ISOLABLE ZIRCONIUM(IV) CARBONYL-COMPLEXES - SYNTHESIS AND CHARACTERIZATION OF (C5R5)2ZR(ETA-2-COCH3)(CO)+ SPECIES (R=ME, H)

The synthesis and characterization of the first isolable terminal carbonyl complex of Zr(IV) is described. The reaction of (C5R5)2Zr(CH3)(mu-CH3)B(C6F5)3 with CO yields the eta2-acylcarbonyl complexes [(C5Rr5)2Zr(eta2-COCH3)(CO)] [CH3B(C6F5)3] (2, R = Me; 3, R = H). Complex 2 exists as a 9/1 mixture of ''O-outside' (2a) and ''O-inside'' (2b) isomers which differ in the orientation of the eta2-acyl ligand. Analogous 0-outside (3a) and 0-inside (3b) isomers are observed for 3, but the isomer preference is reversed (3a/3b = 1/5). The nu(CO) values for the 0-inside isomers 2b (2152 cm-1) and 3b (2176 cm-1) are higher than the free CO value, indicating that the Zr-CO bond is primarily sigma-donor in character. The nu(CO) values for the 0-outside isomers 2a (2105 cm-1) and 3a (2123 cm-1) are slightly lower than the free CO value, as a result of overlap of a filled Zr-acyl bonding orbital and a CO pi* orbital (i.e. sigma --> pi* back-bonding). 2a can be isolated as a thermally stable solid; however, in CH2Cl2 solution both 2 and 3 lose CO under vacuum. 2a has been characterized by single crystal X-ray diffraction: a = 16.951(5) angstrom, b = 10.642(4) angstrom, c = 22.217(6) angstrom, V = 4008(4) angstrom3, Z = 4 in space group, Pca2(1). Consistent with the nu(co) values, the C-0 distance (1.13(1) angstrom) in the nearly linear carbonyl ligand (Zr-C-O angle 175.1(7)-degrees) is not significantly perturbed from that in free CO. The Zr-C(CO) distance (2.25(1) angstrom) is consistent with the absence of significant Zr-C pi-bonding.