STRUCTURES OF A SERIES OF [4-R-C6H4-CH(OR')2]CR(CO)3 COMPLEXES - EVIDENCE AGAINST A FAVORED CARBONYL ORIENTATION IN (PARA-DISUBSTITUTED ARENE) CHROMIUM TRICARBONYL COMPOUNDS

The structures of eight, (para-substituted benzaldehyde dialkylacetal) chromium tricarbonyl complexes [4-R-C6H4-CH(OR')2]Cr(CO)3 (R=Me2N, Cl, Me3Si, CF3; R'=Me, Et) have been determined by single-crystal X-ray d iffraction. For [4-Me2N-C6H4-CH(OMe)2]Cr(CO)3 (1), triclinic, P1BAR, T = 291 K, a = 6.725 (2) angstrom, b = 7.228 (3) angstrom, c = 16.021 ( 8) angstrom, alpha = 1.38 (4)-degrees, beta = 87.86 (3)-degrees, gamma = 87.77 (3)-degrees, Z = 2; for [4-Me2N-C6H4-CH(OEt)2]Cr(CO)3 (2), mo noclinic, P2(1)/n, T = 123 K, a = 10.905 (7) angstrom, b = 15.670 (9) angstrom, c = 11.158 (9) angstrom, beta = 116.61 (7)-degrees, Z = 4; f or [4-Cl-C6H4-CH(OMe)2]Cr(CO)3 (3), monoclinic, P2(1)/n, T = 293 K, a = 9.896 (3) angstrom, b = 12.872 (4) angstrom, c = 11.336 (4) angstrom , beta = 108.21 (3)-degrees, Z = 4; for [4-Cl-C6H4-CH(OEt)2]Cr(CO)3 (4 ), triclinic, P1BAR, T = 293 K, a = 8.305 (3) angstrom, b = 9.999 (2) angstrom, c = 11.313 (4) angstrom, alpha = 106.71 (2)-degrees, beta = 101.66 (3)-degrees, gamma = 110.27 (3)-degrees Z = 2; for [4-Me3Si-C6H 4-CH(OMe)2]Cr(CO)3 (5), triclinic, P1BAR, T = 293 K, a = 6.933 (2) ang strom, b = 11.709 (3) angstrom, c = 12.071 (5) angstrom, alpha = 112.5 7 (3)-degrees, beta = 94.32 (3)-degrees, gamma = 100.52 (2)-degrees, Z = 2; for [4-Me3Si-C6H4-CH(OEt)2]Cr(CO)3 (6), monoclinic, P2(1)/c, T = 292 K, a = 11.823 (3) angstrom, b = 12.926 (4) angstrom, c = 14.608 ( 9) angstrom, beta = 113.03 (3)-degrees, Z = 4; for [4-F3C-C6H4-CH(OMe) 2]Cr(CO)3 (7), monoclinic, P2(1)/n, T = 293 K, a = 6.756 (3) angstrom, b = 10.300 (3) angstrom, c = 21.112 (6) angstrom, beta = 95.18 (3)-de grees, Z = 4; for [4-F3C-C6H4-CH(OEt)2]Cr(CO)3 (8), monoclinic, C2/c, T = 123 K, a = 18.992 (5) angstrom, b = 14.065 (4) angstrom, c = 13.23 2 (4) angstrom, beta = 110.53 (3)-degrees, Z = 8. Compounds 1/2 exhibi t an eclipsed carbonyl orientation, presumably owing to the presence o f the strongly donating dimethylamino substituent. Compounds 3/4 exhib it a staggered arrangement, possibly reflecting electronic similarity between the chloro and acetal groups, or dominance of steric effects i n conformation determination. Intriguingly, compounds 5/6 exhibit diff erent geometries, as do 7/8, which by their nature do not appear to re late either to steric or electronic effects. It is proposed that more subtle factors, such as crystal packing forces, dictate the carbonyl o rientation in these cases, suggesting that a priori prediction of the conformation is often unjustified.