Syntheses and studies of carbonylchromium complexes of some metal chelates

Siddiqui, Zahir Uddin (1974) Syntheses and studies of carbonylchromium complexes of some metal chelates. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Complexes obtained by displacement of two and three carbonyl groups from hexacarbonylchromium by transition metal Schiff bases have been prepared and identified. Two carbonyl groups are replaced by a metal Schiff base forming a σ-bonded tetracarbonylchromium complex. Tricarbonylchromium π-complexes of metal Schiff bases are formed by displacement of three carbonyl groups from hexacarbonylchromium. The stereochemistry about the acceptor metal in the tetracarbonylchromium complex approximates to C₂v symmetry; under these conditions, four i.r. active carbonyl bands are expected and observed. Considering the local symmetry of the carbonyl groups, the tricarbonylchromium complexes belong to the C₃v point group, and so display two i.r. active carbonyl bands. -- Carbonyl stretching vibrations are used to see if electronic effects are transmitted from the central metal to the tricarbonylchromium moiety. The i.r. data for the C0 stretching region indicate a very small shift (∿ 10 cm⁻¹ higher in d¹⁰ than d⁴ systems) of νC₀ using different transition metals at the centre. The observed trend of C0 frequency is opposite to that expected in M(R-N-sal)n Cr(C0)₃ (n = 2 or 3) complexes (M, the central metal = Cr(II), Co(III), Co(II), Ni(II), Cu(II) and Zn(II) and R = CH₃, C₆H₅ and –CH₂-). This reverse increase of order of C0 stretching frequencies from that expected suggests that electron density of the ligand is decreasing as the number of d electrons increases in the central transition metals. This can be explained if π-bonding between M(II) and (R-N-sal)₂ is considered negligible compared to σ-bonding. -- β, the ratio of B in complex/B of free ion determined for a few complexes with the help of electronic spectra, was high (0.75-0.85). The high values of β suggest that the bonding is mostly ionic and a very small amount of covalency exists between the central metal and the ligand. -- These complexes are fairly stable toward oxidation by air and, in some cases, are even stable in solution for more than a day. – The overall spectroscopic study (i.r., U.V./visible and near i.r.) of tricarbonylchromium complexes of metal Schiff bases provides the following information: -- 1. The addition of a Cr(C0)₃ group to a metal Schiff base has no significant effect on stereochemistry of metal Schiff bases. -- 2. The electronic distribution of the Iigand (Schiff base) does not change very much by varying the central metal. – 3. There is very little covalent bonding between the central metal and the ligand (Schiff base). -- The reaction between metal phthalocyanine (metal = Cr(II), Cu(II) and Zn (11)) and hexacarbonylchromium yielded tricarbonylchromium complexes, but the purification of the complexes was not achieved.

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