Table 1.

Competitive stability of complexes 1 and 11.

pH	Complex	Gradienta	r2 b	% Gd3+ retentionc
5.0	GdCl3	3.83 × 10−2	0.998	0
5.0	1	1.67 × 10−2	0.971	56.5 ± 3.4
5.0	11	3.34 × 10−2	0.995	12.8 ± 0.4
5.8	GdCl3	3.57 × 10−2	0.993	0
5.8	1	1.82 × 10−2	0.978	49.0 ± 2.8
5.8	11	2.94 × 10−2	0.998	17.6 ± 0.6

^aSlope of the line of best fit resulting from a plot of the average data using 10 different concentrations (2.0–250 μM).

^bRegression analysis of the line of best fit.

^cPercentage of Gd³⁺ retained by the phosphonium complex over a period of 24 h, as determined by the shift in absorbance from 433 to 573 nm, averaged across all data points.