are derived as follows. Using the cr-donor orbitals lying along the x, —x, y, —y, z, and —z axes as our basis functions, the 0 SALC will be totally symmetric ... 

IR data [131] shows a trend to increasing i/(C-0) as the substituents on the phosphine became more electron withdrawing (Table 2.11) so that as the cr-donor power of the phosphine decreases and the 7r-acceptor power increases, the electron density at Ir decreases and electrons are removed from the "-orbital of CO [132]. 

As heavier analogs of carbenes141) stannylenes can be used as ligands in transition-metal chemistry. The stability of carbene complexes is often explained by a synergetic c,7t-effect cr-donation from the lone electron pair of the carbon atom to the metal is compensated by a a-backdonation from filled orbitals of the metal to the empty p-orbital of the carbon atom. This concept cannot be transferred to stannylene complexes. Stannylenes are poor p-a-acceptors no base-stabilized stannylene (SnX2 B, B = electron donor) has ever been found to lose its base when coordinated with a transition metal (M - SnXj B). Up to now, stannylene complexes of transition metals were only synthesized starting from stable monomoleeular stannylenes. Divalent tin compounds are nevertheless efficient cr-donors as may be deduced from the displacement reactions (17)-(20) which open convenient routes to stannylene complexes. 

A T structure with the strongest ct-donor D trans to the empty site (I in Scheme 1) is preferred in the case of three pure cr-donor ligands. The presence of a ir-acceptor ligand also makes the T structure more stable. When one of the ligands is a tt-donor, X, a Y structure of type II (Scheme 1) is observed. This structure permits the formation of a w bond between the empty metal d orbital and the lone pair of X. No such tt bond is present in the T structure since all symmetry adapted d orbitals are filled. This partial M—X multiple bond stabilizes Y over T. 

HOMO-LUMO) interactions the LUMO being the antibonding cr x orbital [45], the HOMO a non-bonded electron pair, formally available at both 90° and about 180° to the C-X bond [46], Much similar work supports this interpretation. Contacts between halogens (X) and electrophilic centres E (all metal ions) [47] fall almost exclusively in the range 9O<0E<12O°, while, for better electron donors Nu, 0Nu generally lies between 150° and 180°. 

There is some uncertainty whether this complex should be described as [Vm(bipy- )3] or as [V°(bipy)3], In fact, given that 2,2 -bipyridine can act either as a cr-donor or a n-acceptor, the metal-ligand bond in these complexes is constituted by a cr-bond between the lone pair of electrons of the nitrogen atom and an unoccupied s-orbital of the metal. Such electron donation, increasing the electron density on the metal, can in turn favour a back-bonding from the d-orbitals of the metal and the unoccupied rt -orbitals of the aromatic pyridine ring. In short, if the metal ion is in a high oxidation state pyridine will act as a a donor, whereas if the metal is in a low oxidation state pyridine will act as a n acceptor. 

The interaction of the Cr atom with the trigonal set of CO groups is governed by the symmetry properties of orbitals under C3 rotations. The set of three a donor orbitals on the carbon atoms will give SALCs of a and e symmetry. It must be noted that in C3 symmetry there is no ex versus e2 distinction both sets of d orbitals, (dsz, dyz) and dxy) belong to the E... 

It appears that the metal-ligand bonds in carbonyl and dinitrogen complexes are similar but somewhat weaker in the dinitrogen complexes. Carbon monoxide is no only a better cr donor but also a better ir acceptor. This is 10 be expected on the basis of whatever polarity exists in the CO molecule (see Fig. 5.18) and the fact that the ir antibonding orbital is concentrated on the carbon atom (see Fig. 5.20), which favors overlap with the metal orbital. The superior r accepting ability of CO also accounts for the instability of carbonyl dinitrogen complexes. Both Cr(CO)sN2 and cis-... 

The large differences in metal-centered redox potentials between chromium and molybdenum have been attributed to the existence of a greater interaction between the Cr dxy orbital and the a and/or k orbitals of the corrole ligand. Such interaction would in fact reduce d electron density at the metal. Another factor considered to cause the potential difference has been a strong covalent interaction between chromium and N-donor ligands. 

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