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The synthesis and characterization of the first isolable terminal carbonyl complex of Zr(1V) is described. The reaction of (C&&Zr(CH3) (~-CH~)B(CGFS)~ with CO yields the +acyl carbonyl complexes [(CsRs)2Zr(l12-COCH3)(C0)1 [ C H~B ( C~F S )~I (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 +-acyl ligand. Analogous O-outside (3a) and O-inside (3b) isomers are observed for 3, but the isomer preference is reversed (3a/3b = 1/51. The vco values for the O-inside isomers 2b (2152 cm-l) and 3b (2176 cm-l) are higher than the free CO value, indicating that the Zr-CO bond is primarily u-donor in character. The vco values for the O-outside isomers 2a (2105 cm-1) and 3a (2123 cm-l) are slightly lower than the free CO value, as a result of overlap of a filled Zr-acyl bonding orbital and a CO n* orbital (i.e. o-n* back-bonding). 2a can be isolated as a thermally stable solid; however, in CHzCl2 solution both 2 and 3 lose CO under vacuum. 2a has been characterized by sin le crystal X-ray diffraction: a = 16.951(5) A, b = 10.642(4) A, c = 22.217(6) A, V = 4008(4) 13, 2 = 4 in space group, Pca21. Consistent with the vco values, the C-0 distance (1.13(1) A) in the nearly linear carbonyl ligand (Zr-C-0 angle 175.1(7)') is not significantly perturbed from that in free CO. The Zr-Cco distance (2.25(1) A) is consistent with the absence of significant Zr-C n-bonding.

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The title compound, [Hf(C7H7)3(C10H15)], adopts a monomeric three‐legged piano‐stool structure. One benzyl ligand is disordered between two sites (44:56%) related by a ∼30° rotation about an axis defined by the Hf atom and the ipso‐C atom of the benzyl ligand.